Every transformer nameplate assumes 40C (104F) ambient air. If the room is hotter, the transformer cannot deliver its rated kVA — and the kW you can actually pull drops with it.
Why temperature matters
A transformer generates heat from copper losses (windings) and core losses (iron). This heat must dissipate into the surrounding air. If the ambient air is hotter, the temperature difference between the winding and the air decreases — less heat dissipates — the winding reaches its thermal limit at a lower load.
The derating table
Ambient Temp % of Rated kVA 500 kVA delivers ───────────────────────────────────────────────────── 30C (86F) 105% 525 kVA 35C (95F) 103% 515 kVA 40C (104F) 100% 500 kVA ← nameplate 45C (113F) 95% 475 kVA 48C (118F) 89% 445 kVA 50C (122F) 87% 435 kVA 55C (131F) 80% 400 kVA 60C (140F) 72% 360 kVA At 60C ambient, the transformer delivers only 72% of nameplate — a 140 kVA loss on a 500 kVA unit.
Real case: data center
Situation: 500 kVA dry-type transformer (Class F, 155C) Server racks added next to electrical room Room temperature rose from 25C to 48C Nobody recalculated capacity Load: 410 kW at PF 0.92 = 446 kVA Nameplate capacity: 500 kVA ← looks fine Derated capacity at 48C: 445 kVA ← NOT fine 446 kVA > 445 kVA → thermal overload trip at 2 PM on the hottest day of the year The kW impact
The kVA derating directly reduces available kW:
kW available = Derated kVA × Power Factor At 40C (full rating): 500 kVA × 0.92 PF = 460 kW available At 48C: 445 kVA × 0.92 PF = 409 kW available At 55C: 400 kVA × 0.92 PF = 368 kW available Lost capacity: 92 kW from a 15C temperature increase Where hot rooms happen
- Electrical rooms next to server rooms - Electrical rooms next to boiler rooms or kitchens - Rooftop pad-mounted transformers in desert climates - Underground vaults with poor ventilation - Rooms where original ventilation was blocked or removed - Any room where heat-producing equipment was added later The fix options
1. Cool the room Add exhaust fan or supply air Target: below 40C during peak load Cost: $1,500-3,000 Payback: immediate (prevents trips) 2. Reduce the load Move circuits to another transformer Shed non-critical loads during peak Cost: $0-5,000 depending on rewiring 3. Upgrade the transformer Replace with higher kVA rating Or replace Class F with Class H (180C limit) Cost: $15,000-40,000 4. Add forced cooling Fans on the transformer itself Some dry-type models allow fan-cooled rating (FA) Typical boost: 33% above self-cooled rating Cost: $2,000-5,000 The lesson
Data center case: Exhaust fan cost: $1,800 Downtime cost: $23,000 Room dropped from 48C to 35C Full 500 kVA restored (actually 515 kVA at 35C) Fan paid for itself before installation finished Before loading a transformer, check the room temperature. The nameplate kVA is only valid at 40C. Above that, derate — or cool the room.
