70 or 80 amp breaker subpanel

3842 Views 3 Replies 3 Participants Last post by Arrow3030, Feb 19, 2016

gt1935

Discussion Starter · Feb 18, 2016

Cutler HammerJM30 200 amp main panel. Using 2-2-2-4ser alum wire to feed a new Siemens subpanel. 200 amp panel is around 35 years old. NEC 310.15b16 shows this alum wire rated for 75amps @60c or [email protected] How does one determine what column to use 60 or 75? Is it based on the main panel rating, breaker rating etc...? Would be using an 80 amp breaker in the main and the Siemens comes with a 100amp for the subpanel.

Also, paraphrasing NEC 240.4b the 75 amp rating could be upsized to the 80amp breaker under certain conditions. The length of cable run is around 10feet and not in conduit or insulation. What other conditions need to be met to use the 80 amp breaker? Thanks for your help.

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mm11

· #2 · Feb 18, 2016

The temperature rating is based on the lowest allowed in the circuit, which typically is the termination point (breaker, lug, etc). If the breaker is rated for 75C, use that column for selecting ampacity; if not, use the 60C column.

I see no issues with using the 80A breaker.

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gt1935

Discussion Starter · #3 · Feb 19, 2016

So the termination point would be the 80 amp breaker not the Cutler Hammer main panel. The 80 amp breaker is marked showing the 75C rating. Thanks for you help.

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Arrow3030

· #4 · Feb 19, 2016

The lowest temp rating in any circuit 100A or less is usually 75C. However, unless you know without a doubt that every component of that circuit is rated at 75C, you're better off using the 60C column. Load centers, breakers, wire nuts, split bolts, polaris connectors, wiring devices, wiring methods and more all have a listed temperature rating. Most are 75C or higher.
Anything over 100A is 75C.

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