We are installing an item and have some concerns. The supplier is claiming we will void the UL listing and warranty but the current configuration seems unsafe.

I am curious if the following would be adherent to UL standards?

The input is 28.3 amps into the item. The cord going into the back of the item is 10AWG wiring with a 50-amp plug. 10awg wiring is not rated for 50 amps therefore installing this on a 50-amp breaker will not protect this wiring.

The item is internally fused.

Two exemplary questions:

  • If instead we had 14 AWG wiring up the 50A plug but the load was fused at 10A would this be cause for concern? There is a large amperage gap between rated load of the wiring and the point at which a short would trip the breaker.

  • Is there any UL enforced relationship between plug rating and wiring rating.


  • \$\begingroup\$ Does this item have an internal fuse? Plugging a 1 or 2 A (or lower) device into a 10 or 15 A circuit is extremely common, for example. \$\endgroup\$ – The Photon Oct 12 '18 at 21:14
  • \$\begingroup\$ Yep, it does, sorry for the omission. \$\endgroup\$ – nate Oct 12 '18 at 21:22
  • \$\begingroup\$ Yes. For comparison this laptop charger drawing 0.5A uses 18awg/3 wire going to it’s 15A plug, not 14awg. \$\endgroup\$ – τεκ Oct 12 '18 at 21:48

You have answered your own question. If the load draws only 28.3 amps continuous, with maybe a turn-on surge twice that for 1/2 second, no harm no foul. The breaker is rated to handle brief start-up currents close to 50 amps, not a continuous 50 amps.

For a real 50 amp load you would need 8 gauge THHN wire, but 28.3 amps is ok for a short run of 10 gauge. If the wire gets warm to the touch that is not a fire hazard. I do not see any UL violations here.

NOTE: To answer your 3 extra questions;

1) 10 gauge wire is too small for 28.3 amps continuous load. It will get hot but certain high grades of insulation would not catch fire, but the voltage drop over 3 yards or 3 meters would be a lot. The load dictates the plug and wire used, not the other way around. For a given type of plug a certain range of wire gauges are allowed and maximum breaker amp limits are dictated. The type of breaker panel is determined by summing up all the possible loads it might have, with 120 VAC loads sharing breakers to keep construction cost down.

2) Yes, there is a UL enforced rating for the plug and socket type and the wire gauge, based on the voltage being supplied and the expected maximum continuous load current. We are not dealing with 3-phase or even single phase 600 VAC here, so we can stay with residential sockets and plugs. Dedicated 30 amp breakers for a large kitchen are custom install work hopefully built into the cost of the building. It is expensive to add-on later as it requires using 10 gauge wire, and your breaker panel needs a spare slot for it.

3) There is not always a large gap between a wires amp rating and the breaker protecting it. The best I can say about this is that 240 VAC high-wattage appliances will at least have its own dedicated breaker. That rule does not apply to 120 VAC loads which may share a 15 amp or 20 amp breaker.

The socket and plug and wire have to meet UL ratings for the voltage supplied and if it is single phase 120 VAC or split phase 240 VAC. 240 VAC outlets are large and have big slots and connection slides to handle up to 50 amps typical. This would be your water heater, clothes dryer, etc. Items that consume several kilowatts of power as normal operation. The wire is 10 gauge to 8 gauge and rated for 250 AC.

You can see why normal 120 VAC outlets are simple things, though a ground pin in the socket is mandatory, even if what is plugged in does not need it. Wires can be as small as 18 gauge for desk or reading lamps or 10 gauge for toasters, microwave ovens, etc. The outlets limit is about 1500 watts, and it may share a breaker with another outlet or ceiling lights.

Yes, your breaker panel may not have enough breakers to cover each outlet and ceiling light, so tripping a breaker is common in the USA because contractors do not have to have dedicated breakers for each load. Something that has always annoyed me.

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  • \$\begingroup\$ Thanks, Sparky, for the reply. I added two clarifying questions above. If you have a chance the clarification would help. \$\endgroup\$ – nate Oct 12 '18 at 21:51
  • \$\begingroup\$ Specifics ahead. This is a lab setting, 3 phase 208 stepped down from mains 3 phase 480V. We are slated to pull 6ga wire to the 50A outlet and install a 50A breaker to satisfy the current plug. We have runs of 30A already installed. Ideally we would switch to a 30A plug. I think if you answer the first question - (tiny wire, tiny load, big plug) - it would help. It seems now that the plug places no constraints o the cord required - your answers seem to suggest the only relevant UL requirement is the load, independent of the plug. \$\endgroup\$ – nate Oct 12 '18 at 22:38

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