code questions, direct bury vs conduit for outdoor rated wire (THWN-2)

[df0rster]

I have been reading and a lot of people will use direct bury wire inside pvc conduit. However I've already read that NEC does now allow direct bury rated wire inside conduit unless for 24" or less. A local electrician also told me to use only individual wires inside conduit to avoid heat build up.

My 2 DC runs will be about 180', 10-325w modules each string, 6 awg wire, and I would prefer to run inside conduit with outdoor rated direct bury wire but not sure if this is allowed. And there may be different rules that apply for DC runs.

If not allowed to use conduit, can I direct bury the DC runs if the wire is rated for this?

[rbclark]

Your direct bury wire will be rated as UF. or USE. I imagine you will need to use USE to bet large enough conductors as UF seems limited in size where I am.

[df0rster]

Your direct bury wire will be rated as UF. or USE. I imagine you will need to use USE to bet large enough conductors as UF seems limited in size where I am.

I was just looking at 6/2 Romex it is UF wire. So it seems I need to bury it 24" with vertical conduit 18" below the grade only.

Is there any code that will allow the use of conduit in a shallower trench? This will be up to 600v DC.

[SunEagle]

I was just looking at 6/2 Romex it is UF wire. So it seems I need to bury it 24" with vertical conduit 18" below the grade only.

Is there any code that will allow the use of conduit in a shallower trench? This will be up to 600v DC.

The code will require you to use either steel conduit or concrete encased conduit if you want to put it shallower than the Direct Burial distances. It all comes down to protecting both the wire from getting crushed and shorted or people that may come in contact with energized wires.

[df0rster]

The code will require you to use either steel conduit or concrete encased conduit if you want to put it shallower than the Direct Burial distances. It all comes down to protecting both the wire from getting crushed and shorted or people that may come in contact with energized wires.

Ok, that's the answer I was after.

Is it worth it for me to buy a copy of the NEC book? Or is there a simpler reference book that lays out the applicable codes I'll need to follow for a DIY solar install?

[foo1bar]

NEC table 300.5 is the reference for trench depth possibilities.

I don't know of a code section that says you can't use UF cable inside a properly sized (meets conduit fill requirements) conduit.

But I think I'd probably take the approach of using conduit and THWN wire. (I expect UF is more expensive)

If it's a significant distance, make sure you look at your voltage drop as well as ampacity for sizing your wires.

[foo1bar]

You can access NEC online via their website for free.

I didn't spend the money on a paper version nor for the PDF. But that was my choice on how to spend my money - if you choose to spend your money on it, IMO it's your money to spend as you see fit.

[df0rster]

NEC table 300.5 is the reference for trench depth possibilities.

I don't know of a code section that says you can't use UF cable inside a properly sized (meets conduit fill requirements) conduit.

But I think I'd probably take the approach of using conduit and THWN wire. (I expect UF is more expensive)

If it's a significant distance, make sure you look at your voltage drop as well as ampacity for sizing your wires.

In this case, the UF is much cheaper. I found a good deal on a 500' roll for about $.70/ft for 6/2. buying the thwn was not even an option at Lowes. Not a lot of choices around here for electrical supplies.

[Sunking]

In this case, the UF is much cheaper. I found a good deal on a 500' roll for about $.70/ft for 6/2. buying the thwn was not even an option at Lowes. Not a lot of choices around here for electrical supplies.

If your town or city is large enough for a Blue Box Store, then you likely have an Electrical Supply House where EC's get their material.

[df0rster]

If your town or city is large enough for a Blue Box Store, then you likely have an Electrical Supply House where EC's get their material.

I wish, but my town is not large enough for either. We have farm supply stores like Tractor Supply. I usually have to drive 45 minutes to Lowes after work when all of the electrical supply stores would be closed. So I'll end up doing my shopping online and then either getting it delivered, or pickup up in the evenings.

[organic farmer]

Anytime I dig a hole a foot deep, the next morning the hole will have at least 6" of standing water.

For my grid-power, I ran 4/0 direct bury cable inside 2" pvc conduit, buried 4' deep, from our power pole/meter 100 yards to our house. After 4 1/2 years that wire shorted. Both the 'line' wires shorted to each other, and they both shorted to ground. So I did it again; new wire, new pvc conduit in a new trench. 5 years later that wire shorted. The best that we could figure out is that the conduit flooded with ground water, and the water migrates through insulation.

My driveway is built-up of crushed rock powder [the gravel yard nearby calls it 'crusher dust'] 2 to 3 foot above grade of the surrounding forest floor. For the third power line I installed, I laid the conduit on the shoulder of my driveway and I buried it with a foot of additional crusher dust. This latest powerline we hope will stay fairly dry.

The DC power from our photovoltaic array leading into our house is MC-4 cable inside 2' pvc conduit in a trench.

When I built the mount to support our photovoltaic array, I dug three 70' trenches, and I had to keep an Ag pump running to keep the water out of those trenches, to allow us to fill the trenches with concrete.

[bcroe]

I have bought a lot of wire on line, and generally saved money in the process. When a
specific number of feet of 1/0 or 2 gauge was needed, Lowes cut off exactly my request.
Bruce Roe

[df0rster]

Anytime I dig a hole a foot deep, the next morning the hole will have at least 6" of standing water.

For my grid-power, I ran 4/0 direct bury cable inside 2" pvc conduit, buried 4' deep, from our power pole/meter 100 yards to our house. After 4 1/2 years that wire shorted. Both the 'line' wires shorted to each other, and they both shorted to ground. So I did it again; new wire, new pvc conduit in a new trench. 5 years later that wire shorted. The best that we could figure out is that the conduit flooded with ground water, and the water migrates through insulation.

My driveway is built-up of crushed rock powder [the gravel yard nearby calls it 'crusher dust'] 2 to 3 foot above grade of the surrounding forest floor. For the third power line I installed, I laid the conduit on the shoulder of my driveway and I buried it with a foot of additional crusher dust. This latest powerline we hope will stay fairly dry.

The DC power from our photovoltaic array leading into our house is MC-4 cable inside 2' pvc conduit in a trench.

When I built the mount to support our photovoltaic array, I dug three 70' trenches, and I had to keep an Ag pump running to keep the water out of those trenches, to allow us to fill the trenches with concrete.

Interesting. My main service (200 amp)wire is 4/0 200' and the PoCo said this is as long as they will allow (it is aluminum). It is direct buried w/o conduit at 24"+. It hasn't shorted yet, but it isn't sitting in water constantly. I wish I had put it in conduit. From what I have seen it is very hard to unroll and install 200+' of wire without nicking something at least a little bit, even if its not all the way through it still creates a weak spot.

[SunEagle]

Interesting. My main service (200 amp)wire is 4/0 200' and the PoCo said this is as long as they will allow (it is aluminum). It is direct buried w/o conduit at 24"+. It hasn't shorted yet, but it isn't sitting in water constantly. I wish I had put it in conduit. From what I have seen it is very hard to unroll and install 200+' of wire without nicking something at least a little bit, even if its not all the way through it still creates a weak spot.

My house power feed is also a 200 Amp rated AL Direct Burial cable from the pole that runs about 180 feet to my meter. Since I am at an elevation of about 170 feet and the soil is very sandy I doubt I will every have an issue with water permeating the cable insulation.

I feel bad for those that live in a high water table area like organic farmer. It almost makes sense to go over head instead of U/G for any power runs.

[DanS26]

In this case, the UF is much cheaper. I found a good deal on a 500' roll for about $.70/ft for 6/2. buying the thwn was not even an option at Lowes. Not a lot of choices around here for electrical supplies.

This may not apply to you......but 6/2 UF has a #10 ground wire.....thus it may not be adequate in your application. Check the code again but I think you need a #6 ground wire.