I'm running a new 60amp sub panel and have been debating what wire to
feed the sub panel with. I have 6/3 NM-B (75 deg) right now, NEC says
this is good to 75 amps or so, but i must use the 60 degree rating of
55 amps. If both my double pole breaker and my lugs are rated for 75
degrees can i use this wire or should i just go to a 50 amp Sub.
You won't get unanimous agreement on this but, since 55 Amp is not
a "standard" rating or setting, you are permitted to increase the
breaker size up to the next standard rating (which happens to be
60 Amps). Art. 240-3 Excep #4
NEC article on NM cable explicitly limits it use to the 60C column for ampacity.
You can use 90C for
ambient temperature derating, but not ampacity. If your calculated load on the
panel is 55A or less,
then you can round up to the standard breaker size of 60A.
That makes sense. My sub is only using 2 circits for receptacles.
Power tools and tread mill nothing industrial (no 5 saws at the same
time, just basic home owner stuff)the reason I went with 60 amps is
for future use for a bathroom and jacuzzi tub(1 1\2 hp at the most),
all of which will never run 60 amps at one time. Since the breaker is
there to protect the wire even if i did get to 60 amps the wire should
still be ok since its rated at 75 amps (even thogh NEC states to use
55 amp). Ill just use the 60 amp breaker. Scince i did not know the
reasoning for the use of 60 degree rating i was just concerned about
using 60 amps even though its 5 extra amps i did not want the hose to
In US practice cable, as opposed to cord, is named by the number of
current carrying insulated conductors. The presence of some form of
Equipment Grounding (bonding) Conductor (EGC) is assumed. 6/3 would
mean three insulated conductors plus an EGC that is sized for the
maximum permissible ampacity of the current carrying conductors. The
reason for this convention is that the nature of the EGC varies by cable
type. In type NM it would be an uninsulated wire. In type AC it would
be the jacket of the cable. In type MC it would be an insulated wire
and so forth. 6/4 NM would have four insulated conductors and an
uninsulated EGC. In the OPs application 6/3 w G would be the correct
designation for the cable used to supply a feeder supplied, single
phase, 120/240 volt, panel that is located in the same structure as the
Over Current Protective Device (OCPD) that protects the feeder supplying
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