Indoor Transformer Fire Protection: IEC 61936-1, FM Global and NEC Compared

Three frameworks shape how liquid-filled transformers may be installed in and next to buildings. All three reward fluids with a fire point above 300 °C – under conditions.

Updated 12 November 2026 · Published by IsoTec

Short answer

IEC 61936-1, FM Global Data Sheet 5-4 and the US National Electrical Code (NEC) allow transformers filled with K-class or less-flammable liquids closer to buildings and inside them with less demanding measures than mineral oil. In return they ask for fast protection, liquid containment and – for the largest relaxations – a robust tank with pressure relief.

The three frameworks

	IEC 61936-1 / EN 61936-1	FM Global Data Sheet 5-4	NEC (NFPA 70) Art. 450
Where it applies	IEC countries, in Europe as EN	Sites insured by FM Global, worldwide	United States
Fluid criterion	Class K: fire point above 300 °C (IEC 61039)	FM Approved fluid: fire point at least 300 °C (Class 6933)	Listed less-flammable liquid: fire point at least 300 °C
Outdoor	Clearances by liquid class and volume, otherwise fire walls	Separation distances or barriers and water spray; reduced for FM Approved transformers	Restrictions of the liquid listing; more for mineral oil
Indoor	Fire-rated room or sprinklers; further relaxations with enhanced protection	Protection by sprinklers, foam-water, water mist or compressed air foam where recommended	Without vault up to 35 kV under conditions; mineral oil generally in a vault
Transformer requirements	Enhanced protection: tank rupture strength, pressure relief, low- and high-current fault protection	FM Approved transformer (Class 3990): tank strength, pressure relief, fault protection	Installation according to the listing of the liquid

Simplified overview. The tables and conditions of the applicable edition are decisive.

IEC 61936-1

IEC 61936-1 covers power installations above 1 kV AC. For transformers it distinguishes liquid classes O, K and L, the liquid volume and whether the transformer has enhanced protection – tank rupture strength, a pressure relief device, protection against low-current faults and protection against high-current faults. Outdoors, it specifies clearances to other transformers and to buildings; where they cannot be met, fire-resistant walls are used. Indoors, it specifies fire-rated rooms or sprinkler protection. K-class transformers benefit from smaller clearances and simpler indoor arrangements than mineral-oil units. Common sumps must prevent a fire from spreading between transformers, for example with gravel layers.

FM Global and FM Approvals

FM Global Data Sheet 5-4 is the loss prevention guideline for transformers at FM-insured sites. It distinguishes mineral oil, less flammable fluids and FM Approved transformers. Two certification standards are behind this:

Class 6933 – transformer fluids. The fire point is tested by ASTM D92 (Cleveland open cup); the average of three tests must be at least 300 °C. Certification includes production quality and surveillance. Contamination with uncertified fluids voids the certification of that lot.
Class 3990 – liquid-insulated transformers. Certified fluid, tanks that withstand at least 7 psi (48 kPa) without permanent distortion and 15 psi (rectangular) or 20 psi (cylindrical) without rupture, a pressure relief device with a nominal opening pressure of 10 psi (69 kPa), and electrical protection against low- and high-current faults – for example current-limiting fuses, and sudden pressure relays above 2,500 kVA.

The 2019 revision of Data Sheet 5-4 lists automatic sprinklers, foam-water sprinklers, water mist and compressed air foam as options for indoor protection.

NEC Article 450

In the United States, Article 450.23 of the NEC covers transformers insulated with listed less-flammable liquids with a fire point of at least 300 °C. Indoors they may be installed without a vault – for example in non-combustible buildings up to 35 kV where no combustible materials are stored, with a liquid confinement area and within the restrictions of the liquid listing, or with an automatic fire extinguishing system and liquid confinement. Oil-insulated transformers installed indoors generally require a vault (Article 450.26).

What all three have in common

A fluid with a fire point of at least 300 °C
Fast electrical protection that limits the arc energy
Pressure relief and a tank that does not rupture easily
Liquid confinement sized for the liquid volume
Fire-rated separation or automatic suppression, depending on the situation
No storage of combustible materials around the transformer

Typical building situations

Basement substations of offices, hospitals and shopping centres
Substations in high-rise buildings and on roofs
Data centers with transformers inside or next to the building
Industrial halls with process equipment close to the transformer

For underground transport infrastructure see transformers in tunnels.

Checklist

Which framework applies: IEC 61936-1, national rules, insurer requirements, NEC?
Liquid class and volume per transformer – and per separate compartment.
Does the transformer meet enhanced protection or FM Approved transformer criteria?
Required clearances, fire walls or fire-rated room.
Detection and suppression.
Liquid confinement and firewater.
Ventilation and the escape of hot gases from the room.

Frequently asked questions

Can a transformer with ester fluid be installed in a basement?

Yes, in many cases. Codes allow K-class or less-flammable liquid transformers indoors with fire-rated rooms or sprinklers, liquid confinement and fast protection. Local rules and the insurer decide the details.

What is an FM Approved transformer?

A transformer certified under FM Approvals Class 3990: it uses a certified fluid with a fire point of at least 300 °C and meets requirements for tank strength, pressure relief and electrical protection.

What does NEC 450.23 require?

Transformers with listed less-flammable liquids (fire point at least 300 °C) may be installed indoors without a vault under conditions, such as up to 35 kV in non-combustible buildings with liquid confinement, or with automatic fire extinguishing and liquid confinement.

Sources and further reading

Clarifying requirements for an indoor substation?

We provide the fluid data that authorities and insurers ask for. IsoTec products are supplied to customers outside Germany.