ECN Forum Forums Code Related Discussion NEC & other Code issues Derating Application

Yeah, Curt and Ian you caught that, I was editing that, sometimes open mouth and insert foot. I have been doing only nine as a general rule for years, and after posting that, figured I should check the math out. Eep, my bad!

GJ,
If your supply is 208/120Y you could pull out 4 of your neutrals to make this work. If these circuits are primarily for PC’s then the load would be considered non-linear and you will either need to reduce your OCP to 15 amps or pull #10’s.

Curt

Unfortunately, changing to #10's would mean replacing the conduit to 1" also.

Guys.....wire fill is not the issue. I agree that it would have been better to have two conduits here. But the installation is code compliant for wire fill.

The issue is derating. Every example that I've ever worked in all the code classes I've taken show that there are only 7 current carrying conductors in this example.

I agree with Electric Ian 310.15(B)(4)(a)
applies unless you have non-linear loads.

The fix suggested was to change out the ocpd to 15a............I just don't agree that is necessary or proper.

But then.......I could be wrong, it has happened before on occasion.

The solution I'd offer is to run three multiwire branch circuits (6 circuits) plus a two wire circuit. You'll have 7 hots, 4 neutrals, and 1 EGC. Total of twelve 12 ga. conductors is OK on fill, and 8 current-carrying conductors would be OK for 20 amps after derating.

Dave

edited...8...9...8 current-carrying conductors...I'm tired

[This message has been edited by Dave55 (edited 11-02-2004).]

GJ, Why don’t you feel the grounded conductors are current carrying in this installation? They are carrying just as much current as the ungrounded conductors. The grounded conductors (neutrals) are only exempt from the count if they are part of multi-wire circuits. If the majority of load on these circuits is linear I would remove 4 of the grounded conductors and keep the 20 amp OCD. Using multi-wire circuits will also make this installation more efficient (less voltage drop and wasted energy).

Norstarr, If this installation was done using mutli-wire circuits, 10 conductors would be needed. (10) 10 AWG THHN conductors are permitted in ¾” EMT.

Caselec,
Only because that was the way I was taught to figure derating.

The boss is generally opposed to sharing neutrals.

Ok, I'm learning here, so I've got that going for me, which is nice.

Hypothetical question, How does the derating picture change if I have a 208v 3 phase service rather than single phase?

Golf, the only way you can call this the "Unbalanced current" is if you do share the neutral and allow the hots to balance the current. Otherwise Mr Kirchoff says the current in the hot will be exactly the same as the current in the neutral, minus the ground faults.
If you boss doesn't like sharing neutrals he is going to have to buy some more pipe.