Conduit Fill

newenergy

Table C.10 in the 2008 NEC (based on Table 1, chp 9) seems to be saying that you can put up to 5 THWN conductors in 1/2" Schedule 40 PVC.

As I understand it there would be a deration to 80% of the ampacity for conduit fill for 4-6 current carrying conductors.

Am I missing something? It just seems like 5 #10s is too much for 1/2".

It's not for a new installation. There's an existing, empty, 1/2" PVC conduit running through concrete.

JohnJ0906

Not every conductor is a CCC, necessarily. You might have a 3 ph MWBC (full boat) with 3 hots, a neutral and an EGC. 5 conductors, but only 3 CCCs.

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John from Baltimore
"One Day at a Time"
All replies based on the 2008 NEC

Speedy Petey

Sometimes it is too much, but it is allowed. That's just the way it is.

What John said about derating. Not every single conductor is counted as a CCC.

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newenergy

Thx - Just to be clear, the limit of 5, which is based on the cross section of the conductor and insulation doesn't care if the conductor is carrying current or not. Whether they carry current or not just affects the deration.

?

So, actually what I might possibly be putting through this conduit is:

2 10-THWN
1 10-bare copper
2 12-THWN
1 12 bare copper

From tables 5 and 8 I get the total area = (13.61 * 2) + 5.26100 + 3.31 + (8.58100 * 2) = 52.953 sq mm

and from table 4 sched 40 is 184 sq mm

and table 1 I can fill up to 40%, so I'm good

Seem right?

Is this a bad practice? The customer is expecting to have to tear things up and install new conduit, but I'm trying to save them some money as long as it's not a bad practice.

Last night I had a dream about pulling this wire. It wasn't that bad in the dream. Will it be a nightmare IRL?

Celtic

Why would you put 2 bare EGC's in a conduit?

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newenergy

One is for the Solar PV system. On the roof everyone uses bare copper. This may only be for ease of installation since you have to ground every single rail and every single module and you run them through lugs.

I was taught that it is a good practice to have the PV grounding be one continuous conductor all the way from the roof to the inverter. I know a lot of people don't do that in practice though because it is difficult to deal with that much wire on the roof.

690.43 - the grounding conductor must be contained in the same raceway as the circuit conductor.

The other wires would be for a 120v AC circuit. The ground wire for those doesn't need to be bare, but bare is smaller.

The grounding wire for the Solar needs to connect to the inverter.

Perhaps it would be better if the AC ground were insulated as I don't think any fault current from the AC circuit should go through the inverter.

Were you asking why I would use bare conductors or why I would have 2 grounding conductors instead of just one?

Celtic

Having 2 bare in one conduit seems odd - especially when they are of different sizes.

How does 1 insulated #8 size up against the bare 12 & 10?

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newenergy

A insulated 8 is considerably bigger than a bare 12 and 10 put together.

Capacity isn't the problem. A bare 10 would be adequate for either purpose and I don't know why it wouldn't be adequate for both.

The problem is that one ground wire is (according to manufacturer specs) supposed to go in a certain terminal in the inverter and the other is supposed to go to the grounding bus in the service panel.

In theory, I don't know why you couldn't bond the equipment ground for the AC circuit onto the ground for the DC circuit at each piece of equipment running along the conduit and then jump off it before the inverter and go into the service panel. I just wouldn't want to mess with the inverter wiring unless I was sure and even then I would probably want something from the manufacturer because something else could go wrong and they could blame it on the AC ground wire.

If I do change that one 12 bare to 12 THWN, I add (8.581 - 3.31) 5.271, giving me 58 sq mm, so I'm still good with that.

newenergy

Interesting discussion and I appreciate everyone's help, but in this case it's moot.

690.4(B) = I can't mix PV circuit with other circuit in a conduit.

Fortunately, I told the owners at first pass that they would need a new conduit run. I was just trying to figure if I could save them that.