A Complete Guide to Transformer Sizing and NEC Standards

Chameera Sampath · 10/15/2026

How to size a transformer: kVA formulas, full-load amps on each winding, NEC 450.3 overcurrent protection and fault current, with two worked examples.

A transformer sizing calculation determines the apparent-power rating, in kilovolt-amperes (kVA), that a transformer must have to supply a given load, together with the full-load current on each winding. Those currents are then used to select the overcurrent protection required by Article 450 of the National Electrical Code (NEC) and the conductors on each side. This guide sets out the formulas, works two complete examples, and explains the protection rules that follow from the result.

Quick answer. For a three-phase load, kVA = (1.732 × V × I) ÷ 1,000. For a single-phase load, kVA = (V × I) ÷ 1,000. Add an allowance for growth, round up to the next standard rating, then calculate the full-load current of the selected transformer on both windings. The transformer sizing calculator performs every step.

Why transformers are rated in kVA

Transformers are rated in apparent power rather than real power because the losses that limit them do not depend on power factor. Winding (copper) losses rise with the square of current, and core (iron) losses depend on voltage. Neither changes with the phase angle between voltage and current, so a 45 kVA transformer can deliver 45 kVA at any power factor, whereas its output in kilowatts varies with the load.

A load stated in kilowatts must therefore be converted to kVA before a transformer is selected:

kVA = kW ÷ power factor

Selecting a transformer from kilowatts without this conversion undersizes it for every load with a power factor below 1, which includes motors, fluorescent and LED lighting drivers, and most electronic equipment.

The core formulas

In three-phase formulas, V is the line-to-line voltage: 208 V on a 208Y/120 V system, 480 V on a 480Y/277 V system. The factor 1.732 is the square root of three. Using the line-to-neutral voltage (120 V or 277 V) in a three-phase formula is one of the most common errors in transformer sizing and understates the required rating by a factor of three.

Because apparent power is the same on both windings, apart from small internal losses, full-load current varies inversely with voltage. A transformer stepping 480 V down to 208 V draws about 43% of its secondary current on the primary side.

Sizing a transformer step by step

1. Establish the load. Use the calculated load from a load calculation, measured demand, or the nameplate ratings of the connected equipment. Express it in kVA, converting from kilowatts with the power factor where necessary.
2. Add an allowance for growth. Twenty to twenty-five percent is common where load may be added later. A fixed, fully known load may need less; a panel expected to pick up electric-vehicle charging or heat pumps may need more.
3. Select a standard rating. Round up to the next rating in the manufacturer's catalog for the phase in question. The steps between ratings widen as the size increases.
4. Calculate the full-load current of the selected rating on the primary and secondary windings, using the voltages of the actual installation.
5. Check the loading. Many designers keep continuous load at or below about 80% of nameplate rating to leave margin for ambient temperature, harmonics and inrush. This is a design practice rather than a code requirement; the manufacturer's loading guidance governs.
6. Size the overcurrent protection and conductors from the full-load currents, as described below.

Common standard ratings

Dry-type distribution transformers are built in preferred ratings that differ by phase. Catalogs vary between manufacturers, so the list for the product actually specified takes precedence.

Worked example 1: three-phase panel

A 208Y/120 V three-phase panel will draw 100 A. The transformer is fed from a 480 V three-phase system, and a 25% growth allowance is required.

Load: 1.732 × 208 × 100 ÷ 1,000 = 36.0 kVA

With growth: 36.0 × 1.25 = 45.03 kVA. This is within half a percent of the 45 kVA standard rating, which is selected.

Primary full-load current: 45,000 ÷ (1.732 × 480) = 54.1 A

Secondary full-load current: 45,000 ÷ (1.732 × 208) = 124.9 A

Loading: the present 36.0 kVA load is 80% of the 45 kVA rating, at the limit many designers use for continuous duty. If the load will run continuously at that level from the outset, the next rating, 75 kVA, is worth considering.

Worked example 2: single-phase load given in kilowatts

A single-phase 240 V load totals 20 kW at a power factor of 0.9, supplied from 480 V, with a 20% growth allowance.

Apparent power: 20 ÷ 0.9 = 22.2 kVA

With growth: 22.2 × 1.20 = 26.7 kVA. The next single-phase standard rating is 37.5 kVA.

Secondary full-load current: 37,500 ÷ 240 = 156.3 A

Primary full-load current: 37,500 ÷ 480 = 78.1 A

Had the load been taken as 20 kVA instead of 22.2 kVA, the growth figure would have been 24 kVA and a 25 kVA transformer would have appeared adequate. It would have run above its rating once the reactive component of the load was accounted for.

Overcurrent protection under NEC 450.3(B)

For transformers rated 1,000 volts or less, NEC 450.3(B) allows two arrangements. Percentages are applied to the rated full-load current of each winding, not to the connected load.

Where 125% of a current of 9 A or more does not correspond to a standard fuse or breaker rating, the next higher standard rating is permitted. The 250% primary limit in the second arrangement is a maximum and is not rounded up.

Applied to example 1 (45 kVA, 480 V to 208 V):

• Primary only: 54.1 × 1.25 = 67.7 A, so the next higher standard rating, 70 A, is permitted on the primary.
• Primary and secondary: primary not more than 54.1 × 2.5 = 135 A, so 125 A; secondary 124.9 × 1.25 = 156.1 A, so the next higher standard rating, 175 A, is permitted.

Transformer protection under Article 450 is only one requirement. The secondary conductors must also be protected under the conductor rules, and in practice this usually means a secondary overcurrent device even where Article 450 alone would permit primary-only protection. A primary device on a delta-wye transformer cannot be relied on to protect the secondary conductors, because current in the secondary does not appear proportionally on the primary for all fault types. The secondary conductor rules in NEC 240.21(C), including the length limits for unprotected secondary conductors, should be checked for every installation.

Conductor sizing

Conductors on each side are sized from the full-load current of the transformer, not from the present load, so that the conductors can carry the full rating of the equipment they serve. The selected conductor must have an ampacity, after correction for ambient temperature and the number of current-carrying conductors in the raceway, that is not less than the rating of the overcurrent device protecting it, subject to the next-size-up allowance in NEC 240.4(B).

The ampacity calculator applies the temperature and bundling corrections, and the voltage drop calculator checks whether a long secondary feeder needs a larger conductor than ampacity alone requires.

Available fault current

The impedance printed on the nameplate, expressed as a percentage (%Z), limits the current the transformer can deliver into a short circuit at its secondary terminals. Assuming an unlimited supply on the primary, which gives a conservative (high) result:

I(short circuit) ≈ secondary full-load current × 100 ÷ %Z

For the 45 kVA transformer in example 1 with a 4% impedance, the result is 124.9 × 100 ÷ 4 ≈ 3,120 A. Panelboards, breakers and other equipment on the secondary must have short-circuit current ratings at least equal to the available fault current. Lower-impedance transformers deliver higher fault currents, which matters when a transformer is replaced with a different model.

Grounding

A transformer that supplies a new neutral on its secondary, such as a 480 V delta to 208Y/120 V wye unit, creates a separately derived system. NEC 250.30 requires a system bonding jumper connecting the neutral to the equipment grounding conductors at a single point, a grounding electrode conductor, and a connection to a grounding electrode. These connections are frequently missed on small transformers fed from an existing building system and should be verified on every installation.

Factors that reduce usable capacity

Common mistakes

• Using kilowatts instead of kVA for loads with a power factor below 1.
• Using the line-to-neutral voltage in a three-phase formula.
• Sizing overcurrent protection from the connected load instead of the transformer's rated full-load current.
• Assuming primary-only protection under Article 450 also protects the secondary conductors.
• Omitting the system bonding jumper or grounding electrode conductor on a separately derived system.
• Replacing a transformer without checking whether its lower impedance increases the available fault current beyond the ratings of downstream equipment.

Frequently asked questions

How is transformer size calculated? Convert the load to kVA, add an allowance for growth, and round up to the next standard rating. For three-phase loads, kVA equals 1.732 × line-to-line voltage × current ÷ 1,000.

What size transformer is needed for 100 amps at 208 volts three phase? The load is 36.0 kVA. With a 25% growth allowance the requirement is 45 kVA, which is a standard three-phase rating.

How are transformer full-load amps calculated? Divide the kVA rating multiplied by 1,000 by the winding voltage, and for three-phase transformers also by 1.732. A 75 kVA three-phase transformer at 480 V has a full-load current of 75,000 ÷ (1.732 × 480), or 90.2 A.

Can a transformer be loaded to 100% of its nameplate rating? It is designed to carry its rated load at the ambient conditions stated by the manufacturer. Many designers plan continuous load at about 80% of nameplate to allow for ambient temperature, harmonics and future growth, but this is a design practice rather than a code limit.

Is secondary overcurrent protection always required? NEC 450.3(B) permits primary-only protection for the transformer itself, but the secondary conductors must still be protected under the conductor rules in Article 240. In most installations that results in a secondary overcurrent device.

Calculate it

The transformer sizing calculator takes the load in amperes or kilowatts, applies the growth allowance, selects the next standard rating for the phase, and returns the primary and secondary full-load currents. Related tools: breaker size calculator, ampacity calculator, voltage drop calculator, generator sizing calculator.

References and scope

This guide refers to the 2023 edition of the National Electrical Code, in particular Article 450 (transformers), 240.4 and 240.21(C) (conductor protection) and 250.30 (separately derived systems). The edition adopted by the local authority having jurisdiction, and its amendments, governs any installation. The figures in this guide are worked examples for study and estimating; they are not a design for a specific installation, which should be prepared or reviewed by a licensed electrician or professional engineer.

Written by Chameera Sampath. Not yet reviewed by a licensed electrician — see the methodology page. To report an error, use the contact page.