what is the answer to these questions?

[Little Bill]

if I'm told there's a 6awg cu THHN feeder without being told the load, arguably I should protect that with a 70A breaker unless I know the load is less than 60A.

I don't know why you would do that! I would automatically use a 60A breaker with no further info.

 

[jaggedben]

I don't know why you would do that! I would automatically use a 60A breaker with no further info.

Then you don't understand the rule.

 

[Tulsa Electrician]

So I'll ask. What did the answer key say. Or did I miss it.
The guys are going to want to know.
Thanks

 

[Little Bill]

Then you don't understand the rule.

I understand the rule just fine!
You said "feeder", so we don't usually figure a load on a feeder at 100%. Meaning we don't expect a full 60A at the end. So if I see a #6 feeder, without knowing the load, it's going on a 60A breaker.

 

[jaggedben]

I understand the rule just fine!
You said "feeder", so we don't usually figure a load on a feeder at 100%. Meaning we don't expect a full 60A at the end. So if I see a #6 feeder, without knowing the load, it's going on a 60A breaker.

And when the breaker trips at 63A? I mean, that's probably being paranoid, but less paranoid than worrying that the wire will be damaged on a 70A breaker.

Also, unless there are continuous loads, we do figure the load on a feeder at 100%.

 

[Carultch]

And when the breaker trips at 63A? I mean, that's probably being paranoid, but less paranoid than worrying that the wire will be damaged on a 70A breaker.

Also, unless there are continuous loads, we do figure the load on a feeder at 100%.

If you have a non-standard rated breaker, by omission of the NEC not covering this case, you need at least as many amps as OCPD, because 240.4(B) only says you can use the next standard size. Not any size up to the next standard size. Not that 63A would swing a design anyway, since it'd be #6 Cu wire whether the trip rating is 60A, 63A, or 70A.

A place where this would make a difference, is with 330A fuses (which I've had come up as part of an inverter with exotic fuse ratings). This leads to the non-intuitive result that a 350A fuse would allow a smaller wire size than a 330A fuse, in the event that your load only requires 310A of ampacity.

 

[Little Bill]

Also, unless there are continuous loads, we do figure the load on a feeder at 100%.

You may figure it, but not expect it.

 

[wwhitney]

If you have a non-standard rated breaker, by omission of the NEC not covering this case, you need at least as many amps as OCPD, because 240.4(B) only says you can use the next standard size.

That's an unreasonably literal reading of 240.4(B). Since the base sentence of 240.4 sets a maximum ("protected against overcurrent in accordance with their ampacities"), 240.4(B) is changing that maximum to the next higher standard size.

Cheers, Wayne

 

[kwired]

Nope.
You are seemingly the second person to overlook that question 2 specified 60C terminals. OP got the right answer in post #14.

My bad, I think I had a 60 amp breaker in my mind for some reason when it does say 50 amp pretty sure I was thinking 60C temp as I recall thinking 55 amp conductor 60 amp protection is allowed.

 

[kwired]

So is the question expecting you to account for the next size up rule? Or is it just asking for the conventional answer that has at least as much wire as OCPD?

If a document tells me I need a 70A breaker without further information, I would specify #4 Cu wire in practice. If I have enough information to know that it's only utilizing 65A or less, I'd specify #6 Cu wire, but not until I have that information.

As worded it is asking minimum size conductor permitted in each scenario, giving overcurrent device setting and either conductor and/or terminal temp rating. With no other conditions one can only assume it is not something like a motor circuit where there is rules other than the general rules that will apply. Next size up rule is part of the general rules.