Replies

1. 
   pm22 | Feb 16, 2005 01:33am | #1

   The conduit is obviously being used for protection.

   The Code has two concerns: heat buildup and ease of pulling. Not that they are really concerned about us poor electricians who have to pull the stuff but they don't want the wires damaged in the process. The jam ratio you speak of has to do with a certain combination of three cables. When you pull three cables around a bend, at the critical ratio, the trembling fear is that the top or outermost cable will squeeze in between the other two and jam up.

   I think the Romex would be considered one cable which would be 53% fill. Unfortunately, this dimension is determined by the total width of the Romex so you have to do the Pi x R squared thing and compare it with the inteneral area of the conduit which is 0.832" sq.". But I don't think you have to worry.

   As for the heat build up issue, you only have two current carrying conductors since the neutral white grounded conductor only carries the imbalance. And you get three free before you have to derate.

   ~Peter

   What's Tulsa spelled backwards?

1. 
   Taylor | Feb 16, 2005 01:53am | #2

   Thanks, I heard in another forum about the 53% fill, so I measured 6 AWG romex ascircumference ~2"
   radius 0.32" (R=C/2.pi)
   area 0.32 in^2 (A=pi.R^2)and 1" Sch 40 PVC conduit asradius 0.5"
   area 0.785which makes the romex 41% of the internal area of the conduit, so I am okay, even though it looks like I misunderestimated the area of the conduit (thanks for the figure on that).

1. 
   pm22 | Feb 16, 2005 03:04am | #3

   The nominal size of pipes is rather interesting. It started out with the nominal internal size of the pipe being more or less what it says and they made the couplings to fit the outside. Then they wanted thicker pipe for various reasons. So they made the walls thicker but kept the same couplings to keep it simple. Thus a Schedule 80 pipe has a thicker wall than a Schedule 40 pipe but the same outside diameter but then also a smaller inside diameter.

   What is the width of your Romex 6-3?

   ~Peter

   Will Pete Dragenic join our cause?

1. 
   Taylor | Feb 16, 2005 02:29pm | #8

   I measured width of romex 6-3 ascircumference ~2"
   radius 0.32" (R=C/2.pi) so diameter is about 2/3". I think I'm overestimating here, but wanted to be conservative. The wires BTW are twisted together rather than flat like say 14-2.

2. 
   CPopejoy | Feb 16, 2005 03:38am | #4

   Peter,The way I read your post, you implied that the neutral isn't a current-carrying conductor.Of course, the neutral (more correctly, the grounded conductor) in any circuit IS considered a current-carrying conductor for the purposes of determining conductor ampacity. The neutral is current-carrying, except in certain circumstances--in a three wire (Edison) circuit, the current in the neutral will be zero only if the values of the currents in each of the phase conductors are the same.The "three free" thing refers to a commonly-used table of ampacities in the Code--three current-carrying conductors in conduit OR cable. Not both.Also, whether the conductor is current-carrying or not has no bearing on the allowable raceway (conduit) fill.Cheers,Cliff

1. 
   DanH | Feb 16, 2005 06:08am | #6

   Dunno about the chapter and verse of code, but in a 3-wire, 240V circuit, there are effectively only two current-carrying conductors, from a purely technical point of view.Any current in the neutral must necessarily be subtracted from one of the other conductors, so any way you divide up the current you get the same effect of having only two conductors.

2. 
   pm22 | Feb 16, 2005 06:19am | #7

   I don't want to make waves about this thing and especially I don't want to go into the Code book because I've already passed my test and am sick of it. But my reasoning is this: he has 60 amps on his 6-3/g cable. The neutral carries the imbalance. Thus if the black carries 60 amps and the red carries 40 amps, then the white neutral would carry the balance or 20 amps. Each wire releases heat according to its amperage, which is what the derating is all about. Thus you would have 60 + 40 + 20 amps of whatever heat. If the circuits were balanced 60/60 then the neutral would carry 0 amps. The total would still be 120 amps of waste heat.

   But it doesn't matter since at most he has only 3 current carrying wires and he doesn't have to derate. I'm still waiting for the width of the 6-3/g Romex.

   Indeed, the neutral usually does carry some current which has been discovered by me by cutting it with the switch turned off but the lights were on in another room and I got a spark and the lights went out in the other room in a coincidence [as my cat would say when things fall down after he's just jumped on a typical pile]. But I was cheated out of the gratis wire stripping alteration to my dykes.

   ~Peter

   Queen Elizabeth II seemed to enjoy Sutter's Fort.

3. 
   decornut | Feb 17, 2005 02:36am | #9

   While in general the grounded conductor is considered current carrying, I think PM22 is correct in this specific example.  In my 1999 NEC, 310-15(b)(4)(a) says that " a neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted..." for derating purposes.

   Edited 2/16/2005 6:37 pm ET by r

2. 
   davemica | Feb 16, 2005 04:50am | #5

   Using the conduit for physical protection of the romex in this manner is perfectly appropriate. Stripping off the romex jacket is not necessary.
   Dave