What size transformer and switchgear does a DC fast charging site need?

A DC fast charging site is one of the few loads where the number printed on the equipment is not the number to size to. A 350 kW dispenser is rated by what it delivers to the vehicle. The service, the transformer and the switchboard have to carry what it draws from the grid, and that is roughly 6 percent larger. NEC Article 625 then removes the escape hatch most engineers reach for, because EV charging loads are continuous and no demand factor applies, so the calculated load is the full connected load taken at 125 percent. A twelve-dispenser plaza built from four 150 kW and eight 350 kW units lands at 3,615.1 kVA of AC input, 5,000 kVA of transformer capacity and two 3,000 A lineups at 480 V. This article works that site end to end under both NEC and CEC, then shows what an energy management system removes from it. These are the rules behind the EV charging service capacity calculator.

What is the actual electrical load of a DC fast charger?

A DC fast charger is a rectifier. Its advertised rating is DC output to the vehicle; the AC side carries that output plus conversion losses, at whatever displacement power factor the front end presents. The load that belongs in the service calculation is the AC input rating on the nameplate, which NEC 625.42 reaches when it requires that the equipment have sufficient rating to supply the load served.

Before submittals arrive, screen the AC side with two declared assumptions:

S = P ÷ (η × PF)

with η the AC-to-DC conversion efficiency and PF the input displacement power factor. This article uses η = 0.95 and PF = 0.99, written on the one-line as assumptions rather than facts. They are not load-bearing in the way they look. Running the same 3,400 kW plaza at η = 0.93 gives 3,400 ÷ 0.9207 = 3,692.8 kVA, and at η = 0.97 gives 3,400 ÷ 0.9603 = 3,540.6 kVA — a spread of about ±2 percent that disappears entirely into the rounding to the next standard transformer rating. What does move the answer is using DC output kW as if it were kVA, which understates the site by just under 6 percent in this case.

Once the AC input kVA is known, four more relations carry the rest of the design:

S_transformer ≥ S ÷ 0.80

I = S × 1000 ÷ (√3 × V)

OCPD ≥ 1.25 × I_continuous

I_secondary,max = 1.25 × I_rated

No code clause states an 80 percent transformer loading limit in those words. It is the inverse of the 125 percent continuous-load requirement in NEC 215.3 carried onto the transformer, and it is the convention used throughout how to size a transformer. What varies between projects is the secondary voltage and whether an energy management system is in play; the cost side of those choices is worked in DC fast-charger installation cost.

What do NEC 625 and CEC Section 86 require?

The two codes reach the same place by different routes. The NEC applies a 125 percent multiplier to the continuous load; the CEC caps the continuous load at 80 percent of conductor ampacity under Rule 8-104(6), rising to 100 percent under 8-104(5) only where the equipment is marked for 100 percent continuous operation. Equipment without that marking defaults to 80 percent. Both routes land on the same conductor and device.

Two entries carry the design. NEC 625.42 offers no diversity — as Consulting-Specifying Engineer puts it, “there are no demand factors involved. All EVSE loads are considered to be continuous loads” — and CEC 86-300 makes a shared branch circuit conditional on an EVEMS, the Canadian mirror of the NEC 625.42(A) allowance.

Worked example - a 12-dispenser highway plaza at 480 V

Given: four 150 kW dispensers and eight 350 kW dispensers, a 13.8 kV utility primary, and a 480Y/277 V secondary.

1. Connected DC output. 4 × 150 kW = 600 kW, and 8 × 350 kW = 2,800 kW, so the site totals 600 + 2,800 = 3,400 kW.
2. AC input. S = 3,400 ÷ (0.95 × 0.99) = 3,400 ÷ 0.9405 = 3,615.1 kVA.
3. Transformer nameplate. Continuous loading holds the unit at or below 80 percent, so 3,615.1 ÷ 0.80 = 4,518.9 kVA. The next standard three-phase rating is 5,000 kVA, supplied as one unit or as 2 × 2,500 kVA. Each 2,500 kVA unit then carries half the site, 3,615.1 ÷ 2 = 1,807.5 kVA, which is 1,807.5 ÷ 2,500 = 72.3 percent of nameplate.
4. Service current. I = 3,615,100 ÷ (√3 × 480) = 3,615,100 ÷ 831.38 = 4,348.3 A, or 2,174.2 A per lineup on a two-transformer arrangement.
5. Main device and bus. 2,174.2 × 1.25 = 2,717.7 A, so the next standard rating under NEC 240.6(A) is 3,000 A, on a 3,000 A bus.
6. Check the main against the transformer. Rated secondary current of a 2,500 kVA unit at 480 V is 2,500,000 ÷ 831.38 = 3,007.0 A, so a 3,000 A main sits at 3,000 ÷ 3,007.0 = 99.8 percent of rated current, and the continuous load at 2,717.7 ÷ 3,000 = 90.6 percent of the main. Overcurrent protection for the 13.8 kV–480 V unit itself falls under NEC Table 450.3(A), the over-1000 V table whose columns turn on supervision, impedance band and device type — sizing transformer overcurrent protection works that table and the CEC Section 26 equivalents.
7. Branch circuits. A 350 kW dispenser draws 372.1 kVA, or 372,100 ÷ 831.38 = 447.6 A; at 125 percent that is 559.5 A, so 600 A. A 150 kW dispenser draws 159.5 kVA, or 191.8 A; at 125 percent that is 239.8 A, so 250 A.
8. Sanity check on the lineup. Each lineup feeds two 150 kW and four 350 kW dispensers, so the branch devices total 2 × 250 + 4 × 600 = 2,900 A against a 3,000 A bus.
9. The house transformer, where NEC 450.3(B) does apply. A 300 kVA 480–208Y/120 V step-down for site lighting and receptacles has a rated secondary current of 300,000 ÷ (√3 × 208) = 300,000 ÷ 360.27 = 832.7 A. Table 450.3(B) caps its secondary device at 125 percent, or 832.7 × 1.25 = 1,040.9 A, and Note 1 permits the next standard rating, 1200 A.

A single 5,000 kVA transformer at 480 V would have a rated secondary current of 6,014.1 A, above the 6000 A top of common UL 891 switchboard construction and the 5000 A top of typical UL 1558 drawout switchgear. Splitting into two 2,500 kVA units and two 3,000 A lineups is what keeps the design inside ordinary switchboard construction.

What does the same site look like in Canada at 600 V?

CEC 86-302 classes the EVSE connected load as continuous for Rule 8-104, and Rule 8-104(6) expresses the margin as a cap — continuous load not exceeding 80 percent of conductor ampacity, rising to 100 percent under 8-104(5) only where the equipment is marked for it — rather than as the NEC’s 125 percent multiplier. The arithmetic is identical; the sentence is inverted. Rule 8-104(3) sets the threshold for what counts as continuous at 3 hours in 6 above 225 A, which a public charging site clears without argument.

What changes materially is the voltage. At 600 V the same 3,615.1 kVA draws 3,615,100 ÷ (√3 × 600) = 3,615,100 ÷ 1,039.23 = 3,478.6 A, against 4,348.3 A at 480 V. Per lineup that is 1,739.3 A, and 1,739.3 × 1.25 = 2,174.2 A, so the main drops from 3,000 A to 2,500 A. The transformer kVA does not change, because kVA is voltage-independent — only the current, and therefore the gear, bus and cable, do. Transformer overcurrent protection itself sits in CEC Section 26 rather than Section 86, and diverges from the NEC on the primary side; sizing transformer overcurrent protection carries those clause numbers.

How much does an energy management system change the transformer?

This is the largest single lever on the bill of materials. NEC 625.42(A) permits an EMS complying with 750.30 to set the maximum equipment load on the service and feeder, and 625.42(B) extends the same logic to EVSE with restricted access to an ampere-adjusting means under 750.30(C). In Canada, CEC 8-500 permits an EVEMS to monitor the service, feeders and branch circuits and control EVSE loads, provided the controlled load does not exceed Rule 8-104(5) or (6). Cap the plaza at 1,800 kW of simultaneous delivery and the calculated load falls with it:

2,392.3 kVA rounds up to a single 2,500 kVA transformer instead of 5,000 kVA. The uncapped AC input behind that figure is 1,800 ÷ 0.9405 = 1,913.9 kVA, so the service current becomes 1,913,900 ÷ 831.38 = 2,302.0 A at 480 V, and 2,302.0 × 1.25 = 2,877.5 A — one 3,000 A lineup replaces two. The trade is throughput during coincident peaks, which is a utilisation question rather than a code question — EV charging load management covers where that trade is worth making, and the service capacity calculator runs both cases side by side.

Charger-by-charger reference

AC input and branch device for common DC output ratings, at η = 0.95 and PF = 0.99. The DOE Alternative Fuels Data Center notes DC charging units now reach up to 500 kW; the 1,250 kW row represents a megawatt-class dispenser for heavy vehicles and shows where 480 V runs out of room.

And the transformer that follows from total site output, at 80 percent continuous loading:

The same ladder drives the transformer sizing calculator, and how to size a transformer sets out the other four adjustments — demand, growth, altitude and harmonics — that sit alongside the continuous-duty margin used here.

What to specify

• Transformer kVA, from the summed AC input ratings divided by 0.80 and rounded up the ANSI C57.12 ladder. The number comes from the charger submittals, not the marketing rating — 3,615.1 ÷ 0.80 = 4,518.9 kVA on the worked site.
• Secondary voltage and configuration, 480Y/277 V or 600Y/347 V, decided on the current it produces. 3,478.6 A against 4,348.3 A for the same kVA is a whole frame size; choosing a facility distribution voltage works the wider trade.
• Main device and bus rating, at 125 percent of the continuous load — NEC 215.3 for a feeder, 230.90(A) where the device is the service disconnect — then checked against the transformer’s rated secondary current — 3,007.0 A on a 2,500 kVA 480 V unit — and against the right protection table, 450.3(A) above 1000 V or 450.3(B) at 1000 V and below.
• Available fault current and equipment rating, from the utility’s source-impedance letter and the transformer’s %Z, verified against the assembly’s short-circuit rating per NEC 110.9. Available fault current and SCCR covers the calculation.
• Harmonic duty, requested as distortion data from the charger manufacturer and carried into either a K-factor rating or a harmonic-mitigating transformer, with the point of common coupling checked against IEEE 519. See K-rated transformers and IEEE 519 limits.
• EMS or EVEMS setpoint and its enforcement, cited to NEC 625.42(A) and 750.30 or CEC 8-500, with the controlled maximum stated in kW on the one-line so the plan reviewer can trace the reduction.

Common mistakes

• Summing DC output kW and calling it kVA. On the worked site that gives 3,400 kVA instead of 3,615.1 kVA, understating the service by 215.1 kVA, just under 6 percent, and the current by 4,348.3 − 4,089.6 = 258.7 A. The submittal’s AC input rating is the number NEC 625.42 asks for, and it is the first thing a reviewer checks.
• Applying a demand factor to the charger count. There is none for EVSE. A 20 percent diversity assumption drops the basis to 2,892.1 kVA and the transformer to 2,892.1 ÷ 0.80 = 3,615.1 kVA, or a 3,750 kVA unit instead of 5,000 kVA — a full rating step bought with a factor the code does not grant. The only sanctioned reduction is an EMS setpoint under NEC 625.42(A) or an EVEMS under CEC 8-500, enforced in hardware.
• Filing a medium-voltage transformer under NEC Table 450.3(B). That table reaches only transformers rated 1000 V or less; a 13.8 kV–480 V unit is protected under Table 450.3(A), whose columns turn on supervision, impedance band and device type. 450.3(B) governs the 480–208Y/120 V step-down instead, where 832.7 A of rated secondary current caps the secondary device at 1,040.9 A.
• Taking 125 percent of an already-derated number. The 125 percent multiplier and the 80 percent cap are the same margin expressed twice, once by NEC 215.3 and once by CEC 8-104(6). Stacking them yields 5,648.6 kVA on the worked site and an unnecessary rating step.

Where Entogo fits

Entogo builds the transformer, substation and switchgear side of a charging site in its own factory. Three-phase pad-mounted units run 75–2500 kVA in 15/25/35 kV class, with 12.47–34.5 kV primaries and 208Y/120, 240/120, 480Y/277 or 600 V secondaries, so the Canadian 600 V case above is served directly; they are designed and built to IEEE C57.12.34 and DOE 10 CFR 431. Where the medium-voltage switching and the low-voltage section belong in one enclosure, the compact secondary unit substation covers 15/25 kV class up to 2,500 kVA with 208Y/120, 480Y/277 and 600 V secondaries, designed and built to IEEE C37.121 and ANSI C57.12.34 / C57.12.28. Low-voltage switchboards run 800–6000 A at 600 V or less, designed and built to UL 891, and drawout switchgear reaches 5000 A at 65/85/100 kAIC, designed and built to UL 1558 and ANSI C37.20.1. All of it is UL (cULus) / CSA certifiable on request.

Run the site through the EV charging service capacity calculator to fix the kVA and the main, then take the result to a transformer quote or to the pad-mounted transformer and low-voltage switchboard pages for frame sizes. Common ratings have their own size pages, 500 kVA and 2500 kVA among them, and EV charging infrastructure shows how the transformer, switchgear and charger lineup are packaged together. Because the factory is in-house, capacity is scheduled against the project rather than against a queue.

The whole calculation reduces to one substitution and one multiplier. Convert DC output to AC input before anything else, take 125 percent because NEC 625.42 says the load is continuous and grants no diversity, and let the standard rating ladders round the result. The 5,000 kVA of transformer, the two 3,000 A mains and the 600 A branch devices all fall out of those two moves, and an EMS setpoint is the only lever that legitimately moves the answer down.