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Subject - 75 degree verses 60 degree
Electricman Heres something, can 8/3 nmb be fused at 50 amps if the recptacle and panelboard are rated for 75 degrees or must all non metallic cable go by the 60 degree rating for fuse size?
Ryan_J See 110.14(C), and pay close attention to 334.80
JimmyDee
quote:
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. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.
David Hyatt Without looking up Ryans code reference, I asked the same question before. I believe you can as long as both breaker and outlet is rated at 70 degrees. Don't have time to look it up right now. If I am wrong someone will tell me.
Ryan_J 334.80 :)
Electricman Hmmmmmmmmmm.... The reason I asked is because I have encountered a few Hvac installs that are using 8/2 nmb and fused at 50 amps.. I also have found 1 dwelling that used 10/2 fused at 40 amps
Ryan_J
quote:
Originally posted by Electricman

Hmmmmmmmmmm.... The reason I asked is because I have encountered a few Hvac installs that are using 8/2 nmb and fused at 50 amps.. I also have found 1 dwelling that used 10/2 fused at 40 amps


That might be fine. Remember that overload and overcurrent protection are two different things. More on this when I have time. :)
SteveMc Check the HVAC unit nameplate. You will find that it has a maximum OC rating and a minimum ampacity rating. You can, I have been told breaker for the maximum and wire for the minimum. Therefore, you could have a unit with #10 NM on a 35 amp breaker. In fact, it is quite common on AC units. I hope Ryan will explain more on this when he can, but as he wrote, it has to do with overload vs overcurrent protection.
Ryan_J
quote:
Originally posted by SteveMc
I hope Ryan will explain more on this when he can, but as he wrote, it has to do with overload vs overcurrent protection.


Your wish is my command!!! Go here:

http://www.electricalknowledge.com/forum/topic.asp?TOPIC_ID=609
SteveMc Well stated Ryan.
Ryan_J Hey, thanks Steve!