Subpanel Myth or Fact?

[TwoBlocked]

An experienced electrician mentioned today that a subpanel can't be fed from a breaker rated at more than 50% of the main panel breaker. Seems like it might be a good idea, but I can't find any article in the NEC that says so. Any ideas where this comes from?

 

[HEYDOG]

An experienced electrician mentioned today that a subpanel can't be fed from a breaker rated at more than 50% of the main panel breaker. Seems like it might be a good idea, but I can't find any article in the NEC that says so. Any ideas where this comes from?

False! It can be fed with what ever the maximum size breaker the panel says the bus bar tabs are rated for!

 

[jaggedben]

Myth.

People have a weird tendency to believe what their boss or some random inspector told them 30 years ago, instead of reading the book. (Also the book changes every 3 years.)

 

[LarryFine]

Any ideas where this comes from?

Too much drug use in his youth?

 

[roger]

The experienced electrician needs to spend a little more time in the code book.

 

[infinity]

I agree with Roger, Et al. Another sparky believing a non-existent code myth.

 

[TwoBlocked]

Thanks, guys. I'll keep my eye on him. A lot of moving parts to this project. Fracking brine water. His outfit is running conduit and pulling wire. Mine is making control and communications terminations. We both agree that there were some bizarre design decisions.

 

[TwoBlocked]

Too much drug use in his youth?

Probably no more than the rest of us boomers

 

[Tulsa Electrician]

I have heard something close however it delt with raceway fill up to 80% based on 50% loading.

We should start a myth post in campfire chat.
It was a wild discussion I had with that apprentice.

 

[norcal]

Too much time in reading the Urban Legend Code. Found right next to the no wirenuts in panels rule.

 

[GoldDigger]

False! It can be fed with what ever the maximum size breaker the panel says the bus bar tabs are rated for!

Slightly more complicated/variable where tap conductor rules are in play.

 

[drcampbell]

It probably originated with fuse coordination.

If, for example, there were a 100-amp main breaker in the main panel, and a 100-amp branch breaker feeding the subpanel, you couldn't assure that an overcurrent in the branch panel would take out only the branch breaker and wouldn't take out the main breaker instead/also.

But that's a convenience issue, not code or safety.

 

[ramsy]

An experienced electrician (said) sub-panel can't be fed from a breaker rated at more than 50% of the main panel

There is a 50% max load if Fastened in Place, for branch circuits in 210.23(A)(2).

Maybe someone who uses 430.6(A)(1) to interpolate horsepower from current, thinks branch rules interpolate to feeders?

 

[Joethemechanic]

Probably no more than the rest of us boomers

I've never even smoked weed

They warned us about it in school

 

[PaulMmn]

I've never even smoked weed

They warned us about it in school

Reefer Madness is a better 'anti-pot' film. Intended as such, with over-acting and twisted interpretation of events.