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

Fri Jan 18, 2019 1:06 pm

wwhitney wrote: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
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Re: Is a 60A breaker ever OK on 6/3 romex?

Fri Jan 18, 2019 2:03 pm

specialgreen wrote: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
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Re: Is a 60A breaker ever OK on 6/3 romex?

Sat Jan 19, 2019 2:54 am

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.
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LTLFTcomposite
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Re: Is a 60A breaker ever OK on 6/3 romex?

Sat Jan 19, 2019 3:55 pm

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.
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LeftieBiker
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Re: Is a 60A breaker ever OK on 6/3 romex?

Sat Jan 19, 2019 4:09 pm

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.
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wwhitney
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Re: Is a 60A breaker ever OK on 6/3 romex?

Sat Jan 19, 2019 9:34 pm

LTLFTcomposite wrote: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
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Re: Is a 60A breaker ever OK on 6/3 romex?

Sat Jan 19, 2019 11:08 pm

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.
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specialgreen
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Re: Is a 60A breaker ever OK on 6/3 romex?

Sun Jan 20, 2019 1:46 pm

LTLFTcomposite wrote: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.

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

Sun Jan 20, 2019 6:42 pm

specialgreen wrote: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.
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LTLFTcomposite
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Re: Is a 60A breaker ever OK on 6/3 romex?

Mon Jan 21, 2019 4:18 am

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.
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