Breaker Sizes

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Cableman

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There is a 100A outdoor lighting panel which has a 100A main breaker and is located 40 feet from the switchboard who feeds it. The designer put a 125A breaker feeding the lighting controller. The designer specified a #1 AWG size cable. The load on the lighting panel is 3.5KVA, and is not expected to increase at all. The breakers are molded case.
When I asked why he did not put a 100A breaker instead of the 125A, he said that he did it from a coordination standpoint. He wanted the 100A breaker in the lighting panel to trip first before the breaker in the switchboard, that is why he upsized the breaker in the switchboard a bit larger.

I stated that there is really no chance for the breaker in the lighting panel to trip due to an overload (thermal) and if there is a short/fault (magmatic)there will probably be a race between the two breakers, and they most likely will trip at the same time, not guaranteeing that the breaker in the lighting panel will trip first.

Was wondering as to your feedback.

Thank you.
 
Cableman said:
There is a 100A outdoor lighting panel which has a 100A main breaker and is located 40 feet from the switchboard who feeds it. The designer put a 125A breaker feeding the lighting controller. The designer specified a #1 AWG size cable. The load on the lighting panel is 3.5KVA, and is not expected to increase at all. The breakers are molded case.
When I asked why he did not put a 100A breaker instead of the 125A, he said that he did it from a coordination standpoint. He wanted the 100A breaker in the lighting panel to trip first before the breaker in the switchboard, that is why he upsized the breaker in the switchboard a bit larger.

I stated that there is really no chance for the breaker in the lighting panel to trip due to an overload (thermal) and if there is a short/fault (magmatic)there will probably be a race between the two breakers, and they most likely will trip at the same time, not guaranteeing that the breaker in the lighting panel will trip first.



Was wondering as to your feedback.

Thank you.
Click to expand...

Fail...Can't do that. You need to change the breaker feeding the sub(lighting) panel to 100A or smaller. 408.36
 
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You are correct as there is no guarantee as to which breaker will trip under a fault condition, I have seen a 30 amp air conditioner circuit trip a 100 amp main in a fault condition it actually tripped both breakers. another time had a 240 volt 20 amp garage air compressor short out and trip not only the two pole 60 amp main in the garage panel but the 100 amp main in the house also.

There is little to no coordination in these smaller breakers like this.
 
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jumper said:
You edited it but original install is legal. The panel is properly protected.
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What am I missing? If you feed a sub panel, you have to size the breaker equal to or less than the panel OCPD you are feeding. He (OP) said he was feeding the 100A sub panel with a 125A breaker. Wouldn't he need to feed it with 100A or less to be compliant with 408.36?
 
Little Bill said:
What am I missing? If you feed a sub panel, you have to size the breaker equal to or less than the panel OCPD you are feeding. He (OP) said he was feeding the 100A sub panel with a 125A breaker. Wouldn't he need to feed it with 100A or less to be compliant with 408.36?
Click to expand...

You are thinking of MLO.

The#1 wire is protected by the 125 amp breaker. The 100 amp panel is protected by the 100 MCB.

408.36 Overcurrent Protection. In addition to the requirement
of 408.30, a panelboard shall be protected by an
overcurrent protective device having a rating not greater
than that of the panelboard. This overcurrent protective device
shall be located within or at any point on the supply
side of the panelboard.
 
Little Bill said:
What am I missing? If you feed a sub panel, you have to size the breaker equal to or less than the panel OCPD you are feeding. He (OP) said he was feeding the 100A sub panel with a 125A breaker. Wouldn't he need to feed it with 100A or less to be compliant with 408.36?
Click to expand...
The conductors are protected by the breaker that feeds them and the sub- panel is protected by the main in it. Everything is on on the up and up, no?
 
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jumper said:
You are thinking of MLO.

The#1 wire is protected by the 125 amp breaker. The 100 amp panel is protected by the 100 MCB.

408.36 Overcurrent Protection. In addition to the requirement
of 408.30, a panelboard shall be protected by an
overcurrent protective device having a rating not greater
than that of the panelboard. This overcurrent protective device
shall be located within or at any point on the supply
side of the panelboard.
Click to expand...

ActionDave said:
The conductors are protected by the breaker that feeds them and the sub- panel is protected by the main in it. Everything is on on the up and up, no?
Click to expand...

I understand about the conductors being protected. But all this time I have thought (and thought I heard/read) that you couldn't size a sub panel greater than it's rating.
Well, after being "spanked" and slooowly reading 408.36, I guess it is MLO, my brain is foggy!:slaphead:
 
Little Bill said:
I understand about the conductors being protected. But all this time I have thought (and thought I heard/read) that you couldn't size a sub panel greater than it's rating.
Well, after being "spanked" and slooowly reading 408.36, I guess it is MLO, my brain is foggy!:slaphead:
Click to expand...

Yeah, if the 100 amp was MLO, it would be illegal, since then the 125 amp breaker would be the protection of the wire and sub panel.
 
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