electric heat, 125%

[caosesvida]

Does anyone know the official reason for the nec
code 424.3-B electric heaters must be wired for 125%. I thought it was because of heat build up over time? But why not just use the continous rating? thanks.

 

[steve66]

Re: electric heat, 125%

I don't think it has anything to do with the heat from the heater. Sizing breakers and conductors for 125% of load is somewhat standard for any loads that may be continuous. For example, the same requirement applies to lighting branch circuits and motors.

Steve

 

[caosesvida]

Re: electric heat, 125%

I meant the heat from the amperage/resistance through the circuit. If the heater conducts heat to the conductors that is a factor also. As the conductor heats up the resistance goes down, ( as I understand it) allowing more current flow. I have seen 90 amp heaters draw over 100 amps due to using aluminum wire for 90 amps. Upgrading the wire to the 125% level eliminated the problem. Someone questioned me on why electric heaters were always 125% and not just for over three hours like other equipment. I thought it was becuause it might be considered that an electric heater was almost always going to be used for more than three hours at some point. But I wanted some documentation on it. thanks. for the reply.

 

[bob]

Re: electric heat, 125%

caosesvida
The NEC require that electric heat be considered a
continuous load. Breakers that you normally get at the supply house have rating for noncontinuous
load. When you have a continuous load the breaker
rating must be reduced to 80%. An example is suppose you had a load of 100 amps continuous. You can take the load and divide by 0.80 ,100/0.80 = 125 amps, or you can muliply the rating by 1.25 ,100 x 1.25 = 125 amps. You get the same results either way. There are breakers the are rated by the mfg for 100% continuous.

As the conductor heats up the resistance goes down, ( as I understand it) allowing more current flow.

The resistance goes up when the temperature rises and down when it drops.