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

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 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. The interconnection is specified in kVA, because heating follows current and current follows apparent power. The bridge is the power factor 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.

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 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 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) 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.

I_min = 1.25 × I_rated

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 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.

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. 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:

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. 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.

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 fixes the 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 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 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 converts a block size into standard kVA, full-load current and overcurrent protection in one pass, and the transformer configurator carries kVA, voltage, impedance and cooling through to a complete specification. The transformer and substation range, commercial and industrial storage and containerized battery energy storage cover the rest, with coupling topology in DC-coupled versus AC-coupled solar-plus-storage and inverter choice in grid-forming versus grid-following inverters. 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.