conductor sizing once again 310.16

[CopperTone]

This is got me confused and I think I need straighening out. I have a spec for a 60amp sub panel in a residential addition. using 310.16 - 60 degree column says #6 is good for 55amps, 75 degree column says 65 amps. If the sub panel and main panel breaker are both stamped rated at 75 degrees (terminals and lugs) then 6/3 romex (or NM cable) would be fine?

I guess i always thought 6/3 romex was good for 60amps regardless of terminal ratings?

[chris kennedy]

I guess i always thought 6/3 romex was good for 60amps regardless of terminal ratings?

334.80 Ampacity.
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating. The 90°C (194°F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor.What is the calculated load?

[iwire]

NM and now SE used in place of NM are limited to 60 C per 334.80

[infinity]

Accroding to 240.4(B), for a calculated load of 55 amps or less you can use a 60 amp OCPD for this feeder.

[CopperTone]

i didn't do a load calculation for this sub panel. it will have about 6 or 7 - 15 amp 120v branch circuits feeding lights and receptacles in bedrooms in a residential house on the 2nd floor.

[ohm]

i didn't do a load calculation for this sub panel. it will have about 6 or 7 - 15 amp 120v branch circuits feeding lights and receptacles in bedrooms in a residential house on the 2nd floor.

So put it on a 55A 2p breaker and you should be fine.

[chris kennedy]

55A 2p breaker

Where would I find one of these?

[infinity]

So put it on a 55A 2p breaker and you should be fine.


If a 55 amp CB were a standard size [240.6(A)]then you would be prohibited from using a 60 amp OCPD. Have you actually seen a 55 amp CB?

[Dennis Alwon]

.....................

240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers. The standard ampere ratings for fuses and inverse time circuit breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard ampere ratings for fuses shall be 1, 3, 6, 10, and 601. The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.

[CopperTone]

so, a 60amp breaker is ok then for this 6/3 romex sub panel installation?

[iwire]

Yes :smile:

[infinity]

so, a 60amp breaker is ok then for this 6/3 romex sub panel installation?

Start with this:

240.4 Protection of Conductors.
(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes


Your cable has an ampacity of 55 amps according to the 60 degree column of 310.16. 55 amps does not correspond with a standard sized OCPD as Dennis posted in 240.6(A). 240.4(B) allows you to use the next higher standard size OCPD (60 amp) when all of the conditions of 240.4(B) are met. The only problem in this scenario is if the connected load on the feeder is greater than 55 amps then you cannot use a 60 amp OCPD because of 240.4(B)(2).

[iwire]

The only problem in this scenario is if the connected load on the feeder is greater than 55 amps then you cannot use a 60 amp OCPD because of 240.4(B)(2).

I think it would be more accurate to say if the load is greater than 55 amps you can not use 6 AWG NM.

[chris kennedy]

I think it would be more accurate to say if the load is greater than 55 amps you can not use 6 AWG NM.

What would be the article that supports this. Not trying to be a wise guy here, just having a brain freeze.

[ohm]

Where would I find one of these?

I usually just file a corner off of a 60....what was I thinking?

[chris kennedy]

What would be the article that supports this. Not trying to be a wise guy here, just having a brain freeze.

Never mind, I got it.

[frizbeedog]

Never mind, I got it.

...what is 215.2(A)(1)?

[chris kennedy]

...what is 215.2(A)(1)?

Where were you when I needed help? You want to be my friend? Well friends are there when you need help.:grin:

This place is turning into mikesspace.com.:roll:

[frizbeedog]

Where were you when I needed help? You want to be my friend? Well friends are there when you need help.:grin:

This place is turning into mikesspace.com.:roll:

SA

....and it doesn't mean small allpiance.

:D

[infinity]

I think it would be more accurate to say if the load is greater than 55 amps you can not use 6 AWG NM.


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?

[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.

http://electrical-contractor.net/forum/iw/Dennis8.jpg

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

[Len]

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: