Transformer Sizing Calculator: kVA, Load & Voltage Selection

Reading time: ~10 min read

Undersize a dry-type transformer and it runs hot, hums, and dies early. Oversize it and you're paying for core losses around the clock on capacity nobody uses. The right size is one line of math — kW divided by power factor — plus an honest look at what's actually on the load schedule. The calculator below handles it: enter your load in kW with a power factor (add motor horsepower if the load is motor-heavy), choose single or three-phase and a voltage pair, and it returns the required kVA, the next standard transformer size, and full-load amps on the primary and secondary. Below the calculator you'll find the formulas, the voltage-pair cheat sheet, and the NEC 450 protection and inrush notes.

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Transformer Sizing Calculator

The Sizing Formula

Transformer sizing is apparent-power arithmetic:

kVA = kW ÷ PF   ·   FLA (3Φ) = kVA × 1000 ÷ (1.732 × V)   ·   FLA (1Φ) = kVA × 1000 ÷ V

Power factor is where estimates go wrong. Lighting and receptacle loads in modern commercial buildings run 0.9–0.95 with LED drivers and electronic ballasts; older magnetic-ballast and motor-heavy floors can sit at 0.8. If you don't have a measured PF, 0.9 is a defensible default — and it's what the calculator starts with. Motors convert at 0.746 kW per horsepower, then divide by efficiency (~0.85 for a small NEMA motor) and motor PF (~0.85 at full load). A 10 HP motor is not 7.46 kVA on the transformer — it's closer to 10.3 kVA before the demand factor.

The demand factor is your honesty check: not everything runs at once. Office floors demand 70–85% of connected load; a machine shop with duty cycling can be 50–70%. Apply demand to the sum, then apply your growth margin — 20–25% is standard for commercial tenant spaces. The result rounds up to the next standard size, never down.

Standard Sizes and Where Each One Lives

Size (3Φ)	Typical job	Secondary FLA @ 208Y/120
15 kVA	Small tenant panel, lighting circuit group	41.6 A
30 kVA	Small office suite, retail bay	83.3 A
45 kVA	Mid-size tenant space, small kitchen	124.9 A
75 kVA	Office floor, restaurant, light shop	208.2 A
112.5 kVA	Multi-tenant floor, small industrial bay	312.3 A
150 kVA	Large commercial floor, light manufacturing	416.4 A
225 kVA	Industrial bay, warehouse with equipment	624.6 A
300 kVA	Manufacturing floor, data room support	832.7 A

Single-phase standard sizes (3–167 kVA) cover residential services, irrigation panels, and small commercial: 15–25 kVA feeds a house or small shop, 37.5–50 kVA handles a large home or duplex, and 75–167 kVA serves small commercial services and farm loads.

Voltage Pairs You'll Actually Spec

Pair	Use case
480 Δ → 208Y/120	The workhorse: steps 480 V distribution down to 208 V three-phase with 120 V line-to-neutral for panels, receptacles, and lighting.
480 Δ → 240 Δ	Machine shops and industrial equipment needing 240 V three-phase; add a high-leg or a separate 1Φ unit for 120 V.
480 → 120/240 (1Φ)	Single-phase panels in 480 V buildings: offices, break rooms, small services.
600 Δ → 208Y/120	Canadian and heavy-industrial 600 V distribution stepping down to utilization voltage.
480 Δ → 480Y/277	Creating a neutral for 277 V lighting where the service is delta — or isolating a lighting load group.

One trap worth naming: a 480→208Y/120 transformer fed from a 480Y/277 system is fine, but always confirm whether the primary is delta or wye on the nameplate and whether the secondary neutral needs to be solidly grounded per NEC 250.30 — separately derived systems have their own grounding electrode requirements. Our grounding guide (NEC 250) covers the electrode side of that install.

Inrush, Impedance, and Protection (NEC 450.3)

Energizing a transformer looks like a dead short for the first few cycles: magnetizing inrush runs 8–12× full-load current depending on where in the waveform the contacts close. That's why NEC Table 450.3(B) permits primary-only protection at 125% of primary FLA (for currents of 9 A or more), with the next standard OCPD size up allowed when 125% lands between ratings — and why secondary protection at 125% opens the door to 250% primary protection. If a correctly sized breaker still trips on energization, the fix is a time-delay fuse class or a breaker with adjustable instantaneous pickup, not a bigger transformer.

Impedance (%Z) sets fault current on the secondary. Typical dry-type units run 2–5% Z. Available fault current at the secondary is roughly FLA ÷ %Z — a 75 kVA, 208 V, 3% Z unit can deliver about 208 A ÷ 0.03 ≈ 6,940 A before you add motor contribution and utility stiffness. Every downstream panel and breaker must carry an interrupting rating above that number; check it against the gear you're buying, not the gear you wish you had.

K-factor is the last spec check: non-linear loads (VFDs, LED drivers, server PSUs) push harmonic currents that overheat standard transformers. K-4 is standard; K-13 for offices dense with electronics; K-20 for data and broadcast. If the load schedule says "computers and drives," price the K-rated unit the first time.

Ready to quote? Send the load schedule with your voltage pair and we'll return a sized transformer, primary/secondary protection, and disconnects from our electrical supplies inventory. Pairing with a backup source? See our generator collection for units matched to transformer-fed services.

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