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Another Derating scenario, plus Boxfill example, too! #42848
09/27/04 08:35 PM
09/27/04 08:35 PM
Scott35  Offline OP
Broom Pusher and
Joined: Oct 2000
Posts: 2,720
Anaheim, CA. USA
Another Derating scenario, plus Boxfill example, too!

A continuation of the Here's the Derating Discussion!!! Yee-Haww Thread, with a few twists!

BTW, I am just picking example scenarios from “Thin Air”, they are not any textbook exam examples, or from any on-line tests I am aware of.
(AKA “You are not doing my Homework for me” kind of thing…)

1: Which of the following examples is compliant per NEC Chapter 3:

[*] Three 3 Phase 3 Wire circuits of #10 THW cu in same raceway, load = 26 Amps per circuit, non LCL, OCPD = 30 Amps,

[*] Six 1 Phase 2 Wire 230 Volt Resistance Heaters – each is 4.5KW rated @ 230 VAC, fed from 1 Phase System with #10 THHN cu in same raceway; non LCL, OCPD = 20 Amps,

[*] Four 1 Phase 3 Wire circuits of #12 THHN cu in same raceway, system is 1 Phase 3 Wire, each Multiwire circuit will be driving Continuous Non-Linear L-N loads, OCPD = 20 Amps,

[*] Four 1 Phase 3 Wire circuits of #12 THHN cu in same raceway, system is 208Y/120 VAC 3Ø 4 Wire, each Multiwire circuit will be driving Continuous Non-Linear L-N loads, OCPD = 20 Amps.
BTW, Ambient Temperature is 30° C for all examples.

2: I ran Two 12/3 MC Cables (w/ Grounding Conductor), into a 4s 1-½” deep box. There will be a 125-Volt, 20 Amp Duplex Receptacle mounted on a 5/8” raised 1 Gang Plaster Ring when it’s trim time. The Receptacle will be connected to one of the Two branch circuits via “Pig Tails”.
Am I OK with boxfill here?

Are these examples OK?

Please submit any answers, questions, comments and/or examples you wish.


Edited faulty Hyperlink

[This message has been edited by Scott35 (edited 09-27-2004).]

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Work Gear for Electricians and the Trades
Re: Another Derating scenario, plus Boxfill example, too! #42849
09/28/04 09:17 AM
09/28/04 09:17 AM
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Okay, I'll take a stab at this.

A. Not compliant. 9 current carrying conductors, requiring derating to 0.7. #10 THW has an ampacity of 35A, so with derating you are only allowed 24.5A per conductor, insufficient for the load served and not protected by the OCPD. Fix by using THHN conductors.

B. Compliant. 12 current carrying conductors, requiring derating to 0.5. #10 THHN has an ampacity of 40A, so with derating you are allowed 20A per conductor, sufficient for the load served, and protected by the OCPD.

C. Possibly Compliant. 8 current carrying conductors, derating to 0.7. #12 THHN has an ampacity of 30A, derated to 21A, and thus protected by the OCPD. _However_ the load served is specified as continuous. In this case, the continuous capability of the OCPD must be considered sufficient for the load served. If the load served is 16A or less, then this is compliant. Note: the specification of non linear loads should send up a red flag for neutral current. However the non-linear loads that cause problems almost invariably introduce _odd_ order harmonics. These are a problem in a three phase system, where triplen harmonics will flow in the neutral. However odd order harmonics will not cause neutral current in a _single_ phase system. 310.15 (B) (4) (c) identifies the neutral in three phase four wire systems with harmonic loads as a current carrying conductor.

D. Not Compliant. Electrically similar to the above, but now the feed is a 3 phase 4 wire wye system, and you have to count the neutral conductor, even without harmonics considered. Derated ampacity is 15A, and 15A OCPD is required. In addition, since the loads are continuous, the maximum permissible load is 12A.

Box fill:
This one is borderline. I count 7 conductors entering the box: 6 for each of the circuit conductors and 1 for all of the grounds. The receptacle counts for another 2 units of box fill. Finally any clamps _inside_ the box (not specified) will count as a unit of box fill. #12 conductors require 2.25 cubic inches per unit. So you need a total of 20.25 or 22.5 cubic inches.
A 4S 1.5 box has a volume of 21 cubic inches (table 314.16(A) ).
If the mud ring has a printed volume on it, then that counts toward available volume, and you are almost certainly good.



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