more that 3 conductors derating

[Daja7]

If there were six conductor in one conduit but only three would be carrying current at a time would you still need to derate? Two 120/240 volt feeders. they will be alternating via contactors so it will be impossible for both to run at the same time. would derating still apply. conduit exists so we are limited to wire size.

 

[raider1]

With the new wording to 310.15(B)(3)(a) IMHO you would have to apply the ampacity adjustments contained in that section.

The term "Number of Current Carrying Conductors" was removed from the New Table in the 2011 NEC and replaced with the term Number of conductors in the header of the left hand column. Footnote 1 to the table also states that the number of conductors is the total number of conductors in the raceway adjusted by 310.15(B)(5) and (6) which allows you to not count the EGC or the neutral in a multiwire branch circuit.

Chris

 

[Daja7]

We are still under 2008 code.

 

[texie]

With the new wording to 310.15(B)(3)(a) IMHO you would have to apply the ampacity adjustments contained in that section.

The term "Number of Current Carrying Conductors" was removed from the New Table in the 2011 NEC and replaced with the term Number of conductors in the header of the left hand column. Footnote 1 to the table also states that the number of conductors is the total number of conductors in the raceway adjusted by 310.15(B)(5) and (6) which allows you to not count the EGC or the neutral in a multiwire branch circuit.

Chris

It would seem to me that taken literally you are correct, but as a practical matter this would seem OK for this application. Kind of reminds me of the situation where you have 1 neutral and, say, 6 switch legs all fed from the same breaker-does this require derating?

 

[jusme123]

It would seem to me that taken literally you are correct, but as a practical matter this would seem OK for this application. Kind of reminds me of the situation where you have 1 neutral and, say, 6 switch legs all fed from the same breaker-does this require derating?

1. are they current carrying?

2. where is the exception to your scenario?

 

[Smart $]

It would seem to me that taken literally you are correct, but as a practical matter this would seem OK for this application. Kind of reminds me of the situation where you have 1 neutral and, say, 6 switch legs all fed from the same breaker-does this require derating?

1. are they current carrying?

2. where is the exception to your scenario?

Technically the derating applies without exception...

However, the derated ampacity is not limted to the branch circuit ampacity as a minimum. It can go as low as the maximum load current.

Using the 6 switch legs, one neutral as an example, say on a #12 THHN, 20A circuit, and the maximum load to be served by each switch leg is 3A. As long as derated ampacity is equal or greater than 3A, you do not have to increase conductor size on the switch legs (still needs to be at least #12 because of ocpd rating). However, the neutral doesn't get any benefit in this example, and may need upsized because of the 7 circuit conductors (i.e. if more than the 7 total in conduit for THHN, or other derating applies).

 

[jumper]

From the ROP for the change to the table, note the bold which not mine-but part of the proposal.

http://www.nfpa.org/assets/files/AboutTheCodes/70/70-A2010-ROP.pdf

Affirmative: 11
_______________________________________________________________
6-57 Log #635 NEC-P06
Final Action: Accept
(Table 310.15(B)(2)(a))
_______________________________________________________________
Submitter:
James M. Daly, Rep. NEC/CEC Ampacity Harmonization Task
Group
Recommendation:
Revise first column heading in Table 310.15(B)(2)(a) from
?Number of Current-Carrying Conductors? to ?Number of Conductors (See
Note 1.)?.
Add a Note below the Table before FPN No. 1 to read:
1
Number of Conductors is the total number of conductors in the raceway or
cable adjusted in accordance with 310.15(B)(4) and (5).
Substantiation:
A companion proposal is being submitted for Table B.310.11.
This Table was originally added to the Code in 1940 and the column
heading remained as ?Number of Conductors? until 1993. During the 1993
NEC revision cycle, the column headings in Table 310.15(B)(2)(a) and
Table B.310.11 were editorially changed to ?Number of Current-Carrying
Conductors?. There was no Proposal, Comment, or Panel Action to make the
change. A chronological history of the changes in the Tables is attached for
reference.
This reinstatement of the original column heading permits the user to take
the total number of conductors in the cable or raceway and subtract the neutral
conductors not required to be counted under 310.15(B)(4) and the grounding or
bonding conductors not required to be counted under 310.15(B)(5). The result
is the number of conductors that may be simultaneously energized and this
number would be used to select the adjustment factor from the Table.
The Note is proposed for clarity since there has been some confusion that the
column referred to the total number of conductors in the raceway or cable.
The correction to the column heading also correlates with the column
headings in Table 400.5 in the NEC and Table 5C in the 2006 Canadian
Electrical Code, Part I.

 

[kwired]

Have not checked into it yet, but I know in past you did not have to count the control conductors as current carrying conductors, say you had a motor and its associated (art 725 class 1) control conductors in same raceway.

 

[charlie b]

Please allow me to disagree with everything that has been said so far in this thread. I submit that derating is not necessary in the installation described in the OP. I further submit that the code is clear, even with the 2011 edition, that my viewpoint is correct. Please take note of the following:

• Despite the fact that the column title has been changed and note 1 has been added, the title of the table retains the phrase, ?for more than three current-carrying conductors.?
• You don?t get to use any table in the code at all, until some text within the code points you to that table, and tells you to apply what that table says. In this case, the article that points to the table is 310.15(B)(3)(a) (again, the 2011 edition). The title of that sub-paragraph and its first sentence both state that they apply when you have more than three current-carrying conductors in the same raceway.
• In the installation described by the OP, you will not have more than three conductors carrying current at the same time. Therefore, neither the article nor the table come into play.
• Q.E.D.

 

[charlie b]

Have not checked into it yet, but I know in past you did not have to count the control conductors as current carrying conductors, say you had a motor and its associated (art 725 class 1) control conductors in same raceway.

Check out 310.15(B)(3)(a), second paragraph (2011 edition).

 

[mwm1752]

Still the conductors are by definition "current carrying" whether they are energized at the same time or not. IMO there are 4 current carrying conductors by definition with this installation.

 

[charlie b]

If I installed a three phase circuit to a motor, so that there are 3 current carrying conductors plus an EGC that does not count as a CCC, and then pulled another wire into the same conduit but did not connect it at either end, would you say that I now have 4 CCCs?

I would say you still have only 3 CCCs. Remember the issue at hand is ampacity, and that is based on limiting the current so as to prevent overheating the wire's insulation system. A wire that does not, and indeed cannot carry current will not contribute any heat to the inside of the conduit. In the OP's installation, there are some wires that must necessarily be disconnected while others are carrying current.

 

[north star]

= + =

Daja7,

What is the size and type of the raceway where these 6 conductors
are installed ?

+ = +

 

[Daja7]

Still the conductors are by definition "current carrying" whether they are energized at the same time or not. IMO there are 4 current carrying conductors by definition with this installation.

Conductors are current carrying only when energized. not the potential for carrying current. The neutral and ground conductor has the potential by virtue of being a coductor, but is electrically connected so it cannot. it does not contribute to potential heating. A little conveluted but relavent.

 

[Smart $]

Conductors are current carrying only when energized. not the potential for carrying current. The neutral and ground conductor has the potential by virtue of being a coductor, but is electrically connected so it cannot. it does not contribute to potential heating. A little conveluted but relavent.

There is some support for this premise in the Code (I won't get into it for the time being). However, you (and Charlie) are also not looking at another aspect... The more conductors you put in a conduit, whether current-carrying, energized, or not, contribute to heat retention (i.e. they hinder heat dissipation).

 

[charlie b]

The more conductors you put in a conduit, whether current-carrying, energized, or not, contribute to heat retention (i.e. they hinder heat dissipation).

If that were relevant, then the code would make us count control wires when we decide whether to derate. But it doesn't. 310.15(B)(2)(a), Exception 1. I can put a 3-phase branch circuit (3-CCCs) in a conduit, and then fill the conduit to 40% with control wires, and not have to derate.

 

[mwm1752]

4 of the conductors do carry current one time or another, None are control wiring so it does not apply per exception. There is no guarantee the wiring will remain as such or be remodeled. Please show me the code section referring to non coincidental loads specifically to 310.15. Do we have to prove the 5 of say, 25 mc cables that are stacked, do not carry energy in order to eliminate derating? NEC 2008 310.15(B)(2)(a) exc 1 still states the derating factor still applies to power & lighting circuits. Your point of if there are any adverse effects due to the non coincidental loads is understood. Code does not support your interpretation specifically.:happyno:

 

[charlie b]

The code, as written, does support my viewpoint. :happyyes:

A conductor that cannot carry current because it is physically disconnected, and cannot be connected, is not a CCC. The NEC does not specifically define "current carrying conductor," but I think it fair to say that that term can be defined as a conductor that carries, or that at least has the possibility of carrying, current. I see this situation as being vastly different from having two single-phase circuits, one serving lights and the other receptacles, sharing a conduit. It is physically possible for both the lights and something plugged into the receptacle to be both carrying current at the same time. So even if there is nothing (at the present moment) plugged into the receptacle, the conduit still gets counted as having 4 CCCs. In the present installation, an interlock physically prevents both sets of conductors from being connected at the same time. The ones that are not connected are, by definition of CCC, not CCCs. Therefore, you don't get to read any deeper into the requirements of 310.15(B)(3)(a) than to recognize that the article does not apply because you don't have more than three CCCs.

 

[charlie b]

I'll address this point separately:

There is no guarantee the wiring will remain as such or be remodeled.

If they get remodeled, it is up to the person doing that work to ensure that the final configuration meets code. We don't design today for what someone will do tomorrow.

 

[Daja7]

= + =

Daja7,

What is the size and type of the raceway where these 6 conductors
are installed ?

+ = +

1 1/2 and the conductors are #6