Wire Size

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Alwayslearningelec

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I have a main panel with breaker(size unknown yet) feeding a sub panel with 60A main breaker.
I am told the 60A MCB panel will only have loads/draw that will not exceed 40A.
So to recap..If my sub panle has 60A main breaker but will only draw 40a:

1. What size wire would I need for the 60A sub panel main breaker? 60A or 40A?
2. Then I size my breaker in the main panel once I figure out the wire size? Yes Im 99% of this..lol
 
horsegoer said:
I have a main panel with breaker(size unknown yet) feeding a sub panel with 60A main breaker.
I am told the 60A MCB panel will only have loads/draw that will not exceed 40A.
So to recap..If my sub panle has 60A main breaker but will only draw 40a:

1. What size wire would I need for the 60A sub panel main breaker? 60A or 40A?
2. Then I size my breaker in the main panel once I figure out the wire size? Yes Im 99% of this..lol
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forget the panel capacity for a second. assuming the 60A panel is fed from a circuit breaker, the wire feeding a load must be sized per the load and the circuit breaker must protect the wire.

if pass through you need to look at the tap rules. if breaker-its your choice to use 40 A or 60A of wire to feed the panel. just make sure the breaker protects both the wire and panel.

if it were me, i would give the 60A panel 60A of wire, not 40.

edit: to be clear, you do a load calculation to determine minimum ampacity then size wire, then size breaker to protect the wire.
 
You have 3 basic choices:
The preferable one (no risk) would be to feed it with a 60 amp breaker in your main panel using a conductor with a 60 amp ampacity,
or:
Feed it with a 40 amp breaker from your main panel using a 40 amp conductor (the subpanel main would be a disconnect)
or
any size breaker and feeder in the main with the subpanel supp[lied by a 60 amp conductor using the tap rules.
 
For a 40 amp feeder you need #8 AWG conductors, for a 60 amp feeder you need #6 AWG conductors. The cost is negligible between the two so go with #6.
 
I agree with infinity and zbang: The cost is negligible between the two so go with #6.
and, yes on a 60 amp breaker.

In almost all situations you would want to do that but I was pointing out some basics.
The load is 40 amps (assuming non continuous) so you would need a #8 conductor MINIMUM
If you used a #8 conductor you would need to protect it with a 40 amp breaker at it's source
The 60 amp breaker in the panel would simply be serving as a disconnect.
 
horsegoer said:
But than you also need a 60a breaker.
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Correct but you've already stated that one of the three panels will require a 60 amp feeder so don't over compliacte it by running a 40 amp feeder to the other trailer. Just run a 60 amp feeder to both. IMO it's just easier if you keep it simple by using the same material for both trailers.
 
infinity said:
For a 40 amp feeder you need #8 AWG conductors, for a 60 amp feeder you need #6 AWG conductors. The cost is negligible between the two so go with #6.
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In Central Florida most Building Departments enforce 110-14(C)(1) of the 2017 edition of the NEC where you have to use the 60 degree column of the 310-15(B)(16) ampacity table (for conductors #14 through #1 AWG), so the 60A feeder would have to be #4 AWG conductors. Reedy Creek (Disney) does not enforce it, but most of the other jurisdictions do.
 
Jerry Solar said:
In Central Florida most Building Departments enforce 110-14(C)(1) of the 2017 edition of the NEC where you have to use the 60 degree column of the 310-15(B)(16) ampacity table (for conductors #14 through #1 AWG), so the 60A feeder would have to be #4 AWG conductors. Reedy Creek (Disney) does not enforce it, but most of the other jurisdictions do.
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If you look at 110.14(C)(1)(3) carefully you will see a very common situation where the 75° amaocity can be used IF the equipment at termination is rated at least 75° which is often the case especially in circuits above 30 amp.
 
Jerry Solar said:
In Central Florida most Building Departments enforce 110-14(C)(1) of the 2017 edition of the NEC where you have to use the 60 degree column of the 310-15(B)(16) ampacity table (for conductors #14 through #1 AWG), so the 60A feeder would have to be #4 AWG conductors. Reedy Creek (Disney) does not enforce it, but most of the other jurisdictions do.
Click to expand...
A few things, as Augie stated terminals are almost always rated for 75° C so they cannot force you use the 60° C ampacity. Secondly at 60° C #6 AWG has an ampacity of 55 amps so in many cases the next size up rule would allow a 60 amp OCPD.
 
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