Calculator
Transformer Sizing Calculator
Size a dry-type transformer the way the Code does it — enter the connected load in kVA, amps, or kW, pick single- or three-phase and your primary and secondary voltage, and get the next standard catalogue kVA rating with primary and secondary full-load amps, NEC Table 450.3(B) overcurrent protection, minimum conductor sizes, and how much spare capacity you are left with. Free, no signup.
Free
Overcurrent protection & conductors
Conductors are sized at 125% of winding full-load amps against NEC Table 310.16 at base conditions — 30°C ambient and no more than three current-carrying conductors. Add ambient and bundling derating for the real raceway.
Recommended transformer75 kVAHealthy spare capacity
60 kVA connected + 25% margin = 75 kVA design load · 480 V → 208 V 3-phase
Spare capacity
20.0%80.0% loaded — target 20–40% spare (60–80% loading)
Primary full-load amps
90.2 A75 kVA ÷ (480 V × √3)
Secondary full-load amps
208.2 A75 kVA ÷ (208 V × √3)
Primary OCPD
225 A250% of primary FLA = 225.5 A — largest standard rating at or below
Secondary OCPD
300 A125% of secondary FLA = 260.2 A — Note 1 next size up
Primary conductors
2 AWG125% of primary FLA = 112.8 A · NEC Table 310.16 @ 75°C
Secondary conductors
300 kcmil125% of secondary FLA = 260.2 A · NEC Table 310.16 @ 75°C
Bonding jumper / GEC
2 AWG CuNEC 250.30 separately derived system — Table 250.66 off the secondary phase conductors
- The 225 A primary device exceeds the 115 A ampacity of the 125% primary conductors. NEC 450.3(B) permits that setting for the transformer, but NEC 240.4 still requires the conductors to be protected — upsize the primary conductors to the device rating or apply the NEC 240.21(B) tap rules.
- Separately derived system: NEC 250.30(A)(1) system bonding jumper and NEC 250.30(A)(5) grounding electrode conductor size from Table 250.66 off the 300 kcmil phase conductors — 2 AWG copper (1/0 AWG aluminium).
Transformer sizing at a glance
Convert the connected load to kVA, add a growth margin, then round UP to the next catalogue rating — dry-type units only come in fixed sizes. Winding current follows kVA × 1000 ÷ V for single-phase and kVA × 1000 ÷ (V × √3) for three-phase, and NEC Table 450.3(B) percentages apply to that transformer-rated current, not to the load.
With primary-only protection the primary device is capped at 125% of primary FLA (167% between 2 A and 9 A, 300% below 2 A). Add a secondary device at 125% of secondary FLA and the primary is permitted up to 250%, which is what lets a real transformer energise without nuisance tripping. Table 450.3(B) Note 1 only lets you round UP on the 125% values — every other percentage is a ceiling.
Standard three-phase dry-type ratings: 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750 kVA.
Industry benchmarks
- 20–40% spare capacity — the usual design target. Below 10% there is no room to add a single circuit without replacing the unit.
- 480Δ → 208Y/120 V is the standard commercial step-down; 240/120 V and 120 V single-phase cover light commercial and control power.
- Inrush on a dry-type unit can hit 8–12× full-load current for a few cycles — that is why the Code allows 250% on the primary once a secondary device is in place.
- A secondary device is not optional when you use the 250% primary rule — and without one, secondary conductors must follow the NEC 240.21(C) tap rules.
What you'll get
Enter the connected load
Give the load in kVA straight off the panel schedule, in amps at the secondary voltage, or in kW with a power factor. Pick single- or three-phase and your primary and secondary winding voltages.
Add a sizing margin
Choose a growth allowance — 20–25% is the usual spec-grade figure. The tool rounds the design kVA up to the next standard dry-type rating and shows the spare capacity you end up with.
Get the code sizing values
See primary and secondary full-load amps, the NEC Table 450.3(B) primary and secondary overcurrent devices with the exact rule applied, minimum conductors at 125% of FLA, and the Table 250.66 bonding jumper for the separately derived system.
How it works
- 1
Enter the connected load
Give the load in kVA straight off the panel schedule, in amps at the secondary voltage, or in kW with a power factor. Pick single- or three-phase and your primary and secondary winding voltages.
- 2
Add a sizing margin
Choose a growth allowance — 20–25% is the usual spec-grade figure. The tool rounds the design kVA up to the next standard dry-type rating and shows the spare capacity you end up with.
- 3
Get the code sizing values
See primary and secondary full-load amps, the NEC Table 450.3(B) primary and secondary overcurrent devices with the exact rule applied, minimum conductors at 125% of FLA, and the Table 250.66 bonding jumper for the separately derived system.
How to size a transformer and its overcurrent protection
Transformer sizing starts with apparent power, not real power. Convert the connected load to kVA — single-phase kVA is V × A ÷ 1000 and three-phase is V × A × √3 ÷ 1000, or kW ÷ power factor if you only have kW — then add a growth margin and round UP to the next standard catalogue rating. Dry-type units only come in fixed sizes: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100 and 167 kVA single-phase, and 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500 and 750 kVA three-phase. Once you have the rating, full-load amps follow from kVA × 1000 ÷ V for single-phase and kVA × 1000 ÷ (V × √3) for three-phase — a 75 kVA unit on a 480 V three-phase primary draws 90.2 A, and its 208Y/120 V secondary carries 208.2 A.
Overcurrent protection comes from NEC Table 450.3(B), and the percentages apply to the transformer-rated current, not the load. With primary-only protection the primary device is capped at 125% of primary FLA when that current is 9 A or more, 167% between 2 A and 9 A, and 300% below 2 A. Add a secondary device at 125% of secondary FLA and the primary is permitted up to 250%, which is what lets a real transformer ride through its magnetising inrush without nuisance tripping. Note 1 to the table only permits rounding UP to the next standard NEC 240.6(A) rating on the 125% values — the 167%, 300% and 250% figures are ceilings you round down from. Conductors on both sides are sized at 125% of winding full-load amps, and because a 250% primary device can far exceed its conductors’ ampacity, those conductors still have to be protected under NEC 240.4 or run as taps under 240.21.
Related tools
More free resources to help you price jobs, stay organized, and capture leads.
Business Templates
Free invoices, estimates, and work orders for your trade.
Browse templatesAI Receptionist
Let Larry answer every call while you focus on the job.
Learn moreContractor Blog
Guides on pricing, hiring, marketing, and operations.
Read guidesElectrical Licensing
State licensing requirements, exams, and renewal rules for electrical contractors.
View statesTransformer Sizing Calculator FAQs
What size transformer do I need for my load?
Convert the connected load to kVA, add a growth margin of 20–25%, then round up to the next standard catalogue rating. A 60 kVA connected load with a 25% margin is a 75 kVA design load, which lands exactly on the standard 75 kVA three-phase rating and leaves 20% spare capacity. Never round down — a transformer loaded past its nameplate overheats and its insulation life drops sharply.
How do I calculate transformer primary and secondary full-load amps?
For single-phase, FLA = kVA × 1000 ÷ V. For three-phase, FLA = kVA × 1000 ÷ (V × √3). A 75 kVA three-phase transformer with a 480 V primary draws 75,000 ÷ (480 × 1.732) = 90.2 A on the primary, and its 208 V secondary carries 75,000 ÷ (208 × 1.732) = 208.2 A. Because the transformer is nearly lossless, Vp × Ip equals Vs × Is — a useful sanity check on your numbers.
What size breaker goes on a transformer primary?
It depends on whether you also protect the secondary. With primary-only protection, NEC Table 450.3(B) caps the primary device at 125% of primary full-load amps and Note 1 lets you go to the next standard size — 90.2 A × 1.25 = 112.75 A, so a 125 A device. If you add a secondary device at 125% of secondary FLA, the primary may go up to 250% of primary FLA, but that is a ceiling with no round-up: 90.2 A × 2.5 = 225.5 A, so 225 A is the largest standard device you may use.
Which NEC edition does this calculator use?
This calculator implements the 2023 NEC (NFPA 70) — Table 450.3(B) for transformers 1000 V and less, the 240.6(A) standard ampere ratings including the fuse-only 1, 3, 6, 10 and 601 A ratings, Table 310.16 ampacity at base conditions, and Table 250.66 for the separately derived system bonding jumper and grounding electrode conductor. It is a design aid, not a substitute for a stamped design: your authority having jurisdiction (AHJ) governs, local amendments may differ, and real installations still need ambient and bundling derating, short-circuit and coordination study, and the NEC 240.21(C) secondary conductor rules.
