How far can you overload a distribution transformer – and for how long?

"Can I run a 630 kVA transformer at 700 kVA for two hours a day?" is one of the questions we hear most often. The answer is usually yes – with conditions. The rating is defined for continuous operation at 40 °C ambient; a transformer is a large thermal mass that heats up slowly, and IEC 60076-7 (loading guide for oil-immersed transformers) uses exactly that to allow temporary overloads.

Three loading regimes in IEC 60076-7

Regime	Current limit (distribution transformers)	Hot-spot limit	Top-oil limit	Life consumption
Normal cyclic loading	1.5 × In	120 °C	105 °C	Same as continuous rated load over the cycle
Long-time emergency loading	1.8 × In	140 °C	115 °C	Accelerated ageing – accepted for days/weeks
Short-time emergency loading	2.0 × In	160 °C	115 °C	Up to 30 min; risk of gas bubbles above 140 °C

What decides the permissible overload

1. The preceding load. A transformer that ran at 50 % overnight starts the day cool; one that ran at 90 % has no thermal reserve.
2. Ambient temperature. At 10 °C the same transformer can carry far more than at 35 °C.
3. Duration. The oil time constant of a distribution transformer is 2–3 hours, the winding time constant a few minutes. Short peaks barely move the oil temperature.
4. Type. Oil-immersed units have a large thermal mass; cast-resin units heat faster and have a lower overload capability without fans.

Typical cyclic overload capability – oil-immersed, 20 °C ambient

Preceding load	1 h overload	2 h overload	4 h overload	8 h overload
50 %	≈ 150 %	≈ 135 %	≈ 122 %	≈ 112 %
70 %	≈ 140 %	≈ 128 %	≈ 118 %	≈ 110 %
90 %	≈ 125 %	≈ 118 %	≈ 112 %	≈ 106 %

Values are indicative for ONAN distribution transformers with normal ageing; the loading guide's equations or the manufacturer's curves give the exact figures for a specific unit.

What limits the overload in practice

• Bushings, tap changer and cables – rated for a current, not for a thermal cycle. Above 1.2 × In their heating must be checked separately.
• The LV circuit breaker setting – often the real limit in an existing installation.
• Losses. At 130 % load the load losses are 1.69 × rated: the overload hour costs more energy than a normal one.
• Cast resin: hot-spot limit 155 °C (class F); without fans the cyclic overload is roughly half of the oil-unit values above. With the AF kit, 140 % becomes a continuous rating.

For photovoltaic and battery plants

PV transformers see a daily cycle – zero load at night, full load around noon. The morning "cold start" gives a real reserve at midday, which is why a 1 000 kVA PV transformer can be matched to 1 050–1 100 kVA of inverter capacity at cos φ = 1 without loss of life. Ask us for the loading curve for your site's climate; all PNACHO-O data sheets state the thermal design values needed for the calculation.