PVC Conduit Fill Rating?

[pbs]

Hi: Can #12 THHN be run through 1' gray PVC Electrical conduit?

If Yes, what is the code fill rate for 1" PVC? ( I believe that 1" EMT can handle 18 #12 THHN maximum.)

Thanks

[gfretwell]

Table C10 says 25 #12 THHN will fit is 1" sch 40 PVC rigid nonmetalic conduit

[pbs]

Thanks. I had also intended to ask if the PVC conduit is permitted for interior residential applications (this would run up through a chase)

[Pete C.]

That would likely be a local code issue, it's not something in the NEC. I doubt you'd have an issue since PVC DWV stacks do the same thing with the same materials.

Pete C.

[PipeDown]

I found that PVC ENT systems have far fewer choices when it comes to J-boxes and those J-boxes tended to have a different standard for hole position so that you had to use the more expensive covers.

Metal may be a bit more work cutting the tubing and connecting ground pigtails at each J box but you will have lots more options at the ends of the tubing to put those fixtures on. Transitioning to cable (romex) is easier too using a simple clamp to hold the cable.

What money you may save using PVC conduit you will loose buying more expensive plastic J boxes, elbows and tees.

On the other hand, you can drill a 3/4" hole in the side of a plastic J box and thread a 1/2" male adapter to create more options (3 and 4 way J-boxes for example). Don't know if that is to code but I don't really see a reason for it not to be as long as you observe other rules about fill and glue all connections.

[Tom Horne, Electrician]

Actually One inch Electro Metallic Tubing has a table fill of 26 Number

12 American Wire Gage (AWG) conductors. Schedule 40 PVC is allowed 25 conductors. The question then becomes why. With more than 20 conductors in the raceway the ampacity of #12 THHN is 13.5 amperes. so #12 could not be used for even a fifteen ampere circuit. You're allowed fifteen #10 AWG wires in 1" PVC and at an allowable ampacity of twenty amperes for 10 to 20 conductors you could use them for twenty ampere circuits.

[volts500]

Table C10 allows 25 THHN conductors in a 1 inch schedule 40 conduit. That's how many THHN conductors that are PHYSICALLY permited. More important is DERATING of those conductors. IOW one _CANNOT_ pull 25 THHN conductors in a 1 inch PVC conduit and expect that they will be rated for the full allowed 20 amps.....they won't. One needs to look at Table 310.15(B)(2)a for conductors with no load diversity (All conductors in the raceway are loaded to their maximum rated load) or Table B.310.11 in Annex B for conductors with 50% load diversity.

Both Tables are the same until one gets to over 10 current carrying conductors. #12 THHN can be derated from the 90 degree column in Table

310.16 which is 30 amps, HOWEVER the #12 conductors _CANNOT_ be fused at over 20 amps. For derating, an equipment grounding conductor is not counted as a current carrying conductor Also, for Residential Single Phase 120/240, a true neutral (shared between two hots from opposite legs) is not counted as a current carrying conductor. A "neutral from a two wire circuit _IS_ counted.

In your case (from the above Tables) for #12 THHN conductors:

4 to 6 conductors must be derated at 80% 30 amps x .8 = 24 amps 7 to 9 conductors must be derated at 70% 30 amps x .7 = 21 amps.

You can safely install 9 current carrying #12 THHN conductors in that conduit.

To install more conductors, the Tables differ.

With no diversity (fully loaded):

10 to 20 conductors must be derated at 50% 30 amps x .5 = 15 amps Now there is a problem!!!!!! If you install more than 9 current carrying conductors (up to and the physically allowed 20 conductors) you MUST either use 15 amp breakers on the #12 THHN conductors or increase the size of ALL wires to #10 THHN since, from the 90 degree column in Table 310.16 one can derate from 40 amps for #10 THHN: 40 amps x .5 = 20 amps (you must still use a 20 amp breaker.)

HOWEVER, if load diversity is 50% (from Table B)

10 to 24 conductors must be derated at 70% 30 amps x .7 = 21 amps.

You must ALSO derate for ambient temperature, especially if more than

10% of the conduit is in an attic. Table 310.16 is only good for ambient temperatures of 30 C (86 F)

To be safe, I would strongly recommend that you use #10 conductors in the 1 inch conduit(s) if you are going to use 20 amp breakers. If you set J-boxes in the basement and the attic, you can use the existing #12 Romex elsewhere, just use #10 in the conduit(s).

This is one reason why Romex is used in houses......it is inherently derated for use in attics, basically being dummy proof as long as one stays with the common #14 Romex = 15 amp breakers, #12 Romex = 20 amp breakers and #10 Romex = 30 amp breakers. When one starts installing conductors in conduit, ya gotta watch your P and Q's.

[pbs]

[pilaschidt]

So then it would make more sense to run 20 #20 wires, and run ONE amp thru each wire (with a one amp breaker on each wire). You'd still end up with 20amps at the end of the run.

Pila

[Thomas D. Horne, FF EMT]

Please don't feed the troll people. That will just encourage it.

[-------------]

The "Troll People" ? Is that a new band like the Village People?