Some wiring questions

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  #1  
Old 10-17-01, 11:32 AM
G
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A coupleof questions related to information required for code calculations. Romex NM-B cable sold at home improvement stores; are the conductors THHN. Need to know to determine derate factors. If not, can I get THHN 10/3 and 12/3 cable at an electrical supply store.

I am pulling several 10/3, 12/3, cables through PVC for homeruns for 2nd floor rewire, the code only gives cross sectional areas related to conduit fill calcs for individual conductors not 10/3 and 12/3 cable. I need the effective cross sectional area of 10/3 and 12/3 wire. Thanks in advance.
 
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  #2  
Old 10-17-01, 03:16 PM
W
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Type NMB cable has THHN 90 degree C conductors in that cable. The only time you can use the 90 degree C column in Table 310-16 is for deration calculations. Once you are done with you deration calculations as per 310-15-B-2-A and the temp deration at the bottom of the chart in table 310-16 you will have to compare your answer to Article 240-3-D concerning wires 10 awg or smaller. If you calculation is with lower ampacity rating than 240-3-D then you must use your derated ampacity for those conductors. However if 240-3-D has a lower ampacity rating than your calculated derated answer then you must use the smaller of the two being the rating in ampacity found in 240-3-D.

Boy when you are stuffing Romex cables in a conduit you will find that 310-15-B-2-A is rough on you in derating those wires quickly per Romex cable added no matter how overlarge your conduit is than is required.

You asked how to caculate Conduit fill when using Romex cable. See the following copy of that section of the NEC that dictates that calculation;

Chapter 9 note # 9
9. A multiconductor cable of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.

Chapter 9 Table 1 controls the maximum amount of fill allowed per conduit. Conduits that contain 2 conductors in a raceway may be filled a maximum of 40 % of the Cross sectional area of the conduit found in Chapter 9 table 4 second to the last column from the right in each chart. Notice that Table 4 of Chapter 9 lists each type of conduit. You must use the table assigned for use with the conduit you are using.

Hope this helps

Wg
 
  #3  
Old 10-18-01, 06:00 AM
J
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If running wire in conduit it makes a whole lot more sense to use individual conductors. They pull easier and as Wg said, the derating is kinder. Often there is no derating to consider. If you intend to split off in different directions with NM-B from where the conduit ends, terminate the conduit run at a junction box. Then install the NM-B at the box with a cable-to box connector (commonly called "romex connector") and go anywhere you want with the cable(s). If you already sunk the money into all cable, consider stripping the outer jacket off the cable and treating it as individual THHN conductors. Just a thought...

Juice
 
  #4  
Old 10-18-01, 03:31 PM
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Wgoodrich
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Juice when it pertains to temperature or number of current carrying conductors in a raceway the NEC does not care whether these conductors are in a conduit with single conductors ran or in a conduit within cables.

Now you are accurate if you were referring to conduit fill. If you run single conductors rather than cabled conductors you can put many more conductors in a raceway. Single conductors takes up much less room than cabled conductors

For example considering conduit fill a 12/2 romex is 1/2" wide. therefore you would have to calculate that one romex cable at 500 kcml calculating that 1/2" as circular as required. In a 1/2" EMT conduit you would be limited to one 12/2wground cable in that 1/2" EMT conduit considering that Romex cable as one conductor allowing only a 50% fill at .161 area square inch. Only one Romex cable would be allowed in that 1/2" EMT conduit.

If you were installing 12 awg single conductor THHN which is the same insulation as the conductors in type NMB Romex, the single 12 awg conductors would be calculated at .0133 in a 1/2" conduit at 40% fill at .122 you would be allowed a maximum of 9 - 12 awg conductors using tables 4 and 5 in Chapter 9 of the NEC. If you used Appendex C as allowed using all the same gauge and type of insulation you would be allowed the same 9 conductors in that EMT conduit.

As you can see running single conductors in a raceway versus running cables in a raceway shows a lot of difference in capacity considering current carrying conductors. However when deration calculations due to temp or more than 3 current carrying conductors in a raceway it makes no difference whether the conductors are single conductors or cabled conductors.

Good Luck

Wg
 
  #5  
Old 10-19-01, 01:47 AM
J
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Wg, I have become confused. You said "...Boy when you are stuffing Romex cables in a conduit you will find that 310-15-B-2-A is rough on you in derating those wires quickly". Now you say deration is not an issue by saying "...when it pertains to temperature ...NEC does not care" if cable of individual conductors. I can't reconcile this contradiction. Enclosing your individual conductors in an overall sheath, and then further enclosing them in a raceway, one will reduce heat dissapation of the conductors.


I initially was thinking of the different criteria with which you size your conduit with a wide, flat NM-B cable, as opposed to cross sectional area. (Your explanation of treating a cable as an individual conductor covered this well.) This is where folks give themselves the shaft choosing cable over individual conductors in raceway.

Here again, you confused me when you said "...when it pertains to ...number of current carrying conductors in a raceway the NEC does not care whether these conductors are in a conduit with single conductors ran or in a conduit within cables." And I was thinking of, say given a 1" conduit in either case, cable of individual conductors, you certainly cannot have as many current-carrying conductors if using a manufactured cable system. No, the NEC doesn't care whether you put cable or individual conductors in a conduit, so long as you have larger conduit for the same number and gauge of conductors if using cable over individual conductors.

I think we both know what we mean and are correct, we're just not connecting language-wise somehow. Just wanted to communicate further to see that we're on the same page with these two aspects of the issue.

Juice
 
  #6  
Old 10-19-01, 12:12 PM
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Juice there is no provision to adjust ampacity higher or lower when current carrying conductors are contained within a cable and then within a conduit. Deration of ampacity due to number of current carrying conductors does not speak of cable sheath at all. However if you are running say 12/3 cables for use as a part of a three way switch each 12/3 cable you have in that conduit would have 3 current carrying conductors in that conduit. If you ran three 12/3 cables for switching in a conduit you would suddenly have 9 current carrying conductor in that conduit. The statement I said was in referrence of the person posting was wanting to instal 5 - 10/3 cables in that conduit. That doesn't sound like much but with using the whites as neutrals he still had 10 current carrying conductor in that conduit. What I was trying to point out was that the number of current carrying conductors add up quickly when installing multi-conductor cables in a conduit. That was all.

As for temperature deration, this is the ambient temperature that is being considered, not the temperature of the conductors in the conduit. Ambient temperature is the room or area temperature of the location of those wires. Such as boiler rooms, or attics, or to your advantage a freezer, etc. Temperature deration has nothing to do with whether there is single conductors in a raceway or sheathed non metallic cables in the conduit. Temperature deration only relates to ambient [room] temperature the conductors are installed in.

As for conduit fill A flat 1/2" wide Romex cable must be counted as a 1/2" round conductor. A 500 Kcml conductor is approximately a 1/2" round conductor. Take a 1/2 conduit you would only be allowed one flat 14/2wGrnd cable in that conduit as per table 4 and 5 of the NEC. When it comes to conduit fill by counting those flat cables as round as required by the NEC fills the capacity of that conduit very quickly.

I guess what I said before probably could have been said in a better manner saving confusion in my intent.

Both considering number of current carrying conductors in a raceway deration and conduit fill capacity calculation, a Romex cable jumps the numbers up to maximum very quickly causing deration due to numbers [multi current carrying conductors per cable] and due to filling the conduit by counting the area square inch of that flat Romex as round.

Ambient [room] temperature deration would make no difference between single conductors or cabled conductors.

Does that help?

Wg
 
  #7  
Old 10-20-01, 03:39 AM
J
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Yes, that helps, Wg. Thanks again.

Juice
 
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