To tap or not?

[Cow]

I was having a discussion with a coworker about a project he's on. He's adding a 500 amp fused disconnect fed from a 2000 amp main breakered switchboard. He ran two 2 1/2" emt conduits between the switchboard and disconnect thinking he'd run parallel 250's tapping after the main. When he realized his wire length at 23' put him into needing 1/3 ampacity for the tap(667 amps) requiring parallel 400's, the emt conduit is too small. I mentioned if he could tap the 2000 amp buss ahead of the main, there were no tap rule requirements.

Our we reading the code correctly? A tap after the main requires 667 amps of conductor, a tap before the main requires running just 500 amps of conductor? I can't find the logic in tapping before the main and having no main OCPD but getting away with smaller conductor?

Thoughts?

 

[iwire]

Just thinking out loud here.

If it's ahead of the main we are talking service conductors and they never have overcurrent protection but have to remain outside the building as per 230.6. They have more of a length restriction than the 25' tap rule once inside the building.

 

[iwire]

Tell your bud not to forget the EGCs with a feeder tap have to be sized to the 2000 amp OCPD.

 

[electrofelon]

Also, it seems like you were planning to use the next size up rule on the tap conductor which is not allowed. The fuses or circuit breaker that the taps terminate on, must protect the tap conductors at their ampacity.

 

[electrofelon]

Looks like 4-400's will fit no? Skip the wire EGC.

 

[Cow]

Just thinking out loud here.

If it's ahead of the main we are talking service conductors and they never have overcurrent protection but have to remain outside the building as per 230.6. They have more of a length restriction than the 25' tap rule once inside the building.

That had slipped my mind, thanks for reminding me.

Tell your bud not to forget the EGCs with a feeder tap have to be sized to the 2000 amp OCPD.

Yes, this is easily forgotten as well.

Also, it seems like you were planning to use the next size up rule on the tap conductor which is not allowed. The fuses or circuit breaker that the taps terminate on, must protect the tap conductors at their ampacity.

250mcm is 255 amp wire x 2 sets. We should be covered for 500 amp fuses?

 

[electrofelon]

250mcm is 255 amp wire x 2 sets. We should be covered for 500 amp fuses?

Oh indeed it is. Im calling my college first thing and asking for my money back for my math degree :rant: