---
title: "Battery storage interconnection sizing: kVA and fault | Entogo"
description: "A 2 MW battery inverter holding IEEE 1547 reactive capability is a 2222 kVA source, so it needs a 2500 kVA transformer, 2673 A of conductor under NEC…"
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Home › Insights › How to size a battery storage interconnection: inverter kVA, transformer kVA and %Z

Energy Storage

# How to size a battery storage interconnection: inverter kVA, transformer kVA and %Z

Entogo September 21, 2026

![Medium-voltage substation and power distribution equipment at dusk, the grid interconnection point for a battery energy storage system](https://entogo.ca/_astro/battery-storage-interconnection-transformer-sizing.B07z-eni_VK898.webp)

Photo — Evgeniy Alyoshin · [Unsplash](https://unsplash.com/photos/a-couple-of-power-lines-sitting-next-to-each-other-FXw3zkbqd0w) · [Unsplash License](https://unsplash.com/license)

In short

A 2 MW battery inverter holding IEEE 1547 reactive capability is a 2222 kVA source, so it needs a 2500 kVA transformer, 2673 A of conductor under NEC 705.28, and 65 kA gear once the 600 V bus fault duty reaches 44,403 A.

## Key takeaways

- IEEE 1547-2018 sets Category B reactive capability at 44 percent of rated apparent power, a figure chosen to match 0.90 power factor, so a 2 MW inverter is a 2222 kVA source.
- Round 2222 kVA up the ANSI/IEEE C57.12 ladder to a 2500 kVA transformer — 88.9 percent loading is correct, and a 3000 kVA unit buys no additional code compliance.
- NEC 705.28(A) and 705.30 put the 125 percent multiplier on conductors and the overcurrent device, not on transformer kVA — 1.25 x 2138 A = 2673 A, so a 3000 A main under NEC 240.6(A).
- At 5.75 percent impedance the 600 V bus sees 41,837 A through the transformer plus 2,566 A from the inverter at 1.2 pu, for 44,403 A — which rules out 42 kA gear.
- CEC Rule 64-112 caps back-fed sources at 120 percent of a busbar rating for non-dwellings and 125 percent for dwelling units.
- CEC Rule 84-012 sizes transformer overcurrent protection under Section 26 with each winding evaluated as a primary, because power flows both ways.

A battery energy storage system is bought in megawatts and megawatt-hours, but everything between it and the utility is bought in kVA, amperes and kA. That conversion is where interconnection packages go wrong. A 2 MW **power conversion system** carrying the reactive capability IEEE 1547-2018 requires is not a 2000 kVA source — at the 0.90 power factor that capability corresponds to it is a **2222 kVA** source, which rounds up the ANSI/IEEE C57.12 ladder to a **2500 kVA** transformer, draws **2138 A** continuously at 600 V, and needs **2673 A** of conductor and a **3000 A** device once NEC 705.28(A) applies its 125 percent. The first number sets every one after it. What follows is the whole chain — inverter kVA, transformer kVA, %Z, fault duty and the clauses behind each step — and these are the rules the on-site [transformer sizing calculator](https://entogo.ca/tools/transformer-sizing-calculator/) runs.

## Why an inverter’s kW is not the kVA the transformer carries

A battery block is specified in kW and kWh — that split is worked through in [how to size a battery energy storage system](https://entogo.ca/insights/how-to-size-a-battery-energy-storage-system-power-vs-energy/). The interconnection is specified in kVA, because heating follows current and current follows apparent power. The bridge is the [power factor](https://entogo.ca/glossary/#kva-rating) the inverter must hold.

**IEEE 1547-2018** is where that requirement lives. Category A must inject reactive power up to 44 percent of its apparent power nameplate rating and absorb up to 25 percent; Category B must do both at 44 percent, across the full ANSI C84.1 Range A voltage rather than only at rated voltage. EPRI’s summary gives the reason: “The 44% capability was chosen to correspond to a 0.90 power factor at rated active power.” Utilities generally specify Category B at megawatt scale — see [what IEEE 1547-2018 requires](https://entogo.ca/insights/ieee-1547-der-grid-interconnection/).

> S (kVA) = P (kW) ÷ PF

A 2000 kW block asked to hold 0.90 power factor is a 2222 kVA machine. Put a 2000 kVA transformer under it and it runs 11 percent overloaded every hour the utility calls for vars.

| What is being sized | Governing clause | The multiplier |
| --- | --- | --- |
| Inverter apparent power from active power | IEEE 1547-2018 reactive capability | ÷ 0.90 (44 percent reactive) |
| Transformer kVA | IEEE C57.12.00 continuous rating | × 1.00, then round up the ladder |
| Output-circuit conductor ampacity | NEC 705.28(A) | × 1.25 of nameplate current |
| Overcurrent device on those conductors | NEC 705.30 | × 1.25 of nameplate current |
| ESS output conductors and device | NEC 706.30 and 706.31 | × 1.25 of ESS nameplate current |
| Back-fed connection to an existing busbar | NEC 705.12(B), CEC 64-112 | 120 percent of busbar rating |
| Transformer protection where power flows both ways | NEC 450.3 with 705.30(F)(1), CEC 84-012 with Section 26 | NEC names one side the primary; CEC takes each side in turn |

## What size transformer does a 2 MW battery block need?

### A worked example — 2 MW / 4 MWh at 600 V into a 12.47 kV feeder

A commercial and industrial site adds a 2 MW / 4 MWh system for peak shaving and var support. Inverter output is 600 V three-phase, 60 Hz; the feeder is 12.47 kV; the interconnection is a dedicated step-up transformer, not a back-feed.

> I = kVA × 1000 ÷ (√3 × V\_LL)

1. **Apparent power the transformer must carry.** The utility specifies IEEE 1547 Category B, so the inverter holds 0.90 power factor at rated output: 2000 kW ÷ 0.90 = **2222.2 kVA**.
2. **Rated continuous AC output current.** 2 222 200 ÷ (√3 × 600) = 2 222 200 ÷ 1039.23 = **2138.3 A**. This is the nameplate current every downstream rule keys off.
3. **Transformer rating.** Round 2222.2 kVA up the standard ladder — 1500, 2000, **2500**, 3000 kVA. Loading is 2222.2 ÷ 2500 = **88.9 percent**, a correct continuous duty point for a machine rated to IEEE C57.12.00 at 30 °C ambient. The 80 percent convention in [how to size a transformer](https://entogo.ca/insights/how-to-size-a-transformer-kva-selection/) covers diversified building load that can grow past its calculated demand; a current-limited inverter cannot, so 2222.2 ÷ 0.8 = 2778 kVA buys a 3000 kVA unit for a risk that is not there.
4. **Conductors and the low-voltage device.** NEC 705.28(A) requires output-circuit conductors rated at least 125 percent of nameplate current, and NEC 705.30 the same of the overcurrent device: 1.25 × 2138.3 = **2672.9 A**. The next standard rating in NEC 240.6(A) is **3000 A**, so the switchboard carries a 3000 A bus and main.
5. **Medium-voltage full-load current.** At the transformer’s rating: 2 500 000 ÷ (√3 × 12 470) = 2 500 000 ÷ 21 598.7 = **115.7 A**. The primary switch, cable and CT ratio follow — a 200:5 CT leaves room for the settings NEC 450.3(A) permits above 1000 V.
6. **Fault duty at the 600 V bus.** Secondary full-load current is 2 500 000 ÷ 1039.23 = 2405.6 A. At a specified 5.75 percent impedance ([which %Z to specify](https://entogo.ca/insights/transformer-impedance-percent-z-selection/)) the infinite-source contribution is 2405.6 ÷ 0.0575 = **41 837 A**, and the inverter adds 1.2 × 2138.3 = **2 566 A**, for **44 403 A**.
7. **Equipment ratings.** 44.4 kA rules out 42 kA gear; the switchboard, its main and every feeder device need a **65 kA** rating — see [available fault current and equipment short-circuit ratings](https://entogo.ca/insights/available-fault-current-sccr-what-to-specify/).

**Step 1 — apparent power the transformer must carry** — `S = P / PF` with P = 2000 kW, PF = 0.9 ratio → **S = 2,222.2 kVA**

**Step 2 — rated continuous AC output current at 600 V** — `I = S * 1000 / (sqrt(3) * V)` with S = 2222.2 kVA, V = 600 V → **I = 2,138.3 A**

> I\_min = 1.25 × I\_rated

**Step 4 — conductor and device floor at 125 percent** — `Imin = 1.25 * I` with I = 2138.3 A → **Imin = 2,672.9 A**

Every step reads from the one above it, and the only measured input is the inverter nameplate. Change the block size or voltage in the [transformer sizing calculator](https://entogo.ca/tools/transformer-sizing-calculator/) and the chain moves together.

## What do the standard block sizes work out to?

The table runs the same chain at 0.90 power factor, with the transformer and the device each rounded up — the ANSI ladder for one, NEC 240.6(A) for the other.

| Inverter active power | kVA at 0.90 PF | Standard transformer | LV | Inverter rated current | 125 % floor | Standard device | Transformer current at 12.47 kV | Transformer current at 27.6 kV |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 500 kW | 555.6 kVA | 750 kVA | 480 V | 668.2 A | 835.3 A | 1000 A | 34.7 A | 15.7 A |
| 1000 kW | 1111.1 kVA | 1500 kVA | 600 V | 1069.2 A | 1336.5 A | 1600 A | 69.4 A | 31.4 A |
| 1500 kW | 1666.7 kVA | 2000 kVA | 600 V | 1603.8 A | 2004.7 A | 2500 A | 92.6 A | 41.8 A |
| 2000 kW | 2222.2 kVA | 2500 kVA | 600 V | 2138.3 A | 2672.9 A | 3000 A | 115.7 A | 52.3 A |

At 1500 kW the 125 percent floor is 2004.7 A — 4.7 A past the 2000 A standard rating, forcing a 2500 A device and matching bus.

## How much fault current arrives at the low-voltage bus?

Two sources feed a fault on that bus, and they behave nothing alike.

> I\_sc (transformer) = I\_FLA ÷ (%Z ÷ 100)

> I\_sc (total) = I\_sc (transformer) + k × I\_PCS

The grid delivers whatever the **percent impedance** allows — the infinite-source screen used in [sizing transformer overcurrent protection](https://entogo.ca/insights/transformer-overcurrent-protection-fault-current/). The inverter delivers only what its semiconductors survive. The IEEE Power System Relaying Committee puts that at “a typical range between 1.1 pu to 2.0 pu” of rated current, and notes that a resource “having its current limit capped at 120%” injects far less negative-sequence current than a synchronous machine does. That 120 percent is an illustrative current limit, not a recommended multiplier; k below is 1.2 because datasheets commonly declare it, and the datasheet is where it should come from.

Impedance is a line on the purchase order, not a property of a rating. On the 2500 kVA, 600 V unit above:

| Specified %Z | Transformer contribution | Inverter at 1.2 pu | Total at the bus | Smallest common LV equipment rating that covers it |
| --- | --- | --- | --- | --- |
| 4.5 % | 53 458 A | 2 566 A | 56 024 A | 65 kA |
| 5.75 % | 41 837 A | 2 566 A | 44 403 A | 65 kA |
| 6.5 % | 37 009 A | 2 566 A | 39 575 A | 42 kA |
| 8.0 % | 30 070 A | 2 566 A | 32 636 A | 42 kA |
| 10.0 % | 24 056 A | 2 566 A | 26 622 A | 42 kA |

**Step 6 — fault duty at the 600 V bus** — `Isc = Ifla / (Z / 100) + k * Ipcs` with Ifla = 2405.6 A, Z = 5.75 %, k = 1.2 ratio, Ipcs = 2138.3 A → **Isc = 44,402 A**

The step between 5.75 percent and 6.5 percent moves the lineup from a 65 kA class to a 42 kA class, and it is spent before the transformer is released for manufacture. It is not free — impedance is also voltage regulation — and nameplate impedance carries a tolerance, so use the measured routine-test value.

## Where Canada differs — CEC Section 64 and Section 84

The arithmetic is identical in Canada; the code path is not. The Canadian Electrical Code splits the job between **Section 64** (renewable energy systems) and **Section 84** (interconnection of electric power production sources).

Three divergences matter. First, the back-feed limit. NEC 705.12(B) allows the sum of the source device and the busbar’s supply device to reach 120 percent of the busbar rating — it sat at 705.12(B)(3)(2) in the 2020 NEC and has moved between cycles, so cite the edition the authority having jurisdiction has adopted. CEC Rule 64-112 writes the same idea with a split: “the sum of the ampere ratings of the overcurrent devices in source circuits supplying power to a busbar or conductor may exceed the busbar or conductor rating to a maximum of 120% of the rating of the busbar or conductor,” and 125 percent for dwelling units. Neither governs the worked example — a dedicated step-up transformer on its own feeder is not a back-feed onto a shared bus.

Second, the transformer’s protection, where the codes genuinely part company. Rule 84-012 sizes it under Section 26 **by considering first one side of the transformer and then the other as the primary** — both windings get the test. The NEC designates a single primary: under 705.30(F)(1) it is the side with the highest available fault current, and Table 450.3(A) above 1000 V or Table 450.3(B) below sets the percentages, which differ from the CEC Rule 26-254 figures as set out in the [overcurrent protection article](https://entogo.ca/insights/transformer-overcurrent-protection-fault-current/). Third, isolation: Rules 84-010, 84-020, 84-022 and 84-024 drive protection from each source and a disconnecting means opening all ungrounded conductors simultaneously, Rule 64-060 does the same on the inverter side, and Rule 84-030 requires a warning notice and single-line diagram at the service.

### The same block on a 27.6 kV Canadian feeder

Steps 1 through 4 are unchanged — 2222.2 kVA, 2138.3 A, a 2500 kVA transformer, a 3000 A main. Only the primary moves: 2 500 000 ÷ (√3 × 27 600) = 2 500 000 ÷ 47 804.6 = **52.3 A**, under half the 12.47 kV case, so a 100:5 CT replaces the 200:5.

**Canadian case — 27.6 kV primary full-load current** — `I = kVA * 1000 / (sqrt(3) * V)` with kVA = 2500 kVA, V = 27600 V → **I = 52.3 A**

## What to specify

- **Inverter rated apparent power in kVA at the required power factor** — 2222 kVA for a 2000 kW block at 0.90 PF, per the IEEE 1547-2018 category named in the interconnection agreement.
- **Transformer kVA from the standard ladder, with loading stated** — 2500 kVA at 88.9 percent here, on the continuous rating basis of IEEE C57.12.00. State ambient and altitude if the site is outside the 30 °C, 1000 m basis.
- **Percent impedance as a specified value** — 5.75 percent gives 44 403 A at a 600 V bus and 65 kA gear; 8 percent gives 32 636 A and 42 kA gear. The figure comes from the short-circuit study.
- **Conductor ampacity and device rating at 125 percent of nameplate current** — 2672.9 A, rounded to 3000 A under NEC 240.6(A), per NEC 705.28(A) and 705.30, or NEC 706.30 and 706.31 where the ESS nameplate governs.
- **Inverter fault contribution as a declared per-unit figure** — 1.1 to 2.0 pu is the IEEE PSRC range; name the limit and its duration so the coordination study has a number.
- **Medium-voltage winding current, BIL and CT ratio** — 115.7 A at 12.47 kV or 52.3 A at 27.6 kV; [voltage class](https://entogo.ca/glossary/#voltage-class) fixes the [BIL](https://entogo.ca/glossary/#bil).
- **Protection and disconnection, both directions** — NEC 450.3 with 705.30(F)(1), or CEC 84-012 with Section 26 and the Rule 84-020 to 84-024 disconnecting means, with settings from the utility.

## Common mistakes

- **Sizing the transformer from kW.** A 2000 kW inverter on a 2000 kVA transformer runs at 111 percent whenever the utility calls for the vars IEEE 1547 requires it to supply. The correct figure is 2222 kVA, rounding to 2500 kVA. The interconnection study’s reactive dispatch requirement catches this.
- **Carrying a load-side margin over to an inverter-side transformer.** 1.25 × 2222 kVA = 2778 kVA, and the 80 percent convention gives the same answer, pushing the order to a 3000 kVA unit. NEC 705.28(A), 705.30 and 706.30 place the multiplier on conductors and overcurrent devices; a transformer built to IEEE C57.12.00 is rated for continuous full load, and the source here cannot exceed its nameplate.
- **Ignoring the inverter’s contribution to bus fault duty.** The transformer-only figure at 5.75 percent is 41 837 A, inside a 42 kA rating. Add the 2 566 A the inverter contributes at 1.2 pu and it is 44 403 A, and that gear is under-rated. A study modelling storage as a passive load misses it.
- **Rounding the overcurrent device down to the nearest standard rating.** At 1500 kW the floor is 2004.7 A — a 2000 A device is 4.7 A short. NEC 240.6(A) lists 2500 A as the next standard rating, and the bus follows it.
- **Citing the 120 percent busbar rule from the wrong cycle, or where it does not apply.** It moved within Article 705 between editions, and it governs back-feeds onto a shared busbar only. A dedicated step-up transformer on its own feeder is sized by NEC 705.28(A), not 705.12(B).

## Where Entogo fits

Entogo, headquartered in Toronto, manufactures transformers, prefabricated substations, switchgear and battery storage equipment in its own vertically integrated source factory. Step-up equipment such as the [solar and storage skid substation](https://entogo.ca/products/solar-storage-skid-substation/) covers voltage classes up to 34.5 kV and is designed and built to IEEE C57.12.34, C57.159 and C37.121; UL (cULus)/CSA certifiable on request. Low-voltage assemblies such as the [low-voltage switchboard](https://entogo.ca/products/low-voltage-switchboard/) are designed and built to UL 891 at 600 V and below, the class of equipment the worked example’s 3000 A bus and 65 kA rating fall into; UL (cULus)/CSA certifiable on request. Those standards are integration context; none of the above states that a product carries a third-party listing.

The [transformer sizing calculator](https://entogo.ca/tools/transformer-sizing-calculator/) converts a block size into standard kVA, full-load current and overcurrent protection in one pass, and the [transformer configurator](https://entogo.ca/products/transformer-quote/) carries kVA, voltage, impedance and cooling through to a complete specification. The [transformer and substation range](https://entogo.ca/products/category/transformers-substations/), [commercial and industrial storage](https://entogo.ca/solutions/commercial-industrial-storage/) and [containerized battery energy storage](https://entogo.ca/products/containerized-battery-energy-storage-system/) cover the rest, with coupling topology in [DC-coupled versus AC-coupled solar-plus-storage](https://entogo.ca/insights/dc-coupled-vs-ac-coupled-solar-storage/) and inverter choice in [grid-forming versus grid-following inverters](https://entogo.ca/insights/grid-forming-vs-grid-following-inverters-battery-storage/). Where all three come from one manufacturer they can be specified against a single short-circuit study.

An interconnection package is a chain of five numbers, and only the first is measured. Take the inverter’s apparent power at the power factor the utility requires, round it up the standard ladder, put the 125 percent multiplier on the conductors and device rather than the transformer, choose the impedance with the short-circuit study open, and add the inverter’s own contribution before anyone orders gear.

Run the numbers

The rules in this article are the ones the calculator applies live. Enter your own load, voltages and code edition, then take the result into a specification.

[Open the calculator](https://entogo.ca/tools/transformer-sizing-calculator/)

## Sources

1. [EPRI DER Settings — Current Status of Equipment and Practices (IEEE 1547-2018 reactive capability)](https://dersettings.epri.com/assets/Current_Status_of_Equipment_and_Practices.pdf)
2. [IEEE Power System Relaying Committee — Impact of Inverter Based Resources on Utility Transmission System Protection](https://www.pes-psrc.org/kb/report/109.pdf)
3. [EC&M — NEC Requirements for Interconnected Power Sources (Article 705)](https://www.ecmweb.com/national-electrical-code/code-basics/article/55136607/nec-requirements-for-interconnected-power-sources)
4. [EC&M — NEC Requirements for Energy Storage Systems (Article 706)](https://www.ecmweb.com/national-electrical-code/code-basics/article/21281692/nec-requirements-for-energy-storage-systems)
5. [Electrical Industry Canada — Guide to the Canadian Electrical Code, Part 1, 25th Edition, Section 64](https://electricalindustry.ca/latest-articles/9516-guide-to-the-canadian-electrical-code-part-1-25th-edition-a-road-map-section-64/)
6. [Electrical Industry Canada — Guide to the Canadian Electrical Code, Part 1, 25th Edition, Section 84](https://electricalindustry.ca/latest-articles/section-84-canadian-electrical-code/)
7. [Mayfield Renewables — Code Corner on NEC 705.12(B)(3) load-side source connections](https://www.mayfield.energy/technical-articles/code-corner-nec-article-70512b36-load-side-source-connections/)
8. [Electrical License Renewal — NEC 240.6(A) standard ampere ratings](https://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=253)

- Energy Storage
- BESS
- Grid connection
- NEC
- IEEE 1547
- kVA sizing

Glossary: [Power conversion system](https://entogo.ca/glossary/#pcs) [Battery energy storage system](https://entogo.ca/glossary/#bess) [60 Hz vs. 50 Hz](https://entogo.ca/glossary/#60-hz-vs-50-hz) [Step-up](https://entogo.ca/glossary/#step-up-collector-transformer) [Switchboard](https://entogo.ca/glossary/#switchboard-ul-891) [Percent impedance](https://entogo.ca/glossary/#percent-impedance)

FAQ

## Common questions

- **What size transformer does a 2 MW battery storage system need?**: Size it from apparent power, not kW. A 2 MW power conversion system holding the 0.90 power factor that IEEE 1547 Category B reactive capability corresponds to is a 2222 kVA source, which rounds up to a standard 2500 kVA transformer at 88.9 percent loading.
- **Why is a 2000 kW inverter treated as a 2222 kVA source?**: IEEE 1547-2018 requires Category B reactive power injection and absorption capability of 44 percent of rated apparent power, a value chosen to correspond to 0.90 power factor at rated active power. Dividing 2000 kW by 0.90 gives 2222 kVA, and a transformer carries apparent power.
- **How many amps does a 2500 kVA transformer draw at 600 volts?**: 2500 kVA at 600 V three-phase draws 2406 A. The arithmetic is 2,500,000 divided by 1039.23, the product of the square root of 3 and 600. The same 2500 kVA draws 115.7 A at 12.47 kV and 52.3 A at 27.6 kV.
- **Does the 125 percent continuous rule apply to transformer kVA?**: No. NEC 705.28(A) and 705.30 apply 125 percent to the output-circuit conductors and the overcurrent device, and NEC 706.30 does the same from the ESS nameplate current. Transformers are continuously rated under IEEE C57.12.00, so 2222 kVA on a 2500 kVA unit is correct.
- **How much fault current does a battery inverter contribute?**: Far less than a rotating machine. The IEEE Power System Relaying Committee puts converter fault contribution in a typical range of 1.1 to 2.0 per unit of rated current. At 1.2 pu a 2138 A inverter adds about 2,566 A, against 41,837 A arriving through a 5.75 percent transformer.
- **How does the Canadian Electrical Code differ on battery interconnection?**: CEC Rule 64-112 caps back-fed sources at 120 percent of a busbar rating for non-dwellings and 125 percent for dwelling units. Rule 84-012 sizes transformer overcurrent protection under Section 26 with each winding evaluated as a primary, and Section 84 drives the disconnecting means.
- **What impedance should a battery interconnection transformer have?**: Impedance is a purchase parameter that sets fault duty. On a 2500 kVA 600 V unit, 5.75 percent puts 44,403 A at the bus and forces 65 kA gear, while 8 percent gives 32,636 A and fits 42 kA gear. Higher impedance costs voltage regulation under load.

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## Structured data (JSON-LD)

```json
{"@context":"https://schema.org","@type":"Article","headline":"How to size a battery storage interconnection: inverter kVA, transformer kVA and %Z","description":"A 2 MW battery inverter holding IEEE 1547 reactive capability is a 2222 kVA source, so it needs a 2500 kVA transformer, 2673 A of conductor under NEC 705.28, and 65 kA gear once the 600 V bus fault duty reaches 44,403 A.","image":"https://entogo.ca/_astro/battery-storage-interconnection-transformer-sizing.B07z-eni.jpg","author":{"@id":"https://entogo.ca/#organization"},"publisher":{"@id":"https://entogo.ca/#organization"},"mainEntityOfPage":"https://entogo.ca/insights/battery-storage-interconnection-transformer-sizing/","inLanguage":"en","datePublished":"2026-09-21T00:00:00.000Z","dateModified":"2026-09-21T00:00:00.000Z","keywords":"Energy Storage, BESS, Grid connection, NEC, IEEE 1547, kVA sizing","abstract":"IEEE 1547-2018 sets Category B reactive capability at 44 percent of rated apparent power, a figure chosen to match 0.90 power factor, so a 2 MW inverter is a 2222 kVA source. Round 2222 kVA up the ANSI/IEEE C57.12 ladder to a 2500 kVA transformer — 88.9 percent loading is correct, and a 3000 kVA unit buys no additional code compliance. NEC 705.28(A) and 705.30 put the 125 percent multiplier on conductors and the overcurrent device, not on transformer kVA — 1.25 x 2138 A = 2673 A, so a 3000 A main under NEC 240.6(A). At 5.75 percent impedance the 600 V bus sees 41,837 A through the transformer plus 2,566 A from the inverter at 1.2 pu, for 44,403 A — which rules out 42 kA gear. CEC Rule 64-112 caps back-fed sources at 120 percent of a busbar rating for non-dwellings and 125 percent for dwelling units. CEC Rule 84-012 sizes transformer overcurrent protection under Section 26 with each winding evaluated as a primary, because power flows both ways.","citation":[{"@type":"CreativeWork","name":"EPRI DER Settings — Current Status of Equipment and Practices (IEEE 1547-2018 reactive capability)","url":"https://dersettings.epri.com/assets/Current_Status_of_Equipment_and_Practices.pdf"},{"@type":"CreativeWork","name":"IEEE Power System Relaying Committee — Impact of Inverter Based Resources on Utility Transmission System Protection","url":"https://www.pes-psrc.org/kb/report/109.pdf"},{"@type":"CreativeWork","name":"EC&M — NEC Requirements for Interconnected Power Sources (Article 705)","url":"https://www.ecmweb.com/national-electrical-code/code-basics/article/55136607/nec-requirements-for-interconnected-power-sources"},{"@type":"CreativeWork","name":"EC&M — NEC Requirements for Energy Storage Systems (Article 706)","url":"https://www.ecmweb.com/national-electrical-code/code-basics/article/21281692/nec-requirements-for-energy-storage-systems"},{"@type":"CreativeWork","name":"Electrical Industry Canada — Guide to the Canadian Electrical Code, Part 1, 25th Edition, Section 64","url":"https://electricalindustry.ca/latest-articles/9516-guide-to-the-canadian-electrical-code-part-1-25th-edition-a-road-map-section-64/"},{"@type":"CreativeWork","name":"Electrical Industry Canada — Guide to the Canadian Electrical Code, Part 1, 25th Edition, Section 84","url":"https://electricalindustry.ca/latest-articles/section-84-canadian-electrical-code/"},{"@type":"CreativeWork","name":"Mayfield Renewables — Code Corner on NEC 705.12(B)(3) load-side source connections","url":"https://www.mayfield.energy/technical-articles/code-corner-nec-article-70512b36-load-side-source-connections/"},{"@type":"CreativeWork","name":"Electrical License Renewal — NEC 240.6(A) standard ampere ratings","url":"https://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=253"}],"about":[{"@id":"https://entogo.ca/glossary/#pcs"},{"@id":"https://entogo.ca/glossary/#bess"},{"@id":"https://entogo.ca/glossary/#60-hz-vs-50-hz"},{"@id":"https://entogo.ca/glossary/#step-up-collector-transformer"},{"@id":"https://entogo.ca/glossary/#switchboard-ul-891"},{"@id":"https://entogo.ca/glossary/#percent-impedance"}]}
```

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```

```json
{"@context":"https://schema.org","@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What size transformer does a 2 MW battery storage system need?","acceptedAnswer":{"@type":"Answer","text":"Size it from apparent power, not kW. A 2 MW power conversion system holding the 0.90 power factor that IEEE 1547 Category B reactive capability corresponds to is a 2222 kVA source, which rounds up to a standard 2500 kVA transformer at 88.9 percent loading."}},{"@type":"Question","name":"Why is a 2000 kW inverter treated as a 2222 kVA source?","acceptedAnswer":{"@type":"Answer","text":"IEEE 1547-2018 requires Category B reactive power injection and absorption capability of 44 percent of rated apparent power, a value chosen to correspond to 0.90 power factor at rated active power. Dividing 2000 kW by 0.90 gives 2222 kVA, and a transformer carries apparent power."}},{"@type":"Question","name":"How many amps does a 2500 kVA transformer draw at 600 volts?","acceptedAnswer":{"@type":"Answer","text":"2500 kVA at 600 V three-phase draws 2406 A. The arithmetic is 2,500,000 divided by 1039.23, the product of the square root of 3 and 600. The same 2500 kVA draws 115.7 A at 12.47 kV and 52.3 A at 27.6 kV."}},{"@type":"Question","name":"Does the 125 percent continuous rule apply to transformer kVA?","acceptedAnswer":{"@type":"Answer","text":"No. NEC 705.28(A) and 705.30 apply 125 percent to the output-circuit conductors and the overcurrent device, and NEC 706.30 does the same from the ESS nameplate current. Transformers are continuously rated under IEEE C57.12.00, so 2222 kVA on a 2500 kVA unit is correct."}},{"@type":"Question","name":"How much fault current does a battery inverter contribute?","acceptedAnswer":{"@type":"Answer","text":"Far less than a rotating machine. The IEEE Power System Relaying Committee puts converter fault contribution in a typical range of 1.1 to 2.0 per unit of rated current. At 1.2 pu a 2138 A inverter adds about 2,566 A, against 41,837 A arriving through a 5.75 percent transformer."}},{"@type":"Question","name":"How does the Canadian Electrical Code differ on battery interconnection?","acceptedAnswer":{"@type":"Answer","text":"CEC Rule 64-112 caps back-fed sources at 120 percent of a busbar rating for non-dwellings and 125 percent for dwelling units. Rule 84-012 sizes transformer overcurrent protection under Section 26 with each winding evaluated as a primary, and Section 84 drives the disconnecting means."}},{"@type":"Question","name":"What impedance should a battery interconnection transformer have?","acceptedAnswer":{"@type":"Answer","text":"Impedance is a purchase parameter that sets fault duty. On a 2500 kVA 600 V unit, 5.75 percent puts 44,403 A at the bus and forces 65 kA gear, while 8 percent gives 32,636 A and fits 42 kA gear. Higher impedance costs voltage regulation under load."}}],"inLanguage":"en"}
```

```json
{"@context":"https://schema.org","@type":"HowTo","name":"How to size a battery storage interconnection: inverter kVA, transformer kVA and %Z — A worked example — 2 MW / 4 MWh at 600 V into a 12.47 kV feeder","step":[{"@type":"HowToStep","position":1,"name":"Apparent power the transformer must carry.","text":"Apparent power the transformer must carry. The utility specifies IEEE 1547 Category B, so the inverter holds 0.90 power factor at rated output: 2000 kW ÷ 0.90 = 2222.2 kVA."},{"@type":"HowToStep","position":2,"name":"Rated continuous AC output current.","text":"Rated continuous AC output current. 2 222 200 ÷ (√3 × 600) = 2 222 200 ÷ 1039.23 = 2138.3 A. This is the nameplate current every downstream rule keys off."},{"@type":"HowToStep","position":3,"name":"Transformer rating.","text":"Transformer rating. Round 2222.2 kVA up the standard ladder — 1500, 2000, 2500, 3000 kVA. Loading is 2222.2 ÷ 2500 = 88.9 percent, a correct continuous duty point for a machine rated to IEEE C57.12.00 at 30 °C ambient. The 80 percent convention in how to size a transformer covers diversified building load that can grow past its calculated demand; a current-limited inverter cannot, so 2222.2 ÷ 0.8 = 2778 kVA buys a 3000 kVA unit for a risk that is not there."},{"@type":"HowToStep","position":4,"name":"Conductors and the low-voltage device.","text":"Conductors and the low-voltage device. NEC 705.28(A) requires output-circuit conductors rated at least 125 percent of nameplate current, and NEC 705.30 the same of the overcurrent device: 1.25 × 2138.3 = 2672.9 A. The next standard rating in NEC 240.6(A) is 3000 A, so the switchboard carries a 3000 A bus and main."},{"@type":"HowToStep","position":5,"name":"Medium-voltage full-load current.","text":"Medium-voltage full-load current. At the transformer's rating: 2 500 000 ÷ (√3 × 12 470) = 2 500 000 ÷ 21 598.7 = 115.7 A. The primary switch, cable and CT ratio follow — a 200:5 CT leaves room for the settings NEC 450.3(A) permits above 1000 V."},{"@type":"HowToStep","position":6,"name":"Fault duty at the 600 V bus.","text":"Fault duty at the 600 V bus. Secondary full-load current is 2 500 000 ÷ 1039.23 = 2405.6 A. At a specified 5.75 percent impedance (which %Z to specify) the infinite-source contribution is 2405.6 ÷ 0.0575 = 41 837 A, and the inverter adds 1.2 × 2138.3 = 2 566 A, for 44 403 A."},{"@type":"HowToStep","position":7,"name":"Equipment ratings.","text":"Equipment ratings. 44.4 kA rules out 42 kA gear; the switchboard, its main and every feeder device need a 65 kA rating — see available fault current and equipment short-circuit ratings."}],"inLanguage":"en"}
```

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