Alwayslearningelec
Senior Member
- Location
- NJ
- Occupation
- Estimator
If you have a 300A breaker with #350's feeding a panel what size does the MCB in the panel need to be? Does it have to be at least 300A or could it be 250A?
MCB Size
- Thread starter Alwayslearningelec
- Start date
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Does the customer want 300 amps or 250 amps available at the panel?
Alwayslearningelec
Senior Member
- Location
- NJ
- Occupation
- Estimator
IDK. But I guess it could be either???
Johnhall30
Senior Member
- Location
- New Orleans, LA
- Occupation
- Engineer
If you want to put a 250 Amp breaker downstream of a 300 Amp breaker with #350s between them, then you can do that. No issue
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Other than it was a waste to size the conductors and upstream feeder for 300A…
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
might not require a main CB at all.
Sparky2791
Senior Member
- Location
- Northeast, PA
- Occupation
- Electrical Design
Agree......unless voltage drop is an issue on the feeder.
408.36 requires a panelboard be protected by a device having a rating no greater than then rating of the panelboard (bus).
That protection can be integral to the board or ahead of the board.
If your supply is protected by a 300 amp breaker and the panelboard is rated 300 amps, no additional protection is required.
If the panelboard is rated less than 300 amps then added protection based on that rating is needed,
That protection can be integral to the board or ahead of the board.
If your supply is protected by a 300 amp breaker and the panelboard is rated 300 amps, no additional protection is required.
If the panelboard is rated less than 300 amps then added protection based on that rating is needed,
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I never use a Main breaker unless it serves some purpose such as for a tap rule or 408.36 issue. Some people seem to like the idea of a "local disconnect" but I don't see what that accomplishes.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Until you have to march 100 yards in a blinding snowstorm to reset the breaker in another building…
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
If it's a separate building it will require a disconnect at the building (I know there is an exception, I've never used it).
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
...uphill both ways too!
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
What makes you think the main breaker will trip first?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Murphy’s Law…
TX+ MASTER#4544
Senior Member
- Location
- Texas
- Occupation
- electrical Code instructor and mentor
2023 NEC
What is the panel's rating according to the label? You can't exceed the panelboard rating.
See 408.36 for your answer.
Here in Texas you will need a main breaker, not an MLO (main lug only).
Or it can be fed remote from a disconnect as an example.
Thanks for reading
Comments accepted
TX+MASTER#4544
TX+ MASTER#4544
Senior Member
- Location
- Texas
- Occupation
- electrical Code instructor and mentor
2023 NEC and 2024 edition of NFPA 70E
Texas
And if y'all conform to NFPA 70E, Standard for Electrical Safety in the Workplace it requires a remote disconnect or a MCB.
TX+MASTER#4544
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
That seems like an overly broad statement. Can you clarify when a main breaker is needed?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
just like augie47 said in post #8.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
When reacting to a short circuit/ground fault, they both may trip.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Worrying about convenience /inconvenience of something like a 200A breaker tripping is not something I even think about whatsoever.