next size down??

[Ponchik]

Say the calculated load comes out to 535A, can you protect that feeder with 500A OCPD? Maybe not a good design but is it a code issue?

 

[iwire]

No you cannot and yes it is.

 

[Ponchik]

No you cannot and yes it is.

215.2 is about minimum feeder ampacity & 240.4(B) "next higher OCPD is permitted to be used" doesn't say "shall be used"

What am I missing?

Thanks

 

[david luchini]

215.2 is about minimum feeder ampacity & 240.4(B) "next higher OCPD is permitted to be used" doesn't say "shall be used"

What am I missing?

Thanks

Try 215.3

 

[kwired]

215.2 is about minimum feeder ampacity & 240.4(B) "next higher OCPD is permitted to be used" doesn't say "shall be used"

What am I missing?

Thanks

If you have a load of 535 amps you need a conductor that will carry at least 535 amps, you will also need at least 535 amps of overcurrent protection or you will likely trip that overcurrent at some time even there is no overload condition, since 535 is not a standard overcurrent device you are allowed to go the next higher standard size.

Question: why, even without minimum ampacity or overcurrent protection rules, would you want to protect a circuit below the level of the connected load?

You may not really be creating much of an electrical safety hazard, but are creating possible nuisance tripping problems. And if we are talking about 500 amp fuses in a 600 amp fuse holder I can promise you most cases the end user will figure out that 600 amp fuses will fit in the fuse holders, which BTW is acceptable anyway, as long as the conductors have an ampacity of 535 amps.

 

[Ponchik]

If you have a load of 535 amps you need a conductor that will carry at least 535 amps, you will also need at least 535 amps of overcurrent protection or you will likely trip that overcurrent at some time even there is no overload condition, since 535 is not a standard overcurrent device you are allowed to go the next higher standard size. I agree

Question: why, even without minimum ampacity or overcurrent protection rules, would you want to protect a circuit below the level of the connected load? I am not working on this installation, this was just a hypothetical question that came up. Which in the real world may be a real installation.

You may not really be creating much of an electrical safety hazard, but are creating possible nuisance tripping problems. Maybe

Thanks.

 

[G._S._Ohm]

BTW, if you run a 500A conductor at 535A you will shorten its (50 yr?) insulation life by a few percent.

 

[kwired]

BTW, if you run a 500A conductor at 535A you will shorten its (50 yr?) insulation life by a few percent.

The insulation can probably handle 535 just fine but the termination temp is likely 75 C and conductor is sized to that, terminations could run over 75C rating and shorten life of devices faster than conductor insulation will notice any failure.