Quick answer: A transformer is sized in kVA from the connected load and voltage: kVA = (√3 × V × I) ÷ 1000 for three-phase, or from the load kW divided by power factor. Size the transformer at roughly 125% of the calculated continuous load so it isn't running at its limit, then round up to the next standard rating (15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA…). Protect and connect it per NEC Article 450 and size the primary/secondary conductors and overcurrent devices accordingly. Get the size fast with the transformer sizing calculator.
Why transformers are rated in kVA, not kW
The first thing to understand about transformer sizing is why the rating is in kVA (kilovolt-amperes) rather than kW (kilowatts). A transformer's limiting factor is heat, and heat in the windings comes from current, while heat in the core comes from voltage — regardless of the load's power factor. kVA is apparent power (volts × amps), which captures the actual current the windings carry. kW is real power (kVA × power factor), which depends on the load. A transformer feeding a 100 kW load at 0.8 power factor must actually handle 125 kVA of apparent power — so it must be a 125 kVA (or larger) transformer, not a 100 kVA one. That's why transformers, like generators, are always rated in kVA: it reflects what the equipment physically has to carry.
The sizing formulas
You size a transformer from the load. There are two common starting points depending on what you know:
From load current (most direct):
Three-phase kVA = (√3 × VLL × I) ÷ 1000
Single-phase kVA = (V × I) ÷ 1000
From load power (kW):
kVA = kW ÷ power factor
where VLL is the line-to-line voltage and I is the load current in amps. If you're working from horsepower, convert first (1 HP ≈ 0.746 kW, adjusted for motor efficiency and power factor). The current on each side of the transformer follows from its kVA:
Full-load amps = (kVA × 1000) ÷ (√3 × VLL) (three-phase)
The 125% continuous-load margin
You don't size a transformer to exactly the load — you leave headroom. For continuous loads (running three hours or more), good practice is to size the transformer at about 125% of the calculated load, mirroring the NEC's continuous-load philosophy. This keeps the transformer operating in its efficient band, allows for load growth, and prevents it from running hot at its rated limit day after day. After applying the margin, you round up to the next standard kVA rating, because transformers are only manufactured in fixed sizes:
| Standard kVA ratings (dry-type / distribution) |
|---|
| 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500… |
Worked example: sizing a transformer for a panel
A 480 V three-phase feeder supplies a panelboard whose connected continuous load is 140 amps, and you need a 208Y/120 V transformer to serve 120/208 V equipment.
- Load kVA: (√3 × 480 × 140) ÷ 1000 = (1.732 × 480 × 140) ÷ 1000 = 116.4 kVA
- Apply 125% margin: 116.4 × 1.25 = 145.5 kVA
- Round up to the next standard size: 150 kVA
So a 150 kVA, 480–208Y/120 V transformer fits. Its secondary full-load current is (150 × 1000) ÷ (√3 × 208) = 416 A, which sets the secondary main and conductor sizing. Run the numbers instantly with the transformer sizing calculator, and size the feeders on each side with the cable size calculator.
Protecting the transformer: NEC 450.3
Sizing the kVA is only half the job — the transformer must be protected per NEC Article 450. NEC 450.3 sets the maximum overcurrent-device ratings, and the rules differ between primary-only and primary-and-secondary protection, and by voltage class. For a common case — a transformer 600 V or less with both primary and secondary protection — the primary device may be up to 250% of primary full-load current and the secondary up to 125% (with specific values for currents of 9 A or more). The reason the primary can be so high is inrush: energizing a transformer draws a large magnetizing current for a few cycles, and the primary device must ride through it without tripping. This is analogous to motor circuits, where the breaker is deliberately oversized to allow starting.
Don't forget the secondary conductors and voltage drop
The transformer's secondary current is often the largest current in the installation, and the secondary conductors and their protection must be sized for it (a 150 kVA 208 V transformer pushes 416 A). Two things engineers sometimes overlook:
- Secondary conductor protection. The transformer's primary overcurrent device generally does not protect the secondary conductors, so secondary protection (or the tap rules) must be applied — a frequent code-compliance gap.
- Voltage drop on long secondary runs. A step-down transformer feeding a distant panel can suffer voltage drop that a nominal calculation misses; verify it with the voltage drop calculator.
Load calculation: what actually connects
Sizing from a single “connected load” number can mislead, because not all loads run at once and some carry demand factors. A proper transformer size starts from a NEC Article 220 load calculation that applies demand factors to lighting, receptacles, HVAC, motors and special loads. For motor-heavy panels, remember that the largest motor is taken at 125% of its full-load current when sizing feeders (NEC 430.24) — the same logic flows up to the transformer. Under-counting the load undersizes the transformer; blindly summing nameplates oversizes it and wastes money and no-load losses.
Dry-type vs. liquid-filled, and efficiency
| Type | Where used | Notes |
|---|---|---|
| Dry-type | Indoors, commercial buildings | No liquid, lower fire risk; common up to ~2500 kVA |
| Liquid-filled (oil) | Outdoors, utility, large kVA | Better cooling & efficiency at large sizes; needs containment |
Modern transformers must also meet DOE 2016 efficiency standards, which set minimum efficiencies by kVA and type. Over-sizing a transformer “to be safe” isn't free: an idle transformer still burns no-load (core) losses 24/7, so a wildly oversized unit wastes energy continuously. The 125%-and-round-up approach balances headroom against those standing losses.
Impedance, fault current and the transformer
A transformer's percent impedance (%Z) — stamped on the nameplate, commonly around 5.75% for distribution transformers — does two important things beyond sizing. First, it limits the available fault current on the secondary: the bolted fault current is approximately the secondary full-load current divided by the per-unit impedance. A 150 kVA, 208 V transformer at 5% impedance can deliver a secondary fault current of roughly its full-load amps divided by 0.05 — a large number that the secondary equipment's interrupting rating (AIC) and short-circuit current rating (SCCR) must exceed. Second, impedance affects voltage regulation: a higher impedance means more voltage drop from no-load to full-load. So the same impedance that protects against fault current also affects steady-state voltage. Always establish the secondary available fault current when you size a transformer — see our available fault current guide and cross-check with the short circuit calculator.
K-factor transformers for non-linear loads
If the transformer feeds significant non-linear load — computers, LED drivers, VFDs, UPS rectifiers — a standard transformer can overheat even when loaded below its kVA rating. Non-linear loads draw harmonic currents, and the higher-order harmonics drive up eddy-current losses in the windings roughly with the square of the frequency, producing extra heat the transformer wasn't rated for. The solution is a K-rated transformer (K-4, K-13, K-20…), built with oversized neutrals and extra thermal margin to tolerate a defined level of harmonic loading. K-13 is typical for office and IT spaces, K-20 for very heavy non-linear concentrations. Choosing a standard transformer for a harmonics-rich load is a common way to get mysterious overheating — see our neutral current and harmonics guide for why the neutral and transformer both suffer.
Separately derived systems: grounding and bonding
A step-down transformer usually creates a separately derived system — a new voltage system with its own neutral, electrically isolated from the primary. That triggers specific NEC grounding and bonding requirements (Article 250) that are easy to miss and are a frequent inspection failure:
- A system bonding jumper connecting the secondary neutral to the equipment grounding at one point (the transformer or the first downstream disconnect).
- A grounding electrode conductor from the secondary neutral to a suitable grounding electrode (building steel or the nearest effectively grounded point).
- A properly sized equipment grounding conductor on the secondary feeder.
Getting the separately-derived-system grounding wrong leaves the secondary either ungrounded (a shock and fault-detection hazard) or with objectionable current on the grounding system. This is as much a part of transformer installation as the kVA sizing.
Motor loads and inrush considerations
When a transformer feeds motor load, two extra factors come into play. First, motors draw a large starting inrush (6–8 times running current for a few seconds), and a transformer feeding a large motor across-the-line can see a momentary voltage dip during starts — if the transformer is small relative to the motor, that dip can be enough to affect other equipment or prevent the motor from starting. Sizing the transformer with adequate margin, or using reduced-voltage starting or a VFD, mitigates this. Second, in a mixed panel the load calculation should follow the motor rules — the largest motor at 125% of its full-load current plus the rest — which flows up into the transformer sizing. For the motor circuit side, see the motor FLA calculator and our motor full-load amps guide.
Common transformer sizing mistakes
- Sizing in kW instead of kVA. Always convert to apparent power (kW ÷ PF) — the windings carry current, not real power.
- No continuous-load margin. Running a transformer at 100% of rating all day shortens its life; use ~125%.
- Not rounding to a standard size. Transformers come in fixed kVA ratings — round up.
- Forgetting secondary conductor protection. The primary device doesn't protect the secondary conductors (NEC 240/450).
- Undersized primary protection. It must ride through inrush — NEC 450.3 allows up to 250% on the primary for this reason.
- Ignoring voltage drop on long secondary feeders.
- Gross oversizing. Wastes money and burns no-load losses continuously.
Voltage taps, regulation and future load
Two practical sizing considerations round out a transformer selection. First, taps: dry-type transformers come with primary taps (commonly two above and two below nominal, in 2.5% steps) that let you compensate if the actual supply voltage runs consistently high or low, so the secondary lands at the right voltage. If a site's utility voltage is chronically low, setting the transformer on a lower tap boosts the secondary — a simple fix that avoids chasing voltage-drop problems downstream. Second, future load: transformers are long-lived and hard to replace, so it's common to size with growth in mind. But there's a balance — a transformer sized far above today's load runs at low part-load, where it still burns its full no-load core losses continuously and operates below its best efficiency point. The prudent approach is a reasonable growth allowance (the 125% margin already provides some) plus the ability to add a parallel unit later, rather than installing one enormous transformer that idles for a decade. Where continuity is critical, redundancy (a second transformer that can carry the load if one fails) is a better answer than gross oversizing of a single unit.
Standards and references
| Reference | What it covers |
|---|---|
| NEC Article 450 | Transformer installation & overcurrent protection |
| NEC 450.3 | Max primary (up to 250%) & secondary (125%) protection |
| NEC Article 220 | Load calculation with demand factors |
| DOE 2016 / NEMA TP-1 | Transformer efficiency standards |
| Formula | kVA = √3 × V × I ÷ 1000; kVA = kW ÷ PF |
The bottom line
Transformer sizing is a kVA calculation: find the apparent power from the load current and voltage (or kW ÷ power factor), add a ~125% continuous-load margin, and round up to the next standard rating. Then protect it per NEC 450.3 — remembering the primary device is deliberately large to ride through inrush — size the secondary conductors for the full secondary current, and check voltage drop on long runs. Start with the transformer sizing calculator, size the feeders with the cable size calculator, verify fault duty with the available fault current guide, and have the design stamped by a licensed electrical engineer.
Frequently asked questions
How do you size a transformer?
Size it in kVA from the load. Compute the apparent power from the load current and voltage (three-phase kVA = root-3 times line-to-line voltage times current, divided by 1000) or from the real power divided by power factor (kVA = kW / PF). Add about a 125 percent margin for continuous loads, then round up to the next standard rating such as 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750 or 1000 kVA. Then protect and connect it per NEC Article 450.
Why are transformers rated in kVA and not kW?
Because a transformer's limit is heat, and heat in the windings comes from current regardless of the load's power factor. kVA (apparent power = volts times amps) reflects the actual current the windings carry, while kW (real power = kVA times power factor) depends on the load. A transformer feeding 100 kW at 0.8 power factor must handle 125 kVA, so it must be rated at least 125 kVA. That is why transformers are always rated in kVA.
What is the 125% rule for transformer sizing?
For continuous loads (running three hours or more), good practice is to size the transformer at about 125 percent of the calculated load, mirroring the NEC's continuous-load philosophy. This keeps the transformer in its efficient band, allows for growth, and prevents it from running at its thermal limit all day. After applying the margin you round up to the next standard kVA rating, since transformers are only made in fixed sizes.
How is transformer overcurrent protection sized per NEC 450.3?
NEC 450.3 sets maximum overcurrent-device ratings and differs by voltage class and whether protection is primary-only or both primary and secondary. For a common 600 V or less transformer with both, the primary device may be up to 250 percent of primary full-load current and the secondary up to 125 percent. The primary can be that high because energizing a transformer draws a large magnetizing inrush current for a few cycles that the device must ride through without tripping.
Does the primary breaker protect the secondary conductors?
Generally no. A transformer changes the voltage and current, so the primary overcurrent device does not properly protect the secondary conductors. Secondary protection must be provided (or the transformer secondary conductor tap rules applied), sized for the secondary full-load current. Overlooking this is a frequent code-compliance gap, especially since the secondary current of a step-down transformer is often the largest current in the installation.
How do you find the secondary current of a transformer?
Use full-load amps = (kVA times 1000) divided by (root-3 times secondary line-to-line voltage) for three-phase, or kVA times 1000 divided by voltage for single-phase. For example, a 150 kVA transformer at 208 V three-phase has a secondary current of about 416 amps. That secondary full-load current sets the secondary main breaker and conductor sizing, and it is usually the largest current in the downstream system.
Can you oversize a transformer to be safe?
Only within reason. A modest margin (about 125 percent) is good practice, but grossly oversizing wastes money and burns no-load (core) losses continuously, 24 hours a day, whether the transformer is loaded or not. Modern transformers must also meet DOE 2016 efficiency standards. The 125-percent-and-round-up approach balances headroom and load growth against those standing losses, so avoid picking a size far larger than the load requires.
Is a transformer sizing calculator accurate for design?
A calculator that computes kVA from load current and voltage (or kW and power factor), applies the continuous-load margin, rounds to a standard rating, and returns the primary and secondary currents gives reliable sizing. A complete design also needs a full NEC Article 220 load calculation, NEC 450.3 overcurrent protection, secondary conductor protection, voltage-drop checks on long runs, and available-fault-current coordination, verified by a licensed electrical engineer.