Can I install a larger breaker than the feed breaker in a subpanel?

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Johnny123

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New York
Let's say I have a device that calls for 30 ampere overcurrent protection. In the main panel I install a double pole 30 AMP breaker, and use it to feed a subpanel near the device. The device also requires GFCI protection, but all I have is a 50 AMP double pole GFCI breaker. Can I install the 50 AMP GFCI breaker in the sub panel, and feed the device from it? If so, how do I size the conductors from the 50 AMP breaker to the device? Should the conductors be sized at 30 or 50 amps?
 

GoldDigger

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Even though there is a lower current protection at an upstream breaker, I believe that the NEC does not give you any leeway to oversize the actual branch circuit breaker above the carrying capacity of the branch circuit wiring. (With the limited exception of the "next size up" rule where applicable. If wires require 30A protection, then the branch breaker must be 30A (unless it is a tap or a motor circuit, of course.)
You mentioned overcurrent protection by name, but the actual functions required are three: overload, short circuit, and ground fault. In a motor circuit the overload protection can be at or associated with the motor.
If the case of a tap conductor, all three of the protections can be at the far end of the tap.
 

ActionDave

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Interesting question....... your conductors are protected at their ampacity by the the 30A breaker and only getting GFCI protection from the 50A breaker.

There is no direct code rule that says you can't do this that I can think of, but there is no reason to do it that I can think of either.
 

templdl

Senior Member
Location
Wisconsin
Let's say I have a device that calls for 30 ampere overcurrent protection. In the main panel I install a double pole 30 AMP breaker, and use it to feed a subpanel near the device. The device also requires GFCI protection, but all I have is a 50 AMP double pole GFCI breaker. Can I install the 50 AMP GFCI breaker in the sub panel, and feed the device from it? If so, how do I size the conductors from the 50 AMP breaker to the device? Should the conductors be sized at 30 or 50 amps?
Think about it, you have a 30at breaker as a feeder as I understand. Then you only have a 50at w/ GF as a down stream over current protective device. As such would not any thing that is dowen streasn from the 30at breaker be protected at 30a?
Shouilkd you us that 50at breaker dfowen stream and use cable that is sized for 50a you still are protected by that 30at beaker. As such it is of my opinion that there would be no code violation as everything down stream is protected at 30a. The design may not been practiact or would it make any sense.
Let's say you were to feed a 100at breaker to protect a 100a cable you would still be limited by that 30at breaker feeding it.
The issue may be confusing an unqualified person who may not be able to reason out the system masking indirect assumptions. The same thing is possible if you had a 20at breaker which normally would protect #12 rated wire but because of various deratings you may upsize to #10 wire. The you have an unqualified person see the #10 wire and concide that the breaker is undersized and replace the 20at breaker with a 30at while not having a clue about the #10 wire was used in the first place.
Bottom line feeding a 50at breaker with a 30at breaker would be allowed as long as the cable is protected. Since the cable which is being feed buy the 50at breaker is being protected by the upstream 30at breaker where would there be an issue?
 

jaggedben

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240.21

Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive there supply except as specified in 240.21(A) through (H).

Emphasis mine. (A) thru (H) provides nothing addressing this.

A short or overload on the branch circuit can trip the 30A sub breaker before the 50A branch circuit breaker, and shut down all loads on the sub even if only the one with the oversized breaker has a problem. That's great to lose all your lights and outlets in the kitchen because there was a problem that was just with your range. It could be worse than just inconvenient.

Also, it may be unusual, but suppose someone upsizes the conductors and breaker for the sub above 30A (in the example above) in order to power other new loads off the sub, and they don't notice or care that the overcurrent protection is wrong on that circuit.

If nothing else, 110.12.
 

ActionDave

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240.21



Emphasis mine. (A) thru (H) provides nothing addressing this.

A short or overload on the branch circuit can trip the 30A sub breaker before the 50A branch circuit breaker, and shut down all loads on the sub even if only the one with the oversized breaker has a problem. That's great to lose all your lights and outlets in the kitchen because there was a problem that was just with your range. It could be worse than just inconvenient.
The conductors are protected.

Also, it may be unusual, but suppose someone upsizes the conductors and breaker for the sub above 30A (in the example above) in order to power other new loads off the sub, and they don't notice or care that the overcurrent protection is wrong on that circuit.

If nothing else, 110.12.
The conductors are still protected.
 

iwire

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Location
Massachusetts
240.21



Emphasis mine. (A) thru (H) provides nothing addressing this.

A short or overload on the branch circuit can trip the 30A sub breaker before the 50A branch circuit breaker, and shut down all loads on the sub even if only the one with the oversized breaker has a problem. That's great to lose all your lights and outlets in the kitchen because there was a problem that was just with your range. It could be worse than just inconvenient.

Also, it may be unusual, but suppose someone upsizes the conductors and breaker for the sub above 30A (in the example above) in order to power other new loads off the sub, and they don't notice or care that the overcurrent protection is wrong on that circuit.

If nothing else, 110.12.


It seems you are struggling to find a code to shoot this down only because you don't like it.
 

GoldDigger

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The conductors are protected.

The conductors are still protected.
Pray tell me how 30A conductors would be protected by two 50A breakers at different points along the way? Unless this is a motor application or device with MCA and MOCP.
Now the OP did offer the option of using 50A conductors instead of 30A conductors for the branch circuit, and in that case the conductors would be protected, but the device would not be. So still non-compliant if the upstream breaker is changed.
 

petersonra

Senior Member
Location
Northern illinois
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engineer
Let's say I have a device that calls for 30 ampere overcurrent protection. In the main panel I install a double pole 30 AMP breaker, and use it to feed a subpanel near the device. The device also requires GFCI protection, but all I have is a 50 AMP double pole GFCI breaker. Can I install the 50 AMP GFCI breaker in the sub panel, and feed the device from it? If so, how do I size the conductors from the 50 AMP breaker to the device? Should the conductors be sized at 30 or 50 amps?

IMO, I don't see any reason you cannot do this.

The conductors leaving the 50A GFCI are already protected by the 30A CB so don't have to be any larger than that.

Personally, I would just go buy a 30A GFCI to use as a feeder breaker and not screw around with the subpanel, unless you need it for some reason, such as a local disconnect.
 

jaggedben

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Northern California
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It seems you are struggling to find a code to shoot this down only because you don't like it.

I don't know why you say 'struggling'. I think 'at the point of supply' is pretty clear. The point of supply for the branch circuit is the sub, not the feeder.
 

jaggedben

Senior Member
Location
Northern California
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Solar and Energy Storage Installer
The way I read the OP there are only two breakers and the 30A is first in line. The 50A is only for GFCI protection.

No, the 30A is for a sub. If the two breakers were both in series on a branch circuit and the 50A is only for GFCI then I wouldn't say this is wrong. But the 50A is the branch circuit protection the way he described it. He is not protecting the branch circuit properly.

Here's another way this could go wrong; I come along and install a 30A solar source in the sub.
 

david luchini

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Connecticut
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But the 50A is the branch circuit protection the way he described it. He is not protecting the branch circuit properly.

The OP didn't describe using 30A branch circuit conductors. He asked how he should size them.

I agree with you, 240.21 says that the conductors should have overcurrent protection at the point they receive their supply, which is the branch circuit breaker. The branch circuit conductors should be sized per the branch circuit breaker.
 

donaldelectrician

Senior Member
No, the 30A is for a sub. If the two breakers were both in series on a branch circuit and the 50A is only for GFCI then I wouldn't say this is wrong. But the 50A is the branch circuit protection the way he described it. He is not protecting the branch circuit properly....




I agree with JAGGEDBEN . This is 50a CB is the " Branch Circuit Protection " , not the 30a protecting the Sub Panel . In this case using the 50a as ONLY for the GFCI protection required for the 30A circuit and not protecting the circuit at the Sub Panel where the Branch Circuit Originates is wrong .

What the OP says will work but it is not Kosher .


IWIRE - the conductors are sized for amperage first and a VD conductor size increase will still carry the circuit amperage .




Don
 
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petersonra

Senior Member
Location
Northern illinois
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engineer
The OP didn't describe using 30A branch circuit conductors. He asked how he should size them.

I agree with you, 240.21 says that the conductors should have overcurrent protection at the point they receive their supply, which is the branch circuit breaker. The branch circuit conductors should be sized per the branch circuit breaker.

240.21 Location in Circuit. Overcurrent protection shall
be provided in each ungrounded circuit conductor and shall
be located at the point where the conductors receive their
supply...

The code does not specify that the supply has to be a branch circuit.

I think you could make that argument though and not be really out there real far.
 
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