number of current carrying cond.

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hbruce408

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Table310-15(b)(2)(a) . There seems to be confusion about what is derated here. The conductor or the breaker. If the size of the cond. is increased, to compensate, the same amount of heat is generated and not dissipated any faster due to the size of the cond., because the insulation is the same, and it is stacked in the raceway just as the lesser sized conductor.
Need a reading on this.
 

iwire

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Location
Massachusetts
Re: number of current carrying cond.

The derated wire has a lower ampacity and accordingly must be used on a smaller breaker.

If you want to feed 20 amp duplex receptacles with a 20 amp breaker the only option is to increase the wire size to accommodate the adjusted ampacity of the wire.
 

iwire

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Location
Massachusetts
Re: number of current carrying cond.

Here is an example of having five 2 wire circuits in a single raceway.

5 two wire circuits in one raceway would be subject to derating problems.

14 AWG = 25 amps 310.16

5 two wire circuits = 10 current carrying conductors

10 conductors gets a 50% downward adjustment Table 310.15(B)(2)(a)

25 x .5 = 12.5 amps

You should not feed 15 or 20 amp duplex outlets with a 10 amp breaker.

You can not move up to the next breaker size. 240.4(B)

This would mean you would have to use 12 AWG on the 15 amp breaker.
 

charlie b

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Re: number of current carrying cond.

Table310-15(b)(2)(a). . . what is derated here . . . the conductor or the breaker
A breaker is never ?derated.? The manufacturer gives it a rating that tells us the amperage above which it will trip.
. . . the same amount of heat is generated
I think this is where you are getting confused. Ten amps flowing through a #12 will generate less heat than the same ten amps flowing through a #14. The formula for heat generation is I^2 times R. Therefore, with the same ?I,? the larger the wire, the lower its resistance "R," and the lower the heat generated.

The design process starts by calculating the load. Then you pick a conductor that can safely carry that load. If you plan to put many wires in the same raceway, then the notion of ?safely carrying the load? becomes influenced by the mutual heating of the wires. The last step is to pick a breaker that can trip in time to protect the wire from the heat generated by too much current.

Does this help clear up the puzzle?
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
Re: number of current carrying cond.

Originally posted by stew: hey iwire is a shared neutral counted?
Yes. If the two circuits are not loaded to the same value, then the shared neutral will carry current. Granted, you will never get all three conductors (two hots and the shared neutral) carrying full current at the same time. But it is not possible to analyze every possible combination of shared neutral configurations and loading conditions. So the simple approach is to say that all three wires will carry current, so all three will be counted.
 

iwire

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Location
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Re: number of current carrying cond.

hey i wire is a shared neutral counted?
You may count it as Charlie said, but we are not in most cases required to.

310.15(B) (4) Neutral Conductor.

(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).

(b)In a 3-wire circuit consisting of two phase wires and the neutral of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(2)(a).

(c)On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.
I run a lot of 3 phase multiwire branch circuits all 3 phases and one neutral and one of my reasons for doing so is to cut down on current carrying conductors.

In this case 3 current carrying conductors instead of 6.

Typically we will run two 2" EMTs out of a 42 circuit panel, if I can keep all the circuits 3 phase multiwire branch circuits I end up with 21 current carrying conductors in each EMT instead of 42 current carrying conductors in each EMT.

In my state (MA) 21 current carrying conductors only gets a 70% adjustment this means if I use all 10 AWGs in the EMTs I still can use 20 amp breakers.

NEC would adjust these 21 conductors 45% meaning I would need 8 AWGs to do this.
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
Re: number of current carrying cond.

Good answer, iwire (certainly better, and more complete, than mine). When I read the question, I envisioned only the situation covered by sub-paragraph (b).
 

ryan_618

Senior Member
Re: number of current carrying cond.

As far as counting the nuetral goes, I wonder how many people consider the neutral on a typical commercial flourescant lighting circuit, since flourescant is considered to be a non-linear load.
 

chicar

Senior Member
Location
Lancaster,Pa
Re: number of current carrying cond.

iwire' I believe 310.15-4a is for a 3 phase with neutral type load. I do not believe you can apply this to a 42 circuit panel with 20 amp individual breakers. Please let me know if I'm not understanding correctly. :)
 

iwire

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Location
Massachusetts
Re: number of current carrying cond.

Originally posted by chicar:
iwire' I believe 310.15-4a is for a 3 phase with neutral type load. I do not believe you can apply this to a 42 circuit panel with 20 amp individual breakers. Please let me know if I'm not understanding correctly. :)
I am not sure what you are asking.

Are you saying that 310.15(B)(4) is only to be used for three phase loads that use a neutral?

As an example a piece HVAC equipment that has a 3 phase heat load and a line to neutral fan load?

What I describe above you would have to count the neutral as a current carrying conductor as it does not only carry the imbalance of the phase conductors.

310.15(B)(4) tells us that a neutral conductor that carries only the unbalanced current from other conductors of the same circuit is not a current carrying conductor.

That is exactly what happens when you have both hots and one neutral in a single phase system or all 3 hots and one neutral of a single phase system.

It makes no difference if fed from single pole breakers or a common trip breaker, as long as it includes all opposing phases.
 

scott thompson

Senior Member
Re: number of current carrying cond.

Also wanted to throw in my 2? regarding derating Conductors on 1? 3 Wire systems with 2 and 3 wire Circuits, plus on 3? 4 Wire Wye systems with 2,3 and 4 wire Circuits.

In these scenarios, I will be using #12 THHN cu for Circuit Conductors, and an Ambient temperature of 30?C (86?F). Loads will be of the "Non-Linear" (Not Pure Resistance) type, U.O.N. (Unless Otherwise Noted).
Also, each scenario is to be in a single common raceway - either Metallic or Non-Metallic, with a length over 24" (figure no less than 10 foot raceway).
Raceway(s) also contain a single #12 cu Equipment Grounding Conductor.

The following may be used for basic descriptive examples, and are open to all discussions.

***PART A: 1? SYSTEMS***

FYI: Derived from either a 1? Transformer with a Center Tap, or the Center Tapped "Lower" Winding of a 4 Wire Delta.

1A: 2-Wire Circuits (L-L or L-N):

* Four (4) 2-Wire Circuits = 8 Current Carrying Conductors. Maximum OCPD = 20 Amps.

* Five (5) 2-Wire Circuits = 10 Current Carrying Conductors. Maximum OCPD = 15 Amps.

* Ten (10) 2-Wire Circuits = 20 Current Carrying Conductors. Maximum OCPD = 15 Amps (maximum limit for #12).

2A: 3-Wire Circuits (L-N-L):

* Four (4) 3-Wire Circuits = 12 total Circuit Conductors - only 8 Current Carrying Conductors. Maximum OCPD = 20 Amps.

* Five (5) 3-Wire Circuits = 15 total Circuit Conductors - only 10 Current Carrying Conductors. Maximum OCPD = 15 Amps.

* Ten (10) 3-Wire Circuits = 30 total Circuit Conductors - only 20 Current Carrying Conductors. Maximum OCPD = 15 Amps (maximum limit for #12).


***PART B: 3? SYSTEMS***

1B: 2-Wire Circuits (L-L or L-C):

* Four (4) 2-Wire Circuits = 8 Current Carrying Conductors. Maximum OCPD = 20 Amps.

* Five (5) 2-Wire Circuits = 10 Current Carrying Conductors. Maximum OCPD = 15 Amps.

* Ten (10) 2-Wire Circuits = 20 Current Carrying Conductors. Maximum OCPD = 15 Amps (maximum limit for #12).

2B: 3-Wire Circuits (L-C-L or L-L-L):

* Three (3) 3-Wire Circuits = 9 Current Carrying Conductors. Maximum OCPD = 20 Amps.

* Four (4) 3-Wire Circuits = 12 Current Carrying Conductors. Maximum OCPD = 15 Amps.

* Six (6) 3-Wire Circuits = 18 Current Carrying Conductors. Maximum OCPD = 15 Amps (maximum limit for #12).

3B: 4-Wire Circuits (L-L-L-C):

* Two (2) 4-Wire Circuits = 8 Current Carrying Conductors. Maximum OCPD = 20 Amps.

* Three (3) 4-Wire Circuits = 12 Current Carrying Conductors. Maximum OCPD = 15 Amps.

* Five (5) 4-Wire Circuits = 20 Current Carrying Conductors. Maximum OCPD = 15 Amps (maximim limit for #12).

---end---

Abbreviations:
L-C = Line to Common,
L-N = Line to Neutral,
L-L = Line to Line,
L-C-L = Line to Common to Line (multiwire),
L-N-L = Line to Neutral to Line (multiwire),
L-L-L = Line to Line to Line (3? 3 wire),
L-L-L-C = Line to Line to Line to Common (multiwire).
Line = Ungrounded Conductor,
Common = "Common" Grounded Conductor of a 4 wire Wye system,
Neutral = Grounded Conductor of a Center Tapped 1? Transformer Secondary Winding.

Please reply with any comments.

Scott35
 

pierre

Senior Member
Re: number of current carrying cond.

Scott
3B. 4 wire circuits (ckt), for 12 AWG THHN

(2) 4-wire ckts = 6 ccc -20 amp OCPD (80% of 30)

(3) 4-wire ckts = 9 ccc -20 amp OCPD (70% of 30)

(5) 4-wire ckts = 15 ccc -15 amp OCPD (50% of 30)

The grounded (neutral) conductor is not counted as current carrying.

Pierre
 

chicar

Senior Member
Location
Lancaster,Pa
Re: number of current carrying cond.

So your saying that if I took 3 hots and 1 neutral from the panel through "Indevidual" 110V breakers to feed 3 receptical circuits 1-3-5
in the building, that I do not count the neutral?
How about (1) 3 phase breaker with 3 hots and a neutral known as circuit (1) feeding a control panel,ect. Is this where the "same circuit" rule apply in 310.15-4a. Would 1-3-5 be considered the same or individual circuits?
 

stew

Senior Member
Re: number of current carrying cond.

just curious why one would need the neutral out to a 3 phase load? And full boat type single breakers to each load with a shared neutral the neutral does not have to be counted as it then carries only the unbalanced current right?.And the only time you need the neutral(and count it) is when you have a combination load as described in an HVAC application with the 3 phase heater load and the single phase fan as I understand it.

[ March 08, 2004, 05:17 PM: Message edited by: stew ]
 

iwire

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Location
Massachusetts
Re: number of current carrying cond.

Yes you got it, but it does not have to be consecutive breakers (1,3,5) it can be all over the panel as long as you hit phases A, B, and C. :)
 

iwire

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Location
Massachusetts
Re: number of current carrying cond.

Chicar I get it now, yes the NEC considers this one circuit or 2 or 3 it is kind of left open.

Article 100

Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.
210.4 Multiwire Branch Circuits.
(A) General.

Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire branch circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panelboard.
 
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