A 480V EV site still needs a 208Y/120V step-down the moment Level 2 chargers enter the plan, because 208V is just the line-to-line voltage of a 120V wye, and most AC chargers cap out there. Size it in four moves: total the 208V load in VA, multiply by 1.25 for continuous duty, divide by 0.80, then round up to a preferred kVA rating.
That is the whole method in two sentences. The rest of this page is the arithmetic behind it, the four places the arithmetic goes wrong, and the specification language that keeps a supplier from quoting you the wrong unit.
If you have already settled on a kVA number and want frame dimensions, options, and accessory lists, the EV charger transformer sizing guide and the 500 kVA frame page handle that. This article sits one layer underneath them. It deals with the voltage conversion itself, and with the fairly common situation where a site fed at 480V ends up buying a second transformer it never budgeted for.
The short answer on why 480V alone does not work
Three-phase 480V is the right service voltage for a charging site. It carries more power per amp, it keeps conductor counts sane, and every DC fast charger on the market wants it.
Then a Level 2 pedestal shows up with a nameplate that reads 208 to 240V, single phase. You cannot feed it 480V. You cannot feed it 277V either, unless the specific model is rated for it. So the site needs a second voltage, and the cheapest way to make 208V out of 480V is a two-winding transformer with a delta primary and a wye secondary.
That is the entire reason the second unit exists. It is not redundancy, it is not isolation for its own sake, and it is not a utility requirement. It is a voltage mismatch between the service and the hardware.
The pattern is old enough to be documented in patent literature. A station with both high-power and Level 2 chargers “may need two transformers, one to go from medium voltage to 480V, and another to go from 480V to 208V or 240V.” Plenty of projects discover this after the switchgear is ordered.
What runs at 208V and what stays at 480V
Get the load split right before you touch a kVA number, because the two halves of a mixed site are sized on different rules.
DC fast chargers stay on 480V, three phase
A 150 kW dispenser is a rectifier. Its advertised rating is DC output to the vehicle, not AC draw from the grid. At roughly 95 percent conversion efficiency and 0.99 power factor, it pulls about 160 kVA from the 480V bus. A 350 kW unit pulls about 372 kVA.
None of that touches the 208V transformer. DC fast charging belongs on the 480V side, fed from the main service or from a packaged substation built for DC fast charging sites. Keep it there.
Level 2 chargers are the reason you buy the step-down
A Level 2 charger is a pass-through. It does not convert voltage, it just switches whatever AC you give it into the vehicle, and the car’s onboard charger does the rectifying. Most units accept 208 to 240V single phase. A few commercial models accept 277V.
That is why the 208V transformer exists, and why its size tracks the number of Level 2 stalls rather than the site’s total connected load.
The 120V layer nobody budgets for
Once you have a 208Y/120V secondary, you also get 120V line-to-neutral for free. Payment kiosks, network cabinets, lot signage, gate controls, a maintenance receptacle, sometimes restroom and canopy lighting. It is usually 5 to 15 kVA on a site of any size.
Do not leave it out of the connected load. On small sites it is the difference between a 75 kVA unit and a 112.5 kVA unit, and the price step is larger than the load.
| Load | Voltage | Typical unit size | Amps per unit | Branch OCPD at 125% |
|---|---|---|---|---|
| Level 1 AC | 120V, 1 phase | 1.4 to 1.9 kVA | 12 to 16 A | 15 to 20 A |
| Level 2 AC | 208V, 1 phase | 6.7 to 11.5 kVA | 32 to 48 A | 40 to 60 A |
| Level 2 AC, 277V model | 277V, 1 phase | 8.9 to 13.3 kVA | 32 to 48 A | 40 to 60 A |
| DC fast, 50 kW | 480V, 3 phase | 53 kVA AC input | 64 A | 80 A |
| DC fast, 150 kW | 480V, 3 phase | 160 kVA AC input | 192 A | 250 A |
| DC fast, 350 kW | 480V, 3 phase | 372 kVA AC input | 448 A | 600 A |
The 2.31 times current penalty nobody quotes for
For the same kVA, a 208V three-phase circuit carries 2.31 times the current of a 480V circuit. The ratio is just 480 divided by 208.
That number drives cost in four places, and only one of them is the transformer.
- Conductors. A 225 kVA transformer’s 208V feeder is 625 A. In 600 kcmil copper at 75 degrees C, that is two sets per phase. The same 225 kVA on the 480V side is 271 A, one set per phase.
- Conduit. Two sets of 600 kcmil plus neutral and ground does not fit in a 3 inch raceway. You are into 4 inch, and often into a wireway.
- Switchboards. An 800 A frame at 208V is a different price class than a 400 A frame at 480V for the same delivered power.
- Voltage drop. Drop scales with current, so 208V runs lose voltage twice as fast for the same kVA and the same wire.
A 300 kVA unit pushed to 500 kVA is a good illustration. At 208V the secondary full-load current is 1,388 A. Multiply by 1.25 and you get 1,735 A. The standard device ladder jumps from 1,600 A straight to 2,000 A. You cannot buy a 1,750 A breaker. So a 500 kVA step-down lands you on a 2,000 A device, a 2,000 A bus, and four sets of conductors per phase.
That is why experienced designers stop scaling up at 208V somewhere around 300 kVA and split into two units instead. The device ladder is too coarse to absorb the growth.
Sizing it in four moves
Here is the sequence. Run it on the 208V load only, never on the whole site.
Step 1: Total the 208V load in VA, not in charger count
Add the nameplate VA of every 208V and 120V load. For single-phase loads the arithmetic is amps times volts, not amps times 208 divided by anything. There is no square root of three on a two-wire branch.
Watch the code floor. NEC 220.57, which is new in the 2023 edition and does not appear in the 2020 code, sets the EVSE load in a feeder or service calculation at 7,200 VA or the nameplate rating, whichever is greater. It was written around a 30 A, 240V single-phase circuit.
A 32 A charger on a 208V supply is 32 times 208, or 6,656 VA. Under the 2023 code it still calculates at 7,200 VA. That is 8.2 percent of load that exists only on paper, and on a 40-stall site it is 21.8 kVA of phantom demand.
If your authority having jurisdiction still enforces the 2020 edition, that floor is not there and 6,656 VA stands. Ask which edition governs before you size anything, because the answer moves the number.
Step 2: Multiply by 1.25, because EVSE is continuous by definition
NEC 625.42 states that electric vehicle charging loads shall be considered continuous. Continuous means three hours or more at maximum current, which triggers the 125 percent multiplier in 215.3 and 230.42.
There is no demand factor to rescue you. Article 220 has no provision that reduces an EVSE count the way it reduces receptacle or appliance load. Twelve chargers count as twelve chargers, even if a real site never sees more than seven plugged in at once.
The only legitimate relief is an energy management system. NEC 625.42(A) lets you size the feeder and service to the ceiling the EMS enforces, and 220.70, another 2023 addition, drops that setpoint into the load calculation as a continuous value. If you plan to use one, say so before the transformer is ordered, because it changes the kVA.
Step 3: Divide by 0.80 to get the transformer rating
A transformer run at 100 percent of nameplate for continuous duty runs hot and ages fast. The 80 percent loading convention is not a code requirement, it is an industry habit, and it happens to be the exact inverse of the 125 percent multiplier. So dividing the connected kVA by 0.8 and multiplying the connected amps by 1.25 give you the same answer from two directions. Do one, not both.
Then round up to a preferred rating. Three-phase units come in 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000 and 2500 kVA.
Note that 250 kVA is not on that list. Neither is 400 or 600. If your arithmetic lands on one of those, you go up a step, and you should expect a longer lead time if a supplier offers to build it anyway.
Step 4: Set overcurrent protection on both sides
NEC Table 450.3(B) governs transformers rated 1,000V or less, which covers every 480 to 208 unit in this article. It gives two ways to protect the transformer.
Primary-only protection at 125 percent of primary full-load current means secondary protection is not required. Primary and secondary protection allows the primary device up to 250 percent, but then the secondary device must be at 125 percent or less.
In practice, a 208Y/120V step-down almost always uses both, because the 208V feeder needs a main device anyway.
| kVA | 480V primary FLA | Primary OCPD at 125% | 208V secondary FLA | Secondary OCPD at 125% | Level 2 stalls at 48A, 80% load |
|---|---|---|---|---|---|
| 75 | 90 A | 125 A | 208 A | 300 A | 6 |
| 112.5 | 135 A | 175 A | 312 A | 400 A | 9 |
| 150 | 180 A | 250 A | 416 A | 600 A | 12 |
| 225 | 271 A | 350 A | 625 A | 800 A | 18 |
| 300 | 361 A | 500 A | 833 A | 1,200 A | 24 |
| 500 | 601 A | 800 A | 1,388 A | 2,000 A | 40 |
Two lines in that table are worth a second look. The 300 kVA primary calculates at 451.05 A. A 450 A device is standard but it is below the number, so the next size up is 500 A. And the 500 kVA secondary at 1,735 A has nowhere to land between 1,600 and 2,000.
Worked example: four DC fast chargers and twelve Level 2 stalls
Take a site with 4 x 150 kW DC fast chargers, 12 x 48 A Level 2 pedestals, and 12 kVA of 120V house load.
The DC units do not touch the step-down. Each draws 159.5 kVA at 480V, so 638 kVA total, and they are fed straight from the main switchboard.
The 208V side is 12 times 9,984 VA, or 119.8 kVA, plus 12 kVA of 120V load, for 131.8 kVA connected. Divide by 0.8 and you get 164.8 kVA. Round up to 225 kVA.
On the 225 kVA unit: 271 A primary, 350 A primary device, 625 A secondary, 800 A secondary device. The 480V main sees 638 plus 131.8, or 769.8 kVA, which divides to 962 kVA and lands on a 1,000 kVA service transformer.
So the 208V layer is 17 percent of the site kVA but 100 percent of the voltage problem. That mismatch is why it gets its own paragraph in the design narrative and its own line in the budget.
Where 208V designs go wrong
Four failure modes account for most of the rework on these projects.
Voltage drop eats the margin before the charger sees it
Three percent is the usual design target for a branch circuit, from the informational notes in 210.19(A). On a 208V circuit that target is unforgiving, because the current is high and the percentage is calculated against a smaller base.
Take a 48 A charger on a 208V branch, 6 AWG copper at 75 degrees C. Ampacity is fine at 65 A. Now run the voltage drop: 3 percent of 208 is 6.24 V, and the resistance of 6 AWG copper at 75 degrees C is 0.510 ohms per 1,000 ft. Solving for length gives about 125 ft one way.
At 125 ft, 6 AWG is already at the limit. A 250 ft run from a remote electrical room needs 2 AWG. That is a conductor cost multiplier of roughly four, on every stall.
| Conductor | Resistance at 75 C, ohms per 1,000 ft | Max one-way run at 3%, 48 A, 208V |
|---|---|---|
| 6 AWG Cu | 0.510 | 125 ft |
| 4 AWG Cu | 0.321 | 200 ft |
| 2 AWG Cu | 0.201 | 320 ft |
| 1/0 AWG Cu | 0.127 | 510 ft |
This table is the single best argument for putting the transformer near the stalls instead of near the service.
It is a separately derived system, and the bonding matters
A 480 delta to 208Y/120 transformer creates a new grounded system. NEC 250.30 applies in full: a system bonding jumper at the transformer or at the first disconnecting means, a grounding electrode conductor sized from Table 250.66, and a grounding electrode.
Two mistakes show up repeatedly. The first is landing X0 on the neutral bar and never bonding it, which leaves the 208V system ungrounded and floating. The second is bonding X0 at the transformer and again at a downstream panel, which puts normal return current on the grounding conductor and creates a parallel path.
Pick one point and bond there. Document it on the drawing, because the next electrician will not guess correctly.
Phase balance on a wye is not intuitive
Level 2 chargers are single phase, so you distribute them across A-B, B-C and C-A. Four chargers per pair, 48 A each, does not give you 48 A per phase.
Each phase conductor carries two pairs, 120 degrees apart, so the line current is the square root of three times the per-pair current. Four chargers per pair at 48 A gives about 333 A per phase, not 192 A. Check any balance calculation against total kVA divided by root-three times 208, and the two should agree.
An unbalanced string is worse than a slightly oversized transformer. It derates the unit through negative-sequence heating and it makes one phase’s breaker trip first.
The 200 percent neutral rule does not apply here
You will read that nonlinear EV loads require a 200 percent neutral. That advice is usually wrong on a 208Y/120 serving line-to-line chargers.
A single-phase 208V charger is connected line to line. Its harmonic currents circulate between two phases and never enter the neutral. Triplen harmonics only pile up in the neutral when the load is connected line to neutral, which on this system means the 120V house load: kiosks, network gear, switch-mode power supplies, electronic lighting drivers.
So a full-size neutral is the right call for the charger load by itself. Upsize to 200 percent only when the 120V leg is dense with switch-mode equipment, and say so explicitly rather than letting a supplier pad the price.
480V delta primary, and the 277V bypass
The standard configuration is a delta primary and a wye secondary, written Dyn1 on the nameplate. The delta primary has no neutral, which is fine because you do not need one, and it happens to trap zero-sequence harmonic current before it reaches the 480V bus.
Before you buy the unit, check whether your chosen Level 2 hardware is rated for 277V. A 480Y/277 service already has 277V line to neutral, so a 277V-rated charger can run straight off the main service with no step-down at all. Same power at 277V instead of 208V means 25 percent less current, which pays for itself in conductor and conduit on any run over 150 ft.
The catch is availability. Most Level 2 hardware is 208 to 240V only, and the 277V models are a smaller slice of the catalog. Confirm on the actual submittal, not on a sales sheet.
One 225 kVA unit or four 75s
This is the decision that shapes the site layout, and it usually gets made by accident.
| One central 480 to 208 unit | Distributed 45 to 75 kVA units | |
|---|---|---|
| Cost per kVA | Lowest | 15 to 30 percent higher |
| 208V conductor runs | Long, heavy, voltage-drop limited | Short |
| Single point of failure | Yes, whole L2 fleet | No, one cluster only |
| Floor or wall space | One room | One enclosure per cluster |
| Grounding work | One derived system | One per unit, each with its own GEC |
| Expansion | Forklift and a crane | Add a unit |
The tiebreaker is distance. Run the voltage drop table backwards: if the farthest Level 2 stall is beyond about 250 ft from where the central unit would sit, distributed wins on copper alone. Below 150 ft, central wins on first cost and on the fact that you bond one system instead of four.
Between those numbers it is a maintenance argument, not an electrical one, and the operations team should make it.
Harmonics and K-factor on the 208V side
Level 2 chargers are nonlinear loads. The onboard charger in the vehicle is a rectifier, so it draws current in pulses rather than a clean sine.
The K-factor convention from IEEE C57.110 lets you match a transformer’s harmonic tolerance to the load. Level 2 AC charging typically sits around K-4. DC fast charging runs hotter, K-13 to K-20, depending on the rectifier topology.
Here is the part suppliers do not volunteer. A 480 to 208 transformer serving only Level 2 pedestals does not need K-13. Specifying K-13 on a load that measures K-4 buys you a thicker neutral, more derating and a higher price for margin you will never use.
Specify K-4 or K-9 for an L2-only unit, and reserve K-13 for a unit that also carries DC conversion equipment. Ask for the harmonic study or the charger’s current distortion data, and size from that instead of from a rule of thumb.
What the losses actually cost
A step-down transformer runs 24 hours a day. The chargers do not.
Take a 225 kVA dry-type unit with 500 W of no-load loss and 3,100 W of load loss at 75 degrees C. At a 40 percent average load, total loss is 500 plus 3,100 times 0.16, or about 996 W. That is 8,725 kWh a year, roughly 1,222 dollars at 14 cents, and about 15,160 dollars of present value over 30 years at 7 percent.
At a 25 percent load factor, which is realistic for a workplace or retail site, the total loss drops to 694 W, but the no-load component is now 72 percent of it. That is the number that matters for EV work: on a site that idles most of the day, core loss dominates and load loss barely registers. An amorphous core or a lower-loss grain-oriented design earns its premium here in a way it would not on a continuously loaded industrial feeder.
Any three-phase dry-type unit from 15 to 2,500 kVA sold in the US falls under the DOE efficiency table in 10 CFR 431.196, so the baseline is regulated. The next compliance step lands in 2029, which means a unit bought today may sit one tier below the requirement by the time it is mid-life. Ask for loss values at 50 percent and 100 percent load, not just a nominal efficiency percentage.
What to put in the RFQ
Vague inquiries get vague quotes, and on this product class a vague quote is a change order waiting to happen.
- Rating: 225 kVA, three phase, 60 Hz, on the preferred ladder.
- Primary: 480V delta, with tap links at plus or minus 2 by 2.5 percent.
- Secondary: 208Y/120, four wire, neutral brought out at full capacity.
- Impedance: 4.0 to 5.75 percent, stated with tolerance, plus the resulting fault current at 208V.
- Temperature rise and insulation: 150 degrees C rise on a 220 degree C system for standard life, or 80 degree rise if you want the extra thermal margin and can pay for the larger core.
- Harmonic rating: K-4 or K-9 for L2-only; state the basis.
- Efficiency: DOE 10 CFR 431.196 compliance declared, with no-load and load loss in watts at 50 and 100 percent load.
- Construction: copper or aluminum windings named explicitly, plus an electrostatic shield if network or AV equipment shares the secondary.
- Enclosure: NEMA 1 indoors, NEMA 3R outdoors, or a cast-resin unit for a parking structure where oil is prohibited. Our dry-type range and cast-resin units cover both indoor cases, and the oil versus dry comparison handles the pad-mounted outdoor decision.
- Listing: UL 1561 for a dry-type unit at 600V class or below, cULus mark, file number on the submittal.
- Sound: dBA at one meter, measured per the standard test method, not a catalog “typical.”
- Site conditions: ambient above 40 degrees C, altitude above 1,000 m, seismic anchorage, and salt or coastal exposure all change the build. Say so up front.
For a commercial site where the transformer is one line item inside a larger electrical package, the commercial infrastructure solution set is the right starting conversation, and our engineering desk will run the kVA and OCPD numbers against your charger submittals before anything is quoted.
Cite this article. APA: TransNorth Electric. (2026). Transformers for EV Charging Stations: Sizing 480V to 208/120V Loads. Retrieved from https://www.transninelectric.com/resources/ev-charging-station-transformers-sizing-480v-to-208-120v/. MLA: “Transformers for EV Charging Stations: Sizing 480V to 208/120V Loads.” TransNorth Electric, 2026, www.transninelectric.com/resources/ev-charging-station-transformers-sizing-480v-to-208-120v/. Chicago: TransNorth Electric, “Transformers for EV Charging Stations: Sizing 480V to 208/120V Loads,” accessed October 29, 2026, https://www.transninelectric.com/resources/ev-charging-station-transformers-sizing-480v-to-208-120v/.
FAQ
Why does a 480V EV charging site need a 208/120V transformer at all?
Because Level 2 AC chargers are built for 208 to 240V single phase, and 480V would destroy them. A 480V three-phase service has no 208V available unless you create it. A delta-wye transformer with a 480V primary and a 208Y/120V secondary supplies both the 208V line-to-line voltage for the chargers and 120V line-to-neutral for kiosks, network gear and lighting. DC fast chargers do not need it, since they run on 480V three phase directly.
What size transformer do I need for 20 Level 2 chargers at 48 A?
20 chargers times 9,984 VA is 199.7 kVA connected. Divide by 0.8 for continuous loading and you get 249.6 kVA, which rounds to 300 kVA on the preferred three-phase ladder. Note that 250 kVA is not a preferred rating, so a supplier who offers one is quoting a custom build with custom lead time. Add 120V house load before you do the division, since it can push you past the 300 kVA line.
Does NEC 625.42 really block every demand factor on the 208V side?
It blocks the ones people reach for. EV charging load is continuous by definition, so the 125 percent multiplier in 215.3 applies, and Article 220 has no demand factor that reduces an EVSE count. The one legal reduction is an energy management system under 625.42(A), which lets the feeder and service be sized to the EMS-imposed ceiling. A diversity factor based on observed usage needs an engineer’s stamp and an approving authority, and it is not a code allowance.
Should the 480 to 208 transformer be dry-type or oil-immersed?
Dry-type indoors, in a parking structure or an electrical room, because there is no oil to contain and fire codes are friendlier. Oil-immersed on a pad outdoors, where the unit will be cheaper per kVA and the oil gives better overload tolerance in summer heat. Cast-resin dry-type is the usual answer for underground garages and any location where a leak would reach storm drainage or a public surface.
Can I run Level 2 chargers at 277V and skip the step-down?
Sometimes. A 480Y/277 service already provides 277V line to neutral, and a charger rated for 277V can run directly off the main service with no transformer. You get the same power at 25 percent less current, which saves on conductors. The limit is product availability, since most Level 2 hardware is 208 to 240V only. Confirm the rating on the actual submittal before you delete the transformer from the drawing.
What K-factor do I need for Level 2 chargers on the 208V side?
K-4 covers most Level 2 AC charging, because the vehicle’s onboard charger is a fairly gentle rectifier. K-9 gives margin if the same transformer also carries 120V switch-mode equipment. K-13 and above belong on units serving DC fast charging or heavy IT load. Asking for the charger’s current distortion data beats guessing, and it usually saves money against a default K-13 specification.
What fault current should I expect on the 208Y/120V secondary?
Divide the secondary full-load current by the per-unit impedance. A 225 kVA unit at 208V has 625 A of full-load current, and at 5.0 percent impedance that gives about 12.5 kA. Use the minimum impedance inside the tolerance band, since IEEE allows roughly 7.5 percent deviation and the low end is the worst case. That puts the number near 13.5 kA, so 22 kA rated panelboards and breakers are the sensible floor for this size.
