Is a 60A breaker ever OK on 6/3 romex?

[LTLFTcomposite]

I used to live by the old rule of thumb that 6 was for 50, 8 for 40 and so on, but have come to learn that it's more complicated depending on the C rating. Is romex (or NM) always 60C? Seems like that should be 55A max, is a 60A breaker "close enough"?

[wwhitney]

NM cable is limited to the base 60C ampacity (*). So #6 NM typically has an ampacity of 55A. Since 55A is not a standard breaker size, it is permissible to protect it with a 60A breaker, provided that the calculated load is 55A or less.

As an example, suppose you had an EVSE that can be set to a pilot signal of 44A. Then the required ampacity is 55A, and you could use 6/2 NM on a 60A breaker. But if the pilot signal is the more typical 48A, the required ampacity would be 60A, and you'd need to use either 4/2 NM or #6 conductors with a 75C wiring method (for a 65A ampacity).

Cheers, Wayne

(*) The actual conductors in NM cable are 90C conductors. For purposes of temperature or bundling "derating", that means you can start at the 90C ampacity when applying the correction factors. The allowable ampacity is then the lesser of the base 60C ampacity and the corrected 90C ampacity.

[LTLFTcomposite]

Wayne, thanks very much for the reply and as always complete answer.

[camasleaf]

NM-B is rated at 90C base load. When I looked at Home Depot pretty much all they had in store was NM-B. Unless is an older cable it, might actually be NM-B.

Still need to use 75C column since most breakers terminals are only rated 75C.

Either way 60A breaker will work, but with NM-B 48A EVSE would be OK.

[wwhitney]

On the contrary,

334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor. The 90°C (194°F) rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final derated ampacity does not exceed that of a 60°C (140°F) rated conductor.

Cheers, Wayne

[camasleaf]

I stand corrected. I was thinking about NMD90.

[LTLFTcomposite]

...provided that the calculated load is 55A or less.

One more question, this is feeding a small (four space) sub panel, is the "calculation" the sum of the amps of the breakers in the sub panel? As a practical matter none of these loads is continuous, eg a boat lift. Basically we have 6/3 romex feeding a string of LED Christmas lights on the dock

[wwhitney]

One more question, this is feeding a small (four space) sub panel, is the "calculation" the sum of the amps of the breakers in the sub panel?

No, the calculation is done per Article 220 of the NEC. As a practical matter, the sum of the breakers will be an overestimate of the load, often a very large overestimate. On the other hand, if the run is long, you will need to upsize the conductors to account for voltage drop.

As a practical matter none of these loads is continuous, eg a boat lift. Basically we have 6/3 romex feeding a string of LED Christmas lights on the dock

This 6/3 Romex is only for the indoor portion of the run? Because NM cable is only for use in dry locations, and any outdoor location (even inside conduit) is considered a wet location.

Cheers, Wayne

[wwhitney]

I stand corrected. I was thinking about NMD90.

That's a Canadian term, right? I think the Canadian Electrical Code is a bit more enlightened in this regard and doesn't restrict NM cable to the 60C ampacity, although I don't know the details.

Cheers, Wayne

[LTLFTcomposite]

Yes, the 6/3 is just to feed the sub panel on the side of the house where two GFCIs feed individual wires in carlon to the dock area, a 20A single pole for outlet and lights and double pole for the lift. The circuit feeding the sub panel was oversized for possible addition of a pool. The 120v circuit is #12 and the lift is #10 per manufacturer specs (I suppose on account of voltage drop concerns, it's nearly 100' to the lift).
One thing I puzzled over was the ground wire, it seemed like it should have been sufficient to run a single ground wire for both circuits to the dock, maybe #8, but it was cheaper to do a 10 and a 12 so each has it's own ground.