Electrical – Wire Protection for Underground Line to Detached Garage

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I’m running a line from my 100amp main panel to a 40amp sub panel in my detached garage.

Can I run 8/3 NMD90 Romex through my the overhead joist system in my basement and out of the house through a hole in the block wall, then connect to individual sheathed 8ga wires in the 90 degree conduit box on the exterior of the block wall?

Pushing Romex through the buried (schedule 40 PVC, 3/4”) conduit underground seems difficult. I’d rather use individual wires inside the conduit all the way to the sub-panel, but individual wires through my basement joist system seems unsafe.

Is the 90 degree PVC conduit box the right place for that connection?

Also, do I need to run the PVC conduit through the block wall or is sheathed wire acceptable?

Best Answer

In Canada:

This corrugated flexible plastic conduit cannot be used as protection above or in ground where protection required, it's only allowed in concrete:

There may be variants that are allowed, and there may be other locations where it is allowed, thus you may see the blue variant a.k.a. smurf tube (Smurfs are blue) in basements.

Transition to THWN or NMWU can be in box outside against exterior wall. There is some discussion as to whether this is a matter of code or inspector tolerance.
Burial depth is 24in for NMWU direct burial or 18in in conduit or hard covered by e.g. 1x4 PT.

I am not an electrician but this is what I know from an electrician in Canada about a similar situation. I might have the subtleties wrong, but since this can be useful and debated in the comments, I am leaving it here as a supplemental answer rather than a comment.

Image from rona.ca

Overview

You'll install a 20 ampere breaker, in the panel in the house. From there you can either go straight into conduit, or you can run any other approved cable. From the question, it sounds like you want to use nonmetallic sheathed cable from the panel, which is just fine. You'll connect the 12/2 with ground NM cable appropriately in the panel, and run it out to a junction box near where the conduit will leave the house.

Then you'll run conduit from that junction box, outside, underground, then up and into the garage, ending at another junction box. You'll want to make sure you install expansion fittings where appropriate, especially if you're entering the buildings above ground.

Next you'll pull three, 12 AWG THWN wires through the conduit. Connect the wires from the NM cable, to the THWN conductors in the junction box in the house.

Finally, you'll run cable from the junction box to the outlets in the garage.

Single, or Multi-wire branch circuit

National Electrical Code allows a garage to be supplied by a single, or multi-wire branch circuit without much trouble. If you don't plan on using much power, and don't mind having the lights and receptacles on the same circuit. You can simply run a single circuit out to the garage, and power everything with it. However, if you want to separate the lighting load from the receptacles load, you can install a multi-wire branch circuit without much extra effort.

If you decide to install a multi-wire branch circuit, you'll have to install a double pole breaker instead of a single pole breaker. So to do this, you'll have to have two slots open in the panel. You'll also have to install an additional conductor, so you'll have to use 12/3 NM cable and pull and extra wire through the conduit.

Conduit Size

Whether you decide to install a single or multi-wire branch circuit, you'll need to use at least 1/2" conduit if you're using schedule 80 PVC. According to Table 5 of Chapter 9 of the National Electrical Code, each 12 AWG THWN conductor has an approximate area of 0.0133 in.². Table 4 of the same chapter, says that 1/2" schedule 80 PVC has a total internal area of 0.217 in.². However, since there are more than 2 wires in the conduit, you can only fill the conduit to 40%.

0.0133 in.² * 3 conductors = 0.0399 in.²
0.217 in.² * 0.40 = 0.0868 in.²

Three 12 AWG THWN conductors will take up 0.0399 in.², while 40% of the total internal area of 1/2" schedule 80 PVC is 0.0868 in.². So there's no problem fitting the 3 conductors through the conduit.

0.0133 in.² * 4 conductors = 0.0532 in.²

Even if you decide to run a multi-wire branch circuit, you'll still have plenty of room in the 1/2" conduit.

Conduit Minimum Cover

According to Table 300.5 of the National Electrical Code, direct burial nonmetallic raceways not encased in concrete or other raceways is required to have a minimum cover of 18" (450 mm). However, if it's a residential branch circuit, 120 volts or less, GFCI protected, with 20 ampere or less overcurrent protection, it can have a minimum cover of 12".

So if you're installing a single branch circuit to supply the garage, you can install a GFCI breaker and you'll only have to bury the conduit 12". Otherwise, you're going to have to bury the conduit 18".

NOTE: Minimum cover is based on the conduit running under nothing but grass and dirt. Minimum cover may vary if run under concrete, walkways, streets, parking lots, etc.

Grounding and Bonding

Since you'll only be installing a single or multi-wire branch circuit, you'll only be required to run an appropriately sized grounding conductor along with the current carrying conductors (250.32(A)Ex. 1). You'll extend this grounding conductor to each outlet, and connect any devices to it. There are no other grounding or bonding requirements, as long as there are no metallic pathways connecting the two structures.

Means of disconnect

As Speedy Petey points out, you'll also need a means of disconnect inside or outside the building near where the circuit enters (225.31, 225.32). A means of disconnect is simply any approved method of disconnecting all ungrounded conductors. This could be simple snap switches, a pullout disconnect, a safety switch, etc.

Outlets required

Once you supply a garage with electric power, you'll also have to install a few required outlets. First you'll need at least one receptacle outlet (210.52(G)(1)), which will have to be GFCI protected (210.8(A)(2)). You'll also have to install one switch controlled lighting outlet inside (210.70(A)(2)(a)), and one switch controlled lighting outlet on the outside to provide lighting for the entrance/exits (210.70(A)(2)(b)).

^{Single branch circuit supplying garage}

^{Multi-wire branch circuit supplying garage}

Notes:

This answer is based on National Electrical Code 2014, and may not be applicable to areas that do not follow NEC.

Electrical – US-Michigan—Subpanel install—Can I use PVC conduit and THHN in-wall to go from disconnecting means to panel inside the garage

Sch 40 is fine, this time at least

Since this conduit run is inside a wall, its generally not subject to physical damage, which means Schedule 40 PVC is fine for this job. I'd simply bring it out of the back of the box with a LB body, by the way, and then it can just pipe straight into the side of the indoor subpanel, or into the bottom or top with a LL or LR body if that is more convenient.

As to other means? PVC is really your best choice here, unless you want to go metallic instead, which puts you in EMT-land. (Rigid is a bigger pain to deal with due to threads, and SE cable is going to be harder to replace later if you want more power in the garage.) I'd just size it generously -- 2.5" is a good size for this.

Wire derating is not needed here

Insulation isn't an issue when it comes to derating for wire in conduit as long as the ambient temperature isn't roastingly hot (i.e. not greater than 30°C/86°F).

Don't forget the grounds

Since you're providing a feeder to a detached outbuilding, you'll need to both run an equipment grounding conductor back to the source of the feeder (it can be 6AWG copper) to provide a fault path, and run some more 6AWG copper back to a grounding electrode (ground rod) for the outbuilding to deal with lightning-type problems. Don't forget to pull the bonding screw in the disconnect while you're at it!