pulling wire; what gauge?

[lukati]

To install a 240V/40A circuit to my garage, my electrician recommends running two #8 hot wires and one #10 ground wire through a to be installed underground conduit at a depth of 6 inches below ground. He didn't say anything about the type or size of the conduit or the type of wire. I assume it will be THWN rated wire through a 3/4" rigid metal conduit.

Note that you are only allowed one circuit or feeder per structure (under most conditions). So a single circuit for your EVSE will be fine if your garage doesn't already have an electrical circuit or feeder. If it does, you need to run a single feeder for all the electrical loads, which would require including a neutral conductor and installing a grounding system at the detached garage, if there isn't one already.

Thanks for the information! I already have a buried 1/2" conduit with four #12? wires (1 hot, 2 travelers, 1 neutral) for the 120V/15A garage feed. My electrician didn't mention anything about this ~30 year-old conduit needing to be joined with the new one. He did say that pulling through the old conduit wasn't going to work. So if a new all-inclusive conduit needs to be installed, should it have 2 #8 hots, 1 #10 neutral, 1 #10 ground, and 2 #12 travelers? And be at least 1"? All on the same 40A breaker? I don't know anything about a grounding system at the garage.

Is there any advantage using rigid vs intermediate metal conduit or going to a smaller or bigger size? Thanks.

For this application, I don't seen any advantages for rigid versus intermediate, so just use whatever is cheaper, probably rigid. Plus, depending on the distance, it might be noticeably easier to pull your conductors through a 1" conduit than through a 3/4" conduit.

The conduit is about 25 feet. Sounds like I need more than 1", given the number of wires that need to be pulled. Can you still bend that by hand?

 

[wwhitney]

So now two things confuse me about what you said. I assumed that all four wires would be in a single cable (you know, standard for that application, pick your service needs, size it up and purchase all in one like the 7 wire cable I used for my sprinkler valve controls). However, it sounds like you are choosing individual wires and running all four down the conduit. Is either method acceptable?

Either method is acceptable, but if you have gone to the trouble of running a complete conduit system, then it is advantageous to run individual conductors. It may be cheaper, it gives you greater flexibility in choosing the conductor sizes, and it is probably an easier pull (more flexible, no outer jacket). Plus the cable will have a larger area than the sum of the areas of the conductors, so the cable might exceed the allowable conduit fill. The main advantage of a cable is in using direct burial cable where you don't need a complete conduit system, just the sweeps (large radius 90 degree bends) where the cable emerges from grade for physical protection of the cable.

Second question on the ground. I'm not planning on doing any of this work, just interested. Yes, I am having a new foundation poured and assumed that a ground would be required. So I would assume that the ground spike is connected to the sub-panel ground bus.

If you are pouring a new foundation, and part of the footing is in direct contact with the earth, then you need to use the foundation footing as one of your electrodes (a concrete encased electrode, or CEE). Once you have the CEE, you don't need any more electrodes, so you don't need a ground rod. Now if your entire footing is separated from the earth by plastic, then it doesn't qualify as a CEE, but that is a good reason not to do that. To connect the CEE, you connect your Grounding Electrode Conductor to either 20' of #4 or larger rebar, continuously tied together, or to a 20' #4 Cu conductor embedded in the concrete.

Then the ground wire in the conduit must connect the main panel ground bus and sub-panel ground bus. Is that correct? Boy that seems like a simple question now that I re-read it, so I guess it shows how simple my mind really works.

That is correct. In particular, in the panel in the detached structure, you keep grounds and neutrals separate.

Cheers, Wayne

 

[wwhitney]

Thanks for the information! I already have a buried 1/2" conduit with four #12? wires (1 hot, 2 travelers, 1 neutral) for the 120V/15A garage feed. My electrician didn't mention anything about this ~30 year-old conduit needing to be joined with the new one. He did say that pulling through the old conduit wasn't going to work. So if a new all-inclusive conduit needs to be installed, should it have 2 #8 hots, 1 #10 neutral, 1 #10 ground, and 2 #12 travelers? And be at least 1"? All on the same 40A breaker?

OK, first, your 30 year old conduit should have a ground in it, unless it is a metallic conduit which serves as the ground. Second, if the only thing that your 120V/15A circuit serves in the garage is a light that is controlled from both the garage and the main house, then that is one exception that would allow you to have more than one circuit feeding the garage, and you could just run your 40A EVSE circuit as you planned. However, since your conduit has a hot in it, I expect that it also serves other loads, like receptacles, in which case you can't do that.

Now if your existing conductors to the garage really are #12 Cu and are in good condition, you could run a 16A 240V EVSE on your existing wiring, with everything in the garage on one 240V/120V 20A circuit. That would require repurposing one of your travelers for an extra hot (and running that hot phase from the house panel to where the traveller currently originates) and controlling your light via some other method, like RF switches or perhaps a pair of smart switches that can communicate over only one "traveler" (if such things exist). And you couldn't run any other significant load while the EVSE is charging. I don't really recommend this idea.

So that brings us to running a new feeder, and moving everything other than the light over to the feeder. I'd just keep the light on the existing circuit and utilize the existing travelers, since that is an allowed case of a second circuit. Your feeder options are:

Direct burial UF cable, needs to be 24" deep (18" under a driveway), 60C rating, so for Cu #8 = 40A, #6 = 55A
Non-metallic conduit, needs to be 18" deep, can use 75C rated conductors, for which Cu #8 = 50A, #6 = 65A
Rigid conduit, only needs to be 6" deep, except 18" under a driveway, can use 75C rated conductors.

If you want a 40A EVSE circuit, then you'll need a minimum 55A feeder. You could also install a 20A EVSE, since that is all the LEAF can use, and then you'd just need a minimum 35 A feeder. Note that if you conductor ampacity is not a standard breaker size, then you can go up to the next higher breaker size, as long as the load served is less than the ampacity.

In either case, you'll need a #10 ground. A #10 neutral is the minimum size in each case, and will be plenty since the EVSE doesn't use the neutral. [Correction: if you use #6 Cu 75C conductors in conduit on a 70A breaker, you'll need #8 neutral and ground.]

I don't know anything about a grounding system at the garage.

If you only have one circuit to the garage, you don't need a grounding system. As soon as you have circuit breakers in the garage, then you need a grounding system. The simplest thing to do is to drive two 8' ground rods near the garage and run a grounding electrode conductor to the new panel in the garage. Grounds and neutrals are kept separate in that panel.

The conduit is about 25 feet. Sounds like I need more than 1", given the number of wires that need to be pulled. Can you still bend that by hand?

I think 1" conduit would be fine if you just have two sweeps and no other bends, but I don't have any actual experience with it. If you are using rigid, I believe it can be bent by hand, but I wouldn't try, I would just use manufactured sweeps, threaded fittings, and if I had to cut a piece, compression fittings..

 

[aqn]

FWIW...

JThe panel is at least 50 feet from where the outlet will be.

AFAICT, for 110V charging we need:
- 20A breaker
- 12 gauge wire preferred (rather than 14), to reduce voltage loss
- standard polarized, grounded outlet (NEMA 5-15)

When I first got my LEAF, I charged on a 15A outlet in the garage with a 50-foot 12-gauge extension cord. Did that for three months with no ill effects.

If we mod or buy a 220/240V EVSE, we would need:
- 40A breaker
- 8 gauge wire?
- whatever outlet the EVSE needs

My Schneider is hardwired to a 40A breaker. The panel is on the other side of the house from the garage, probably 40-50 feet apart. The electrician used "8/3 with ground" cable (four conductors altogether). I've been charging on the Schneider for about a month now, with zero problems.

 

[highcountryrider]

The electrician used "8/3 with ground" cable (four conductors altogether). I've been charging on the Schneider for about a month now, with zero problems.

You only really need 8/2, at least for the AV unit. I used 8/3 as well because my local
hardware store doesn't carry 8/2. I have a spare white wire.

 

[lukati]

OK, first, your 30 year old conduit should have a ground in it, unless it is a metallic conduit which serves as the ground. Second, if the only thing that your 120V/15A circuit serves in the garage is a light that is controlled from both the garage and the main house, then that is one exception that would allow you to have more than one circuit feeding the garage, and you could just run your 40A EVSE circuit as you planned. However, since your conduit has a hot in it, I expect that it also serves other loads, like receptacles, in which case you can't do that.

It is unclear what the underground conduit is. The electrician suspects that the cable is directly buried and the ground connected to the thin wall tubing coming out of the ground. Apparently buried EMT conduit was never code. Pulling on a wire would give the answer. The hot serves additional garage lights, door opener, and outlets.

Now if your existing conductors to the garage really are #12 Cu and are in good condition, you could run a 16A 240V EVSE on your existing wiring. I don't really recommend this idea.

I proposed that to the electrician but he didn't think it would be a good idea. No point doing anything borderline workable. Better be safe. .

Your feeder options are:
Direct burial UF cable, needs to be 24" deep (18" under a driveway), 60C rating, so for Cu #8 = 40A, #6 = 55A
Non-metallic conduit, needs to be 18" deep, can use 75C rated conductors, for which Cu #8 = 50A, #6 = 65A
Rigid conduit, only needs to be 6" deep, except 18" under a driveway, can use 75C rated conductors.

Rigid conduit would require the least digging. I'd like to run thermally insulated 1" PEX tubing along the electric conduit through the trench for my planned thermal solar panels on the garage roof. For that purpose I also need to run some low voltage wiring to measure the temperature in the panel. Is it code to add that to the conduit or do I need to run a separate conduit? If I need a separate conduit I would like to use PVC tubing. I assume the 18" depth requirement for PVC is to protect it from digging damage and would not apply to low-voltage wiring. Is this correct?

I think 1" conduit would be fine if you just have two sweeps and no other bends, but I don't have any actual experience with it. If you are using rigid, I believe it can be bent by hand, but I wouldn't try, I would just use manufactured sweeps, threaded fittings, and if I had to cut a piece, compression fittings..

Will do. Very useful advice. Thanks, Wayne!

 

[wwhitney]

It is unclear what the underground conduit is. The electrician suspects that the cable is directly buried and the ground connected to the thin wall tubing coming out of the ground. Apparently buried EMT conduit was never code. Pulling on a wire would give the answer.

Given that information, I suggest just abandoning the old wiring and pulling two new #12 Cu travelers and a #12 Cu neutral in with your feeder. It will be very little additional trouble or expense.

Rigid conduit would require the least digging.

PVC conduit would require more digging, but is quite a bit cheaper, so it is a tradeoff. In either case, I suggest 2 #6 hots, 1 #10 ground, 1 #8 neutral, and 3 #12 conductors for the switched light circuit, with a 60 amp breaker for the feeder. Per NEC Chapter 9 for THWN or THWN-2, #6 has an area of 0.0507 in^2, #8 0.0366, #10 .0201, and #12 0.0133, for a total area of 0.1981 in^2. Schedule 80 PVC (required when PVC conduit is exposed to damage) in 1" trade size has an allowable fill of 0.275 in^2; 1 1/4" has an allowable fill of 0.495 in^2. Rigid Conduit in 1" trade size has an allowable fill of 0.355 in^2 ; 1 1/4" has an allowable fill of 0.610 in^2.

I'd like to run thermally insulated 1" PEX tubing along the electric conduit through the trench for my planned thermal solar panels on the garage roof. For that purpose I also need to run some low voltage wiring to measure the temperature in the panel. Is it code to add that to the conduit or do I need to run a separate conduit? If I need a separate conduit I would like to use PVC tubing. I assume the 18" depth requirement for PVC is to protect it from digging damage and would not apply to low-voltage wiring. Is this correct?

This gets into the requirements of NEC Article 725 and Article 800, which I am not so familiar with, so you may wish to take a look yourself. In any case you need to keep the conductors separate from the feeder conductors. If these are telecommunications conductors or class 2 or class 3 power limited conductors, then the burial depth requirements don't apply. If they are class 1 power limited conductors, they do apply.

Cheers, Wayne

 

[wwhitney]

I suggest just abandoning the old wiring and pulling two new #12 Cu travelers and a #12 Cu neutral in with your feeder. It will be very little additional trouble or expense. . . . In either case, I suggest 2 #6 hots, 1 #10 ground, 1 #8 neutral, and 3 #12 conductors for the switched light circuit, with a 60 amp breaker for the feeder.

Whoops, I forgot the downside of running a feeder and another circuit within the same conduit--once you exceed 3 current carrying conductors in a conduit or cable, you need to derate the ampacities of all the conductors. In your case, if you use conductors with a 90C wet insulation rating for your feeder hot and neutral, e.g. THWN-2, then you will still have a 60 amp capacity. The ground is unaffected (not a current carrying conductor in normal circumstances), and the #12 conductors have a lot of reserve capacity with respect to derating, so they are fine with a 75C wet insulation rating, e.g. THWN.

Cheers, Wayne

 

[lukati]

This gets into the requirements of NEC Article 725 and Article 800, which I am not so familiar with, so you may wish to take a look yourself. In any case you need to keep the conductors separate from the feeder conductors. If these are telecommunications conductors or class 2 or class 3 power limited conductors, then the burial depth requirements don't apply. If they are class 1 power limited conductors, they do apply.

I looked into this. The temperature will be measured using a simple thermistor on a ~3-5V circuit fed with AA or AAA batteries. This is clearly not class 1, so burial depth requirements don't apply. It looks like the wiring doesn't even need to be in a conduit code-wise. I am thinking of running speaker wire in flexible PVC tubing.

 

[wwhitney]

It looks like the wiring doesn't even need to be in a conduit code-wise. I am thinking of running speaker wire in flexible PVC tubing.

Sounds good as long as you use a wet location rated wire. Good luck with your project.

Cheers, Wayne

 

[Reddy]

Wayne: Thanks so much for all the info. I'll let you know how it all works out.

Reddy

 

[lukati]

Wayne: Thanks so much for all the info. I'll let you know how it all works out.

+1!!

 

[lukati]

I finished the garage upgrade today. New underground conduit and wires. Everything exceeding code. Also installed two runs of highly insulated 1" PEX tubing in the same trench for a future solar thermal project on the garage roof. All 120V circuits and outlets working. 240V outlet installed and working, ready for the 240V/16A modified EVSE. A fun and educational DIY project. Thanks again Wayne for all your help and patience.