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]

unfinished business

unfinished business

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.

 

[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?