Conduit Fill calc w/ different size conductors

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gwomanness

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I have a 3/4" sch 40 PVC conduit under driveway 18' long. Can I put 3 #8 and 1 #10 bare conductors for a 30 amp circuit about 45' run PLUS a 20 amp circuit with 3 #10 and 1 #12 ground for a 20 amp circuit in the same 3/4" without problems? It would sure save me a LOT of effort.

What about removing one of the #8 wires since it would be an extra in case expansion is needed later.

The 30 amp will only be used very rarely for an RV. The 20 amp run will have 3 outlets and be about 120' run.

To summarize; 3 #8, 1 #10 bare ground, 3 #10 and 1 #12 in a 3/4" PVC 18' long.

Thanks in advance.

[ February 08, 2004, 03:18 AM: Message edited by: gwomanness ]
 
Re: Conduit Fill calc w/ different size conductors

Using tables 4 and 5 in Chapter 9, 3/4 PVC schedule 80 has .164 sq in available, and the conductors you want to install take .2075 sq in.
The conductors would probably fit in the raceway, but it is not permitted and would be a very tough pull - most likely damaging the conductors.
PVC schedule 40 has .203 sq in available, that is still not permitted by the NEC.

You are not required to install both equipment ground conductors. Remove the 12 AWG conductor and you will have a compliant installation for PVC schedule 40, not for schedule 80. Although it will still be a little tough to pull.

Pierre

[ February 08, 2004, 07:52 AM: Message edited by: pierre ]
 
Re: Conduit Fill calc w/ different size conductors

OK I will take a shot at this but I will have to make some assumptions which is always dicey at best. ;)

You said underground so conductors for wet locations are required by 300.5(D)(5), I will assume THWN conductors.

I will also assume you are using 8 AWG for 30 amps and 10 AWG for 20 amps for voltage drop, great idea but 250.122(B) requires that the EGC be increased in size in proportion with the size increase of the ungrounded conductors.

This means you will need a 8 AWG EGC for the 30 amp circuit and a 10 AWG EGC for the 20 amp circuit.

There is no reason you can not use the same EGC for both circuits.

So if I was doing this job I would be running;

3 - 8 AWG THWNs for the 30 amp circuit

3 - 10 AWG THWNs for the 20 amp circuit

1 - 8 AWG Bare for the grounding conductor.

From Table 5

8 AWG THWN = .0366 sq.in. x 3 = .1098 sq. in.

10 AWG THWN = .0211 sq. in. x 3 = .0633 sq. in.

From Table 8

8 AWG Bare = .017 sq. in. x 1 = .017 sq. in.

The conductors total .1901 sq. in.

Go to Table 4 section for Schedule 40 PVC and use the over 2 wires (40% fill) column and find the smallest raceway with over .1901 sq. in. listed.

Your in luck 3/4" PVC schedule 40 is listed as .203 sq. in. at 40% fill.

It seems you can fit in all that you need in the existing 3/4" PVC.

Anyone is welcome to jump in as I do not usually figure mixed conductor fill, I typically only have one size in the raceway.

When I have mixed sizes I just use the tables and size the raceway as it was all the larger size.

[ February 08, 2004, 08:03 AM: Message edited by: iwire ]
 
Re: Conduit Fill calc w/ different size conductors

Bob (Iwire), you are correct in your assumptions but I goofed up and left out the second (red) #8 which is not needed for this job anyway and still have a #10 and #12 EGC's. Any problems with this? I have never pulled one wire while other wires are already there. Do I need to go back and redo or am I OK with the two EGC's. I am using the #8 for the 30 amp RV and the #10 for the 20 amp.

Thanks for the info boys.

Elaine
 
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