Feeder to cranes can be undersized?

[Isaiah]

Does Table 610.14.14(A) allow you to run #4AWG from a 100A CB if you have 60 minute motors at 90degree C? It’s shows #4 is good for 111A
HOWEVER. Table 310.16 shows #4 is good for 85A
Thanks
Isaiah


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[winnie]

NEC 310.16 gives the _continuous_ ampacity for general purpose conductors.

610.14(A) gives time limited ampacity for conductors used in a specific application.

This takes advantage of the fact that it takes time for the conductors to heat up and get damaged. If the load is of necessity time limited, then you can permit the conductors to carry more current during that limited time.

Jon

 

[Isaiah]

NEC 310.16 gives the _continuous_ ampacity for general purpose conductors.

610.14(A) gives time limited ampacity for conductors used in a specific application.

This takes advantage of the fact that it takes time for the conductors to heat up and get damaged. If the load is of necessity time limited, then you can permit the conductors to carry more current during that limited time.

Jon

So basically you’re saying it’s ok to run #4AWG off a 100A CB due to non-continuous load using 610.14(A)


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[Engser18]

So basically you’re saying it’s ok to run #4AWG off a 100A CB due to non-continuous load using 610.14(A)


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I would say it is depend on how you evaluate this circuit and its wiring method. You also need to see 110.14 (C) Temperature Limitation and 310.14 (A) (2) Selection of Ampacity.

 

[winnie]

So basically you’re saying it’s ok to run #4AWG off a 100A CB due to non-continuous load using 610.14(A)

That is what the code says. I've not done an independent analysis, however it seems reasonable to me.

In other parts of the code there are allowances about higher breaker ratings allowed for a given wire size on the basis of short term overload; look at the allowed breaker sizes for welders and X-ray machines.

In my work we are specifically engineering motors which are used for short duration overloads.

Lots of other examples of time limited operation allowing higher current than usually expected for a given conductor size.

-Jon