NEC Article 100 - Continuous Load and duty cycle

I am looking for clarification on the definition of "continuous load" for the purpose of sizing conductors and overcurrent devices. Some components are quite obviously one or the other...

What if you have heating elements that keep a process at temperature, but rarely exceed 50% duty cycle (solid state switching) except at startup - which is <30 minutes?

I was unclear whether "3 hours of operation per day" means a continuous 3 hours or a sum of three hours and if duty cycle even factors into these calculations. Any input is appreciated. Thanks,

bimmersix,

i would say that heating elements are continuous because they are solid state controlled. My opinion.

you must also consider the number of conductors (derating factor), ambient temperature (derating factor), terminal rating (this rates the wire amperage reguardless of what you use), type of wire, location, and other things as well.

regards,
james

James Mcquade said:

bimmersix,

i would say that heating elements are continuous because they are solid state controlled. My opinion.

you must also consider the number of conductors (derating factor), ambient temperature (derating factor), terminal rating (this rates the wire amperage reguardless of what you use), type of wire, location, and other things as well.

regards,
james

Click to expand...

Thanks, James.

Yes, I have factored those in (6 conductors in a raceway, 75C terminals, 50C ambient). I am using MGT 450C wiring. As we speak I am reading and highlighting through the entire NFPA 79 (2015 Edition). I'll probably need to visit NFPA 70 and at least NEC 240.

Personally I'd call the "<30 minutes" 100% load warm up continuous for sizing purposes. Especially with a 50*C ambient and 75*C terminals. Not a whole lot of margin for error there.

You need not look any further than NFPA70 Article 424.3(B)

Which states:

"Fixed electric space heating equipment shall be considered continuous load"

BCS