conductor sizing once again 310.16

[iwire]

If there are two loads one 40 amps the other 17 amps they may or may not operate at the same time is the 60 amp OCPD permitted on a feeder rated at 55 amps?

In your above example the load is under 55 amps.

If the load is above the rating of the conductor you can not use that conductor.

Lets say the non continuous load is 56 amps, you can use a 60 amp breaker but you would need to use 4 AWG NM or 6 AWG pipe and wire.

 

[infinity]

In your above example the load is under 55 amps.

If the load is above the rating of the conductor you can not use that conductor.

Lets say the non continuous load is 56 amps, you can use a 60 amp breaker but you would need to use 4 AWG NM or 6 AWG pipe and wire.

Bob, what am I missing 40 + 17 = 57, are you referencing some sort of load calculation?

 

[iwire]

Bob, what am I missing 40 + 17 = 57, are you referencing some sort of load calculation?

I misread your post, all I saw was 'not operate at the same time'



I have to be honest, I don't really understand what we are talking about. 310.16 tells us the conductor rating .... end of story as fas as I can see. :-?:-?:-?:-?

240.4(B) has absolutely no influence on conductor ratings.

 

[frizbeedog]

....240.4(B) has absolutely no influence on conductor ratings.

It would seem to me that the increase to the next standard size OCPD, by implication, would have no adverse effect on the conductors if the load were increased without any change to the initial installation. Otherwise, I would think that this practice would not be allowed. Knowing full well that we will choose conductors for the load, and the next standard size as allowed, and later someone will increase that load, and make no other changes.

Meaning, there must be sufficient margins built into the tables to allow this at 800 amps and below.

 

[iwire]

It would seem to me that the increase to the next standard size OCPD, by implication, would have no adverse effect on the conductors if the load were increased without any change to the initial installation. Otherwise, I would think that this practice would not be allowed.

I disagree with that entirely.

The NEC is counting on us, the installers, not to overload the conductors.

For another area where this is abundantly clear check out 230.90 Exception 3.

I can have six 800 amp breakers 'protecting' 100 amps of conductors as long as the calculated load is under 100 amps.

 

[frizbeedog]

For another area where this is abundantly clear check out 230.90 Exception 3.

I can have six 800 amp breakers 'protecting' 100 amps of conductors as long as the calculated load is under 100 amps.

What about exception 2.....?

Is 800 the next higher for a 100 amp load for 100 amp conductors?

Adjustable trip?

Sneaky.

 

[iwire]

What about exception 2.....?

Is 800 the next higher for a 100 amp load for 100 amp conductors?

Adjustable trip?

Sneaky.

Do not read anything into my choice of 800 amp breakers, it could be two 100s, a 300 and a 1000 all supplied by conductors that exceed the calculated load but are well under the combined rating of the breakers.

Here is a service a friend of mine did, there are six 100 amp breakers 'protecting' the roughly 225 amp riser. All that matters is his calculated load was under 225 amps.

The NEC is counting on future installers not to overload the riser.

 

[Len]

rating

rating

Under 100 amps and smaller than #4 wire you size by 60c column over 100amps or larger than #4 you use 75c column.

 

[iwire]

Under 100 amps and smaller than #4 wire you size by 60c column over 100amps or larger than #4 you use 75c column.

Unless the equipment is otherwise marked and it often is marked 75 C. :smile: