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 :-D

 

[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 :-D

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.

 

[specialgreen]

protect it with a 60A breaker, provided that the calculated load is 55A or less.

For a hardwired EVSE, wouldn't the "continuous load" and the "calculated load" be the same thing? That is, the 60A breaker supports maximum 48A of "continuous load," so you wouldn't use an EVSE which could pull more than 48A. So its Calculated Load would also be 48A or less.

Calculated load and continuous load could definitely be different if the breaker served a receptacle which may have intermittent loads above 80% of breaker rating (e.g. arc welder).

 

[wwhitney]

For a hardwired EVSE, wouldn't the "continuous load" and the "calculated load" be the same thing?

(2011) NEC 210.19(A)(1) says in part

Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

So the "calculated load" for conductor sizing is 125% times the "continuous load".

Cheers, Wayne

 

[Oilpan4]

It's fine if it's on a dedicated welder circuit.
My little plasma cutter is on a 60 amp breaker with NM-B 6/2 with ground.
But welding equipment is covered under nec table 630.11a I believe.

 

[LTLFTcomposite]

Related question, I've seen people warn of dire consequences of having too small a breaker on a continuous load (eg 30A instead of 40A on a 32A evse) that the breaker can overheat and start a fire. That sounds wrong to me, I'd expect the consequence to be nuisance trips, but don't pretend to know.

 

[LeftieBiker]

Nuisance trips would be the normal result. The problem is that breakers sometimes stick on, and in that case you want a heavier breaker there, not an undersized one.

 

[wwhitney]

Related question, I've seen people warn of dire consequences of having too small a breaker on a continuous load (eg 30A instead of 40A on a 32A evse) that the breaker can overheat and start a fire. That sounds wrong to me, I'd expect the consequence to be nuisance trips, but don't pretend to know.

Nuisance trips are the primary problem. If it happens repeatedly, and you keep resetting the breaker, it can wear out the breaker, which could leave the circuit underprotected should another problem arise.

Cheers, Wayne

 

[Oilpan4]

Modern breakers are magnetic and thermally protected.
If the surrounding ambient temperature is warm enough then if you run 30 amps or nearly 30 amps through a 30 amp breaker for 2 or 3 hours it may trip.

The old thermal only ones could weld closed if hit with a dead short.

Zinsco breakers were known for doing this.

 

[specialgreen]

Related question, I've seen people warn of dire consequences of having too small a breaker .

I made an offer on a house once that had an undersized breaker serving the AC compressor. The city inspector noted it, but said that there were no prohibitions against voluntarily using a smaller breaker.

When I put in solar, the electrician was planning to undersize the main utility breaker on our house's panel from 100A to 80A, in order to ensure that the sum of the utility + solar feed-in breakers was below the panel's busbar maximum. He ended-up getting approval from the inspector to leave the main breaker at 100A, because my solar array faces due east and will produce about 17% below nameplate (barring some unexpected change to the earth's orbit or rotation). I thought that was lucky: usually trivial things like physics have no impact on code interpretation.

 

[Oilpan4]

When I put in solar, the electrician was planning to undersize the main utility breaker on our house's panel from 100A to 80A, in order to ensure that the sum of the utility + solar feed-in breakers was below the panel's busbar maximum. He ended-up getting approval from the inspector to leave the main breaker at 100A, because my solar array faces due east and will produce about 17% below nameplate (barring some unexpected change to the earth's orbit or rotation). I thought that was lucky: usually trivial things like physics have no impact on code interpretation.

You don't have to do that any more as of 2017 NEC. Unless there is some outdated local code that still requires it. You can back feed up to 20% of the panels rated amps with out changing the main to a lower breaker.
So if you have a 200 amp panel like I do you can back feed up to 40 amps continuously.
My electric coop doesn't want a residential solar install back feeding more than 10kw so that works out pretty good.

 

[LTLFTcomposite]

Are you sure the breaker was too small for the AC compressor? Afaik AC compressors have different rules and bigger isn't always better. We have a family member at a house where the AC compressor is on a 15A breaker, that looked wrong but turns out that is what's specified for that unit and anything bigger is a no-no.