#6 nmb at 50 amp ? or 60?

Status
Not open for further replies.
ritelec

ritelec

Senior Member
Location
Jersey
Wandering..
Why is #6 nmb rated for 50 amps other than the code requirement saying it has to be rated in the 60 degree if type nmb?
I mean it's thhn in romex now right?

Also, why won't the next higher breaker rule work for 55 amps to 60 ? Or does it, and #6 nmb CAN be installed on a 60 amp breaker?

thank you
 
All NM cable has to used at the 60° C ampacity which is 55 amps for #6. The next size up rule would allow if to be protected at 60 amps with a 55 amp or less load.
 
we argued that one in another thread but basically not all manufacturers are using thhn, it is not individually marked internally, and the packaging of it in the cable along with the standard installation methods and failures during arc testing caused but staple crush problems and other installation errors mean it is being kept at the 60 degree rating for nmb cables so far... I think there was a proposal in the 2020 codes but have not heard how far it has gotten yet...
 
ritelec said:
Than you

Any thoughts why it's not rated as thhn ?
Click to expand...

I don't know, it's a 90° C rated conductor but we still limit it to 60° C doesn't make much sense to me. I guess we could have the same argument about #12 MC cable with 75° C terminations why limit it to the 20 amps or the 60° C ampacity.
 
My personal opinion, is that nm is rated at 60C because no one de-rates the cable when run in attics. There are probably other issues where homeowners tend to over due things with circuits so they rate it at a safe temp. to cover those issues.
 
Also the conductors inside NM have more total thermal insulation from the surroundings than the same conductors run in raceway, even when not bundled or run in insulated walls.

Sent from my XT1585 using Tapatalk
 
Dennis Alwon said:
My personal opinion, is that nm is rated at 60C because no one de-rates the cable when run in attics. There are probably other issues where homeowners tend to over due things with circuits so they rate it at a safe temp. to cover those issues.
Click to expand...

and in insulated walls ? edit ( just saw posted) .... good thought...

I can never get with panels installed in attic space ?
 
ritelec said:
and in insulated walls ? edit ( just saw posted) .... good thought...

I can never get with panels installed in attic space ?
Click to expand...

You see a lot of panels in attics?

Rare IME, unless old existing. Rarely meets headroom requirements.

Somewhere in the NEC it addresses this.

Edit: Found it.

110.26(A)(3)Exception No. 1: In existing dwelling units, service equip- ment or panelboards that do not exceed 200 amperes shall be permitted in spaces where the height of the working space is less than 2.0 m (61⁄2 ft).
 
GoldDigger said:
Also the conductors inside NM have more total thermal insulation from the surroundings than the same conductors run in raceway, even when not bundled or run in insulated walls.

Sent from my XT1585 using Tapatalk
Click to expand...

which was one of the previous arguments.. that though it is still just a two wire or three wire plus ground in a plastic, like thhn in a conduit... the surrounding plastic does not give it space for the heat to go... thus it overheats quicker...

So it must be kept to a lower heat number for calculations.

I think this was in one of those twenty page arguments back and forth over calculations or stripping wire and reusing it..lol... and I am also paraphrasing as how I best understood it.
 
ritelec said:
Than you

Any thoughts why it's not rated as thhn ?
Click to expand...

Because it's a cable assembly and not individual conductors.

Probably saves on Ink and manufacturing costs to not stamp out all the information on each wire.

It also serves to sell a little bit of extra wire seeing that it's a code violation (310.11A) to use a piece of stripped out Romex to make up pigtails or extend wires in a panel or jbox. 100% properly done would be to go grab a piece of THHN off a spool. I'll be the first to admit that 90% of the time, I treat individual conductors of Romex as THHN for purposes of splicing... I wouldn't strip out 100 foot of it and put it in EMT though.
 
ritelec said:
MC and AP are cable assemblies ?
Click to expand...

Metal clad or MC cable is definitely a cable assembly. Not sure what AP is, armor-clad or AC is also a cable assembly. Article 330.1 defines MC as a cable, 320.1 for AC.

Eta: if you are talking about metal clad all-purpose cable, or MCAP, Southwire lists the conductors as THHN however if they are not marked per the above cited code section, it would still be a code violation to use stripped out pieces as pigtails.
 
JFletcher said:
. . . if they are not marked per the above cited code section, it would still be a code violation to use stripped out pieces as pigtails.
Click to expand...
I still do it, and will continue to do so. If 2' of NM conductors can exist in a panel, so can 6" more.
 
infinity said:
All NM cable has to used at the 60° C ampacity which is 55 amps for #6. The next size up rule would allow if to be protected at 60 amps with a 55 amp or less load.
Click to expand...
I agree.

A common place one must watch out is with electric furnaces. Most ~10kW circuits(with blower) are actually 9.6 kW and 60C conductor is fine. If you have actual 10 kW and a large enough blower involved you could end up with minimum conductor ampacity after increasing to 125% for continuous load to get above 55 amps. Then you are too small if it is a 60C conductor but a 75C conductor can still be 6 AWG.

mbrooke said:
I think its time the code make an allowance for 60amps.
Click to expand...
The issue isn't whether 6 AWG NM cable should be 60 amps, it is the fact it must be treated as 60C conductor (which has an ampacity of 55) because it is part of an NM cable.

So the question is why does NEC require us to use NM cable at 60C even though all NM cable has had 90C conductor insulation for many years now?
 
Another place #6 NM is a real problem is with EV car chargers. Tesla's car charger has a setting of "Maximum Current Output of 48A and Circuit Breaker 60A". This is a situation where #6 romex is no good, but #6 MC is fine.
 
Status
Not open for further replies.
Top