Can I use a 90 ampere breaker to protect a circuit rated at 85 amperes?

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Johnny123

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I'm installing a subpanel. The terminals in the service panel supplying the panel are rated 75C, the terminals in the panel are rated 75C, and I'll be using THHN copper conductors which are rated at 90C. Based on Table 310.15(B)(16) (NEC 2011), I'm planning to have a 90A breaker in the service panel, a 90A breaker in the sub, and I'll use 3 AWG THHN conductors between the panel.

A coworker pointed out that according to 240.4(B), I can use 4 AWG THHN instead of #3.

Can I really use #4 and a 90A breaker in this situation?
 
Provided your load does not exceed the coductor's ampacity of 85 amps, you could use the "
next size up" or 90 amp breaker.
I have found that a vast majority of the sub-panels I see are supplied by full size conductors as most folks want to cover for the possibility of added loads.
 
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You see use of the next size up rule quite often when talking higher capacity installations - but of course not over 800 amps where that rule no longer is allowed.

How often do you see 500 copper for a conductor that is protected at 400 amps? 500 copper is only good for 380 amps, or 4/0 aluminum protected at 200 amps, 4/0 is only good for 180 amps. Or two sets of 500 copper for an 800 amp circuit?

You do sometimes see people that forget this rule and try to apply three sets of 500 for a 1200 amp circuit - but that is not allowed as the next size up rule doesn't apply when you are over 800 amps.
 
I have used the rule often with a 60 amp sub panel by running 6/3NM which at 60C is only 55 amps. Generally my loads are pretty low so it has never been an issue. Often the panels are in spots where adding to it would not be practical without removing drywall.
 
Dennis Alwon said:
I have used the rule often with a 60 amp sub panel by running 6/3NM which at 60C is only 55 amps. Generally my loads are pretty low so it has never been an issue. Often the panels are in spots where adding to it would not be practical without removing drywall.
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But be careful, another thing that is easy to overlook is continuous loads, happens with electric heating quite often when supplied with NM cable. If your continuous load is 44 amps or more the minimum conductor ampacity must be 125% which puts the minimum conductor ampacity over 55. Many install such applications with 6 AWG NM cable and a 60 amp breaker without giving much thought to 55 amp limit for 60 deg conductors. The 60 amp breaker is still the correct breaker but the conductor needs to be either larger or switched to one that can be used at 75C.
 
kwired said:
But be careful, another thing that is easy to overlook is continuous loads, happens with electric heating quite often when supplied with NM cable. If your continuous load is 44 amps or more the minimum conductor ampacity must be 125% which puts the minimum conductor ampacity over 55. Many install such applications with 6 AWG NM cable and a 60 amp breaker without giving much thought to 55 amp limit for 60 deg conductors. The 60 amp breaker is still the correct breaker but the conductor needs to be either larger or switched to one that can be used at 75C.
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Of course there are scenarios to be careful about but that is true everywhere. I do this in a residence and I would not put heating loads or anything large on it. BTW the feeder to the panel is not required to be rated 125%, I don't believe so anyway.
 
Dennis Alwon said:
Of course there are scenarios to be careful about but that is true everywhere. I do this in a residence and I would not put heating loads or anything large on it. BTW the feeder to the panel is not required to be rated 125%, I don't believe so anyway.
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I guess it depends on exactly what you mean by 125%, feeder minimum ampacity is supposed to be 125% of continuous load plus (100% of) non-continuous load. 215.2(A)(1)
 
Johnny123 said:
...I'm planning to have a 90A breaker in the service panel, a 90A breaker in the sub...
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The 90A main breaker in the subpanel is not required. There are instances where having both a supply and load breaker for a feeder may be a good design practice. In most cases it adds nothing but extra cost.
 
Smart $ said:
The 90A main breaker in the subpanel is not required. There are instances where having both a supply and load breaker for a feeder may be a good design practice. In most cases it adds nothing but extra cost.
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Smart is correct but if this panel is in a separate structure the 90 amp breaker with hold down kit may be the cheapest way out.
 
90 amp breaker on the supply end and in most cases if a main is needed or desired at the sub panel just use a readily available and likely less cost 100 amp main breaker panel. Nothing wrong with doing it that way, if you load it too much the feeder breaker trips, the conductors are still protected.
 
I agree with Kwired, done it that way several times. The size of OCPD on the load side of the feeder is irrelevant if the OCPD at the line side protects the conductor.

Besides, it's a good way to convince the homeowner you installed "100A" to their shed fed with 6 AWG. :angel:
 
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