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Any electricians out there? Need advice.


HauteHippie

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I'm wiring up a subpanel in my garage and have a question...

The subpanel is being fed through a 60A breaker at the main. The main is grounded, and I've got about 50' of 4-3 +gnd Romex coming into the sub (rated for 65A according to the home store). Now in the subpanel, there will be a 30A, 2x20A, and 15A breakers (85A total). These appliances should never be on simultaneously, though. The question I have, however, is this: Is it safe to only have the 60A breaker back at the main (in the case where 85A was accidentally demanded at the sub), or does the subpanel need to have its own 60A main breaker, or does it not matter?

Thanks.

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It would be best to have a 60A breaker at the sub-panel. A little prior planning and prevention for the anything can happen category :)

But wouldn't anything greater than 60A be squelched by the breaker back at the main anyhow? The 65A-rated Romex should never see any more than 60A, right?

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I'm not an electician but I play one on TV.

I would do a load test on all the panel. Just because you have a 30 amp breaker doesn't mean the load would be that high (not counting shorts). A stove may require a 30 amp breaker but only be able to pull 22 amps. As long as the total possible load on the panel is less than 60 amps you are fine.

I put in a 200 amp service and if I was really letting everything scream I might be able to get 120 amps flowing. Basically, just because I have 200 amps available doesn't mean I could pull it.

With that said, the more breakers the better.

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Chieftang

It would be safe..the 60 in the main panel will trip if the load in the subpanel exceeds

60amps...it's not kosher but it will work...the rule of thumb is

the main breaker should never be exposed to a load which exceeds 85% of its capability

So your subpanel should be limited to a 20 amp and a 30 amp.. anything more is verbotten

but it will work..:)

Remember breakers are there to prevent the wire from overheating..:)

Bill

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Yes, the 60 at the main will protect the line to the sub panel. As mentioned above it really shouldn't be subject to a full load but as long as it is working you should be fine. Although throwing a main breaker in the middle of party might be a real drag. Just don't turn the guitar amps up to 11.

Now the question of code....that might be something different. I had to put a 0000 line from my main to the subpanel. Sometimes the codes make you go way over what you would think. I also had to replace most of my breakers with GF and AF breakers (which is expensive but does add quite a bit of safety).

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Chieftang

It would be safe..the 60 in the main panel will trip if the load in the subpanel exceeds

60amps...it's not kosher but it will work...the rule of thumb is

the main breaker should never be exposed to a load which exceeds 85% of its capability

So your subpanel should be limited to a 20 amp and a 30 amp.. anything more is verbotten

but it will work..:)

Remember breakers are there to prevent the wire from overheating..:)

Bill

Thanks Bill. I guess it's like any other string of outlets in the house that are strung off a 15A breaker. There is no limit on how many outlets you have on one breaker, so you have a 15A breaker and 15A wire, and if/when the demand on the outlets exceeds 15A the breaker trips.... Same scenario with the sub-panel, really.

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Yes, the 60 at the main will protect the line to the sub panel. As mentioned above it really shouldn't be subject to a full load but as long as it is working you should be fine. Although throwing a main breaker in the middle of party might be a real drag. Just don't turn the amps up to 11.

Now the question of code....that might be something different. I had to put a 0000 line from my main to the subpanel. Sometimes the codes make you go way over what you would think. I also had to replace most of my breakers with GF and AF breakers (which is expensive but does add quite a bit of safety).

GF breakers are a darn good idea. I should do that too..... BTW, was that 0000 copper? 50' of 4/3 copper just cost me a small fortune....

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The more I read on the AF breakers the more I liked them. It's amazing how much power you can flow through a 20-30 amp breaker without it throwing. You could weld with the wire as long as you don't pull the amps. Crazy! Plus even if they do trip it will take something like 200-500 cycles. The AF breakers trip in 5-8 cycles. They are now required in all bedrooms but I think I'm going to replace even more than is required.

The 0000 is aluminum. I couldn't even find copper that big, although I think I could go one size smaller with copper. I was told it could be special ordered at a very, very high cost and a long wait. The aluminum stuff cost me a few hundred bucks as it was. The stuff is laughably large and a [censored] to pull.

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I'm trying to figure out the add on ground bus bar. It's over my head. As far as I know all sub panels should have a ground bar and a neutral bar. I've heard of people using service boxes with only a neutral and having to add a ground bar but that has got be be a unusual.

That's why you never trust advice from the forum. I'll burn your house down. f :p

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Chief

What will the load be? IE what's on the 30? whats on the 2 20's and whats on the 15?

Motor loads ( compressors, blowers motors, pumps etc) have an inrush current of 6 times the running amp load

it only happens for a short period of time but it is there and can trip the main if 2 many motors attempt to start at the same time...I am a retired A/C contractor ( 35 years) I ran the first house I bought in 1978 with a 60 amp service for YEARS

I installed central A/C, an electric clothes dryer, a 1.5 HP sprinkler pump, 60 gallon H2O heater and all the rest of the common stuff...worked fine as long as the A/C, the sprinkler pump and the dryer did not all run at the same time..

IF they did it tripped the main...:) I finally upgraded to a 200 amp service but only because I was selling the house

and the real estate agents freaked out when they saw my 1956 60 map fused main...hehehe

Bill

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Chief

What will the load be? IE what's on the 30? whats on the 2 20's and whats on the 15?

Motor loads ( compressors, blowers motors, pumps etc) have an inrush current of 6 times the running amp load

it only happens for a short period of time but it is there and can trip the main if 2 many motors attempt to start at the same time...I am a retired A/C contractor ( 35 years) I ran the first house I bought in 1978 with a 60 amp service for YEARS

I installed central A/C, an electric clothes dryer, a 1.5 HP sprinkler pump, 60 gallon H2O heater and all the rest of the common stuff...worked fine as long as the A/C, the sprinkler pump and the dryer did not all run at the same time..

IF they did it tripped the main...:) I finally upgraded to a 200 amp service but only because I was selling the house

and the real estate agents freaked out when they saw my 1956 60 map fused main...hehehe

Bill

Wow, 60A service....

There's a dryer on the 30A. There's a 220V table saw on the 20A, and a 220V jointer on the 15A. And the other 20A is going to be miscellaneous 110V stuff... lights, etc. Although I do have my eye on a 2 HP dust collector that is speced at 20A, 110V. 220V would really be better for that but I don't know if that motor can be rewired. Theoretically I'll never be drying clothes while running the tools, so the 30A dryer (which is probably not sucking 30A) is mostly out of the picture... And I definitely can't use the table saw and jointer simultaneously. :) I'm just being paranoid is all.

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Wait, I might have figured it out. I think some main panels have the neutral and ground together (or bonded). An add on ground bus bar would be needed to allow the GF to work properly. (I think) :D

Figured it out... The guy clarified actually. He said normally, subpanels do not come with separate neutral and ground bus bars. There are usually two bars in the panel, but they are bonded to each other. You definitely don't want neutral bonded to ground in the sub-panel so you could either unbond them or buy a ground bus bar. But in my panel this wasn't the case... Ground and neutral aren't bonded.

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Yep, I don't think they have combined ground and neutral anymore (unless it is very old or I don't know what I'm talking about). The neutral is a current carrying wire so if you used a single bus you could possibly have current moving up the ground to the appliance, saw or whatever. It is a shock hazard because the outer metal frames of the equipment could have potential difference from a different appliance on a different circuit (or the cement floor in the garage) and it would flow voltage.

Zappola' :black_eye:

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Yep, I don't think they have combined ground and neutral anymore (unless it is very old or I don't know what I'm talking about). The neutral is a current carrying wire so if you used a single bus you could possibly have current moving up the ground to the appliance, saw or whatever. It is a shock hazard because the outer metal frames of the equipment could have potential difference from a different appliance on a different circuit (or the cement floor in the garage) and it would flow voltage.

Zappola' :black_eye:

I'm going to have to figure out how the neutral gets energized since it's connected to ground in the main panel.... Hmm.

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I think the main panel bonding is actually what is required for the breakers but not for the sub panel.

The neutral is a current-carrying conductor, whereas the ground wire is not. As such, the neutral wires are insulated, the same as any other current-carrying conductor. The misunderstanding comes from the fact that the neutral wire is intentionally grounded at the main service panel only. The reason for this action is to allow ground-faults (or "short circuits" to ground) a return path back to the source of power, the utility transformer. Having the ground wires bonded to the neutral makes it possible for the circuit breaker to trip out easily under such conditions.

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Chiefang

An efficient electric clothes dryer pulls about 24 amps or less on the highest setting...

A 220 volt table saw will probably never pull more than 13 amps unless you are attempting to cut a wet PT 4x4 with a dull blade..:)

The jointer will probably never see 10 amps

that's a total of 47 amps

with plenty left over for lights etc...you will be fine

Just as you don't show off your rep watch to an AD

Don't show your electrical panel to the local electrical inspector...:)

Bill

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I think the main panel bonding is actually what is required for the breakers but not for the sub panel.

The neutral is a current-carrying conductor, whereas the ground wire is not. As such, the neutral wires are insulated, the same as any other current-carrying conductor. The misunderstanding comes from the fact that the neutral wire is intentionally grounded at the main service panel only. The reason for this action is to allow ground-faults (or "short circuits" to ground) a return path back to the source of power, the utility transformer. Having the ground wires bonded to the neutral makes it possible for the circuit breaker to trip out easily under such conditions.

Ah yes, of course! Duh. Thanks.

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I'm an electrical PE with 25+ years experience

this is fine...

it would be nice to have a 60A CB in the sub-panel, but it's not required by the NEC...

what is on each CB in the sub?

add the actual loads up, see what it is, my guess, much less than 60A

or do this, get an clamp in amp meter and turn everything on and measure...

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Thanks Bill. I guess it's like any other string of outlets in the house that are strung off a 15A breaker. There is no limit on how many outlets you have on one breaker, so you have a 15A breaker and 15A wire, and if/when the demand on the outlets exceeds 15A the breaker trips.... Same scenario with the sub-panel, really.

actually there is a limit...

80% of the CB rating

so a 20A circuit can be sized to carry 16A

a receptacle is usually calculated at 180VA load

no. of recept allowed = (120V x 16A)/180VA = 10.7, round down to 10...

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