We had one of our crews run into a situation yesterday with derating.

Final inspection, a new office in a converted industrial space. The guys had run 7 ckts. in a 3/4" emt. 7 hots, 7 neutrals, one ground, #12 thhn, 20a ocpd.

The state inspector failed them for derating. He said that, "Since this was a single phase service all neutrals are counted as current carrying conductors." These circuits are all general use plugs and lighting. No Non-linear loads, no HID lighting.

My repsonse is WTF??????? This is definitely a new one on me. I consider that conduit to have 7 current carrying conductors per the NEC. Please educate me if I'm wrong.

GJ

I don't know how other guys figure this, but I'm pretty sure if I took the neutral out of the service and pinched it's end between my fingers, then, with a different finger, touched the screw where it came from I'd find it to be "current-carrying", so I count it as such.

(Don't try this at home.)

Dave

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

Current may indeed flow through your fingers but I dare say it would be "unbalanced current" and thus 310.15(B)(4)(a)would apply, no?

310.15(B)(4)(a)
A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions
of 310.15(B)(2)(a). (derating table)

I understand that portion of the NEC to apply to multiwire branch circuits. As it is described as 7 hots and 7 neutrals for 7 circuits, I don't think it applies.

Dave

Junkie,
I'm under the impression that you have 7 two wire circuits which means they are all current carrying conductors. Looking at 310.15(B)(2)(a) you find that you must derate to 50% of the thhn value of 30A and the ocpd would be 15A.
Why aren't you installing 3 wire circuits which would reqire you to derate to 70% and your ocpd would be 20A?
Ron

You guys are right. I was thinking 3 wire circuits with shared neutral. Separate neutral with each phase wire - all neutrals are indeed current carrying conductors.

Nailed on derating, and fill factor.

358.22 to Chapter 9 Table 1, and notes to Tables. 40% fill, not 88%.

The only way I could see 15 #12's being allowed, is if were in a 23" conduit, and even then, I wouldn't do it.



[This message has been edited by e57 (edited 11-02-2004).]

I use Annex C, Table C1 Maximum Number of Conductors....in EMT.

Type THHN, THWN for 3/4" EMT is 16 conductors for 12 ga. Of course there would be some derating involved.

Dave

Up to 16 THHN conductors in 3/4" EMT allowed per table C1 in annex C.

Mark,
The tables in Annex C are not 100% fill, they are based on Chapter 9 Table 1. (16) #12 THHN conductors would meet the 40% fill requirement for ¾” EMT according to Annex C Table C1.
The area of #12 THHN is .0133”
16 X .0133 = .2128”
¾” EMT is .213 @ 40% fill

Curt

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.

If you are using the three phase system and a grounded conductor for each circuit the grounded conductor is a current carrying conductor. If you are using 2 phases with a common grounded conductor, the grounded conductor is a current carrying conductor. If you are using a three phases with a common grounded conductor the grounded conductor is not a current carrying conductor if a majority of the load is linear, and is a current carrying conductor if a majority of the load is non-linear.
Don

Caselec, you are correct that it would be 10 conductors but only 8 would be current carrying thus derating would be figured at 70% allowing the use of 20 Amp circuit breakers. If this was a three wire single phase system you would have 11 conductors but only 8 would be current carrying and you could still use 20A olcp.
Ron

By the way, hats off to the inspector that caught this, I know a lot of inspectors and electricians would have missed it.

I'm not very fond of multiwire circuits either, so run another pipe.

And thanks to resqcapt19 for explaining the 3-phase multiwire circuit (3 wire). I hadn't thought about those two phases not being opposing, therefore having current in the neutral. Learn something new every day!

Dave

edited for spelling

[This message has been edited by Dave55 (edited 11-03-2004).]

Any time an inspector sees a lot of wire in any single pipe on his inspection it is a red flag that he should be thinking about fill and derating. You can usually "eyeball" fill after a while and get close enough to see whether you need your calculator but derating requires some deeper investigation, particularly if you don't see any multiwires. They bring up their own issues that require a closer look.