Conduit for PV Wire

[zorrotm]

I'm running PV wire from panels 100-150 ferry underground 2'. I will have 8 10awg PV stands running in 3" pipe. It is rated for direct burial but for extra peace of mind I want to put it in conduit. Anyone see any negatives or code issues with using DWV drain PVC?

https://www.lowes.com/pd/3-in-x-10-ft-Sewer-Drain-Pipe-PVC-Sewer-Pipe/3133165

 

[pvgirl]

Plumbing pipe is not an approved wiring method per the NEC, and that would not be code compliant. With that many wires in one conduit or direct buried in a single trench the amp rating of the wire will be reduced 70%, if your wire is 90c rated 10awg would be derated to 28 amps. You will be fine as long as the required current rating is less then that, required current rating is the total of the Isc of the strings in parallel multiplied by 1.56. You also want a ground wire to be run with the PV wires to bond the panels, and metal panel mounts, to the inverter/charge controller and connected to the building ground system.

 

[zorrotm]

Plumbing pipe is not an approved wiring method per the NEC, and that would not be code compliant. With that many wires in one conduit or direct buried in a single trench the amp rating of the wire will be reduced 70%, if your wire is 90c rated 10awg would be derated to 28 amps. You will be fine as long as the required current rating is less then that, required current rating is the total of the Isc of the strings in parallel multiplied by 1.56. You also want a ground wire to be run with the PV wires to bond the panels, and metal panel mounts, to the inverter/charge controller and connected to the building ground system.

389V and 11amps per PV wire. So it is basically just derating the wire but since it is low amperage (running panels in series) it won't have an effect really?

I just don't like the idea of direct burying, seems too risky to me. We have a lot of rocks in our area and even if I use sand the rocks will move around. So it seems any PVC conduit would be a plus over none.

 

[pvgirl]

With that current you won't have to worry about derating.

If you want extra protection you can use electrical conduit and be code compliant. PVC electrical conduit is available, you can use schedule 40 PVC conduit underground but you will need to transition to schedule 80 PVC conduit before coming above ground. 3in is very large for the number of wires you have 2 inch would be cheaper and easier to find, 1.5 inch would be the minimum based on NEC conduit fill calculations,

 

[zorrotm]

With that current you won't have to worry about derating.

If you want extra protection you can use electrical conduit and be code compliant. PVC electrical conduit is available, you can use schedule 40 PVC conduit underground but you will need to transition to schedule 80 PVC conduit before coming above ground. 3in is very large for the number of wires you have 2 inch would be cheaper and easier to find, 1.5 inch would be the minimum based on NEC conduit fill calculations,

I figured the 3" size based on 8 wires * 0.280" OD from:

10 AWG Solar Photovoltaic Wire 2KV UL 4703 (PV Wire)

10 AWG Solar Photovoltaic Wire 2KV UL 4703 (PV Wire)

www.wireandcableyourway.com

It comes to 2.24" so seems like 2" would be too small or am I missing something?

 

[hwy17]

Fill is calculated by area not diameter. You also need a ground conductor so make it 9 for fill calc.

1.5" would be legal but hard to pull. 2" better. 2.5 or 3" is gonna add a lot of cost.

Conduit Fill Calculator | Southwire

Get accurate conduit fill percentages with Southwire's Conduit Fill Calculator. Perfect for electricians, engineers, and contractors.

www.southwire.com

 

[hwy17]

I wouldn't bother with 10 awg, 12 should be plenty.

 

[pvgirl]

Simple summing the diameters is not the correct way to determine what will fit in a circular area. Using a conduit fill calculator can figure out what you need. https://www.southwire.com/calculator-conduit

If we do the math manually we need to work in area taken up by a cable, 0.28 inch diameter wire has an area of 0.06158 square inches. 9 conductors(8+ground) of this wire is a total area of 0.55422 sq inches. Chapter 9 of the NEC https://up.codes/viewer/colorado/nfpa-70-2020/chapter/9/tables#9 has the allowable area for different sizes and types of conduit, in these tables we will use the 40% column since there are more then 2 wires in the conduit. If we look at the table for schedule 80 PVC conduit, we see that 1.5in is the smallest size that can hold our 0.554 sq inches. 3 inch conduit is more then 4 times bigger then we need.

 

[zorrotm]

Thank you both of you. I should have realized that. Makes sense now that i think about it ?‍

1 1/2" will be much cheaper. Great. One more step figured out. Appreciate it.

 

[hwy17]

Pull force is harder to calculate but something to consider, if you really want to pull 9 10 awg PV wires that might be pretty hard over 150ft in 1.5".

The 40% or so premium for 2" might be worth it. I don't have any experience pulling PV wire to say.

 

[hwy17]

And do you already have the PV wire? Personally I would pull THHN or direct bury PV wire, but not try to pull PV wire in any size. I don't know what's professionally common though, as legal as it is inspectors might not like to see it if it's rare in your area. If you are getting inspections I recommend trying to find out what they're expecting to see.

 

[pvgirl]

I would 2nd the recommendation to use THHN in the conduit if possible, it will be cheaper and doesn't have as thick insulation. You can transition to THHN at junction box at the array. Just be sure to verify the THHN wire you use has THWN rating as well so it can be used underground, most THHN sold today does. Don't forget to use plenty of wire pulling lube to make pulling easier.

 

[zorrotm]

I would 2nd the recommendation to use THHN in the conduit if possible, it will be cheaper and doesn't have as thick insulation. You can transition to THHN at junction box at the array. Just be sure to verify the THHN wire you use has THWN rating as well so it can be used underground, most THHN sold today does. Don't forget to use plenty of wire pulling lube to make pulling easier.

I don't have the PV wire yet, ordering it soon. What kind of wire do you recommend for running the ground from panels, underground in conduit to inverters? I assume it isn't supposed to be bare copper.

 

[pvgirl]

I don't have the PV wire yet, ordering it soon. What kind of wire do you recommend for running the ground from panels, underground in conduit to inverters? I assume it isn't supposed to be bare copper.

In the conduit you can use any 10AWG wire, such as THHN in a green color. Outside of the conduit where connecting to the array and mounting 6 AWG wire is required for protection from damage.

 

[Balrog006]

In the conduit you can use any 10AWG wire, such as THHN in a green color. Outside of the conduit where connecting to the array and mounting 6 AWG wire is required for protection from damage.

As long as that THHN is also dual rated THWN or THWN-2. Straight THHN is not approved for wet locations and buried PVC conduit is a wet location per code.

 

[Nobodybusiness]

With that many wires in one conduit or direct buried in a single trench the amp rating of the wire will be reduced 70%,

Can you explain that to me?

Why would 8#10 awg run together lose 70% ampacity ?

 

[hwy17]

Can you explain that to me?

Why would 8#10 awg run together lose 70% ampacity ?

Heat. 8 heated wires in a tube get warmer than 2.

I haven't checked the exact NEC derating table to confirm, but pvgirl is usually exactly right.

 

[Nobodybusiness]

Heat. 8 heated wires in a tube get warmer than 2.

I haven't checked the exact NEC derating table to confirm, but pvgirl is usually exactly right.

I used direct bury PV wire for my array but I didn’t derate them. Have 5 pairs. 5 pos 5 negative going to 5 MPPT channels.

So what your are telling me I could be getting more power by running # 8 instead for the same pairs?

Are you actually losing %70 or is that just a NEC thing?
I only had a 3% drop when I checked.

 

[hwy17]

I used direct bury PV wire for my array but I didn’t derate them. Have 5 pairs. 5 pos 5 negative going to 5 MPPT channels.

So what your are telling me I could be getting more power by running # 8 instead for the same pairs?

Are you actually losing %70 or is that just a NEC thing?
I only had a 3% drop when I checked.

8 count of wires, and derating is different than voltage drop. The NEC lets you run 70% of the current you could otherwise when they're together that way in conduit. It's about code approval, not actual performance.

 

[Balrog006]

Can you explain that to me?

Why would 8#10 awg run together lose 70% ampacity ?

It’s not a 70%loss. It’s a rating reduction to 70% of the max capacity for the conductor. If it’s basic rating is 10a then under the detecting situation rules the max you should expect it to be able to carry safely is 7a. It’s not about voltage drop either, that’s an efficiency/thru-put type of deal that has to do with how efficiently the length of wire is when trying to deliver what is sent based on its internal resistance based on size length and voltage.