OCP protection question

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Dengineer

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I have a project where there is a switchboard with a 225A breaker feeding a 200MCB panel with 3/0 feeders. Does anyone see an issue here? Now I understand the feeder is being protected by the 200A main breaker, but shouldn't the main breaker upstream be protecting the feeder also? Is that not the supply side and point of origin or am I missing something?
 
Dengineer said:
I have a project where there is a switchboard with a 225A breaker feeding a 200MCB panel with 3/0 feeders. Does anyone see an issue here? Now I understand the feeder is being protected by the 200A main breaker, but shouldn't the main breaker upstream be protecting the feeder also? Is that not the supply side and point of origin or am I missing something?
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You need to match your feeder conductors to the feeder OCPD. 3/0 Cu is only good for 200A. Besides that, you can't feed a panel from a breaker that is rated more than the panel rating. In your case, a 225A feeding a 200A.

Welcome to the Forum!
 
Dengineer said:
I have a project where there is a switchboard with a 225A breaker feeding a 200MCB panel with 3/0 feeders. Does anyone see an issue here? Now I understand the feeder is being protected by the 200A main breaker, but shouldn't the main breaker upstream be protecting the feeder also? Is that not the supply side and point of origin or am I missing something?
Click to expand...
It's problematic unless the 225-amp breaker has an adjustable longtime pickup set to protect the cable at 200 amps.
 
Dengineer said:
I have a project where there is a switchboard with a 225A breaker feeding a 200MCB panel with 3/0 feeders. Does anyone see an issue here? Now I understand the feeder is being protected by the 200A main breaker, but shouldn't the main breaker upstream be protecting the feeder also? Is that not the supply side and point of origin or am I missing something?
Click to expand...
It might come under the tap rules. If so, it would be OK.
 
Little Bill said:
You need to match your feeder conductors to the feeder OCPD. 3/0 Cu is only good for 200A. Besides that, you can't feed a panel from a breaker that is rated more than the panel rating. In your case, a 225A feeding a 200A.

Welcome to the Forum!
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Thank you for the welcome! I have been lingering on the forums for 3 years now and finally decided to join. Hopefully one day I'll be able to help a few people.

Back to the question, sorry I don't understand what you mean here. Are we saying that the upstream breaker takes precedence in protecting the feeder as I assumed? Could I get a code section?

Also lets assume that the panel is rated for 225A, but has a 200A MCB. Does that matter?
 
mayanees said:
It's problematic unless the 225-amp breaker has an adjustable longtime pickup set to protect the cable at 200 amps.
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The electrician I am having a discussion with did mention this, but when I looked up the existing breaker I did not see any specifications for adjustable trip curve. (Not sure if I used the correct term there)
 
Dengineer said:
Also lets assume that the panel is rated for 225A, but has a 200A MCB. Does that matter?
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The panel has a 200 amp main so it likely has 200 amp bus making the OCPD ahead of the feeder irrelevant. You're #3/0's are too small to be protected by the 225 OCPD ahead of them. If 25' or less a tap rule can apply.
 
infinity said:
The panel has a 200 amp main so it likely has 200 amp bus making the OCPD ahead of the feeder irrelevant. You're #3/0's are too small to be protected by the 225 OCPD ahead of them. If 25' or less a tap rule can apply.
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Thank you for the quick replies! greatly appreciated.
 
Dengineer said:
Can you elaborate? and thank you for the reply.
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If the conductors from the 225 amp breaker to the panel are 25' or less they can be tap conductors if all of the 25' tap rule conditions are met {240.21}. If not you'll need #4/0 feeder conductors or protection of the
#3/0 feeder at 200 amps or leas.
 
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.
Exception No. 1: Individual protection shall not be required for a
panelboard used as service equipment with multiple disconnecting
means in accordance with 230.71. In panelboards protected by three or
more main circuit breakers or sets of fuses, the circuit breakers or sets of
fuses shall not supply a second bus structure within the same panelboard
assembly.
Exception No. 2: Individual protection shall not be required for a
panelboard protected on its supply side by two main circuit breakers or
two sets of fuses having a combined rating not greater than that of the
panelboard. A panelboard constructed or wired under this exception
shall not contain more than 42 overcurrent devices. For the purposes of
determining the maximum of 42 overcurrent devices, a 2-pole or a
3-pole circuit breaker shall be considered as two or three overcurrent
devices, respectively.
Exception No. 3: For existing panelboards, individual protection shall
not be required for a panelboard used as service equipment for an individual
residential occupancy.
 
The general rule is that overcurrent protection needs to be upstream of whatever it is protecting (ie, at the point of supply)

There are two different systems that require overcurrent protection in the scenario you're providing - the feeder conductors (art. 215) and the panelboard (art. 408). The feeder OCPD is the 225A circuit breaker in the switchboard. In my opinion, the panelboard OCPD is the 200A MCB at the panelboard. If there wasn't a MCB, I would then say that the 225A OCPD at the switchboard would be the panelboard OCPD. However, just based on nominal mains ratings of panelboards, I don't think this matters as either nominal mains/bus rating (225A @ 208Y/120V or 250A @ 480Y/277V) may be protected by either a 200A or 225A breaker.

As stated by other posters, the #3/0 feeder conductors need to match the OCPD size unless it's a tap. The point of supply for the feeder is at the switchboard and not at the panelboard so the 200A MCB doesn't provide overcurrent protection for that feeder.
 
So you're saying that if the panel has a main breaker of less than the feeder breaker it's allowed?
If it's a main lug, the the 225A can't feed the 200A panel?
 
Little Bill said:
So you're saying that if the panel has a main breaker of less than the feeder breaker it's allowed?
If it's a main lug, the the 225A can't feed the 200A panel?
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Yes. The subpanel bus is 200A and is adequately protected by the 200A main breaker. if the main breaker were not there (if the sub were MLO), then it would be a problem.
 
Little Bill said:
So you're saying that if the panel has a main breaker of less than the feeder breaker it's allowed?
If it's a main lug, the the 225A can't feed the 200A panel?
Click to expand...
Exactly....it is common to feed panels with feeder taps where the upstream OCPD exceeds the rating of the panel, but the panel has a main breaker that does not exceed the panel rating. The key is the main breaker must protect the panel bus.
 
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