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I have old overhead wires that go between my home and a detached garage. The plan is to remove the overhead wires and install a new sub-panel in the garage with an underground (PVC 80) run between the house and garage.

Right now I am trenching the underground portion of the run and need to know what size PVC 80 conduit to install in the trench.

Here are all of the details. The run will be 1 phase 240V and go from a 100amp main panel and travel 130 feet to a 50amp sub-panel. The run will start at the main panel at front of house, dip into the crawl space, run to the back of the house, dip underground and travel to the garage, then run to the sub-panel in the garage. It is 60 feet from main panel through crawl space. It is another 60 feet underground through the yard, and 10 feet inside the garage to the sub-panel.

I'm know the length of the run will impact wire gauge and that in turn will dictate the conduit size due to fill constraints. I am in California. Also, if you answer can you please give a reference to the part of the NEC that you are referencing? I plan to have an electrician do most the work so for now I just need to know what size conduit to drop in the ground.

  • Are you sure it's 3-phase? Do you have an electrician you're working with? Do you know what size wire you'll be using? – Tester101 Apr 17 '16 at 20:03
  • Sorry, its 1-phase 240V. and I think it will be a 50 amp sub instead of 60. I was hoping someone here could supply both wire gauge and conduit required for the required wires. – John Apr 17 '16 at 20:53
  • This answer might be helpful with sizing conduit. While this answer may help calculate voltage drop. – Tester101 Apr 18 '16 at 15:53
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Base Conductor Size

Start out by using Table 310.15(B)(16), and applying any required corrections, to determine what size conductors you'll need. For your situation, we'll assume we can use the 75°C column, that you want to use copper conductors, and there's no other corrections required. So in your case, if you want to install a 50 ampere panel, you'll need at least 8 AWG copper conductors. If you want a 60 ampere panel, you'll need 6 AWG copper conductors.

Voltage Drop

Once you have the base conductor size selected, you'll want to calculate the voltage drop across that size conductors for the length of the feeders. The first step here will be to use Table 8 from chapter 9 of the NEC, to determine the resistance of the conductors you've selected.

In your case, 8 AWG stranded copper wire has a resistance of 0.778 ohms per 1000 ft. 6 AWG stranded copper wire has a resistance of 0.491 ohms per 1000 ft.

Next you'll use the following formula, to calculate the voltage drop across the feeders.

V = L * 2 * R * A
Where:

  • V = Voltage Drop
  • L = Distance along the wire from one breaker to the next.
  • R = Resistance per foot of wire.
  • A = Current running through the conductor.

For a 50 ampere circuit, 130 ft. long, using 8 AWG stranded copper conductors, the calculation looks like this...

V = 130' * 2 * 0.000778 * 50 A
V = 260 * 0.000778 * 50 A
V = 0.20228 * 50 A V = 10.114 V

10.114 V is 4.2% of 240 V. The NEC recommends having a voltage drop less than 3%. To achieve this, you're going to have to use larger conductors.

6 AWG stranded copper conductors have a resistance of 0.000491 ohms per foot, which means the voltage drop would only be 6.383 volts or 2.7%.

For a 60 ampere circuit 130' long, 6 AWG stranded copper conductors would have a voltage drop of 7.6596 volts or 3.2%. While 4 AWG stranded copper would be 4.8048 volts, or 2%.

Conductor Type

Once you know what size conductors you need, you'll have to determine what type of insulation the conductors should have. Since you're burying the conduit, you'll need a wire rated for wet locations. The popular choice in this situation, would be to use THWN wires.

Wire Size

Now that you know what size conductors, and what type of wires you'll use. Then next step is to determine the physical size of the wires, and how much space they'll take up in conduit. For this, you can use Table 5 from chapter 9 of the NEC. There you'll find that 6 AWG THWN wires have an area of 0.0507 square inches, while 4 AWG THWN wires have and area of 0.0824 square inches.

Conduit Fill

Using the size of one wire, you can figure out the area required for all four wires.

0.0507 * 4 = 0.2028 in.sq.
0.0824 * 4 = 0.3296 in.sq.

Use Table 1 from chapter 9 of the NEC, to determine the allowable conduit fill percent. Since you'll have more than 2 conductors, you can fill the conduit to 40%.

Conduit Type

If you know what type of conduit you're using, you can use Table 4 from chapter 9 of the NEC to look up the area fill values for various sizes of conduit.

Conduit Size

Since you've decided to use Schedule 80 PVC, you'll simply find that table in Table 4. Then look down the 40% fill column, until you find an area large enough for all your wires.

In your case four 6 AWG THWN conductors, will require 1" Schedule 80 PVC. While four 4 AWG THWN conductors, will require 1 1/4" Schedule 80 PVC.

Conduit Size Alt.

If you don't feel like calculating wire/conduit area, and all the wires are the same size, you could use Table C.9 from Annex C of the NEC to look up the conduit size required. There you'll find that you can fit five 6 AWG THWN wires throug 1" Schedule 80 PVC, and that you can fit six 4 AWG THWN wires though 1 1/4" Schedule 80 PVC.

tl;dr

  • For 130' long 50 ampere feeder, use four 6 AWG stranded copper THWN conductors though 1" Schedule 80 PVC.
  • For 130' long 60 ampere feeder, use four 4 AWG stranded copper THWN conductors through 1 1/4" Schedule 80 PVC.

NOTES:

  • This answer contains some of the tables used in this answer.
  • If you don't feel like doing any maths, you can surely find a calculator online to do all the work for you.
  • Tester101, for educational purposes could you provide the formula for 120 volt run. Thanks – Kris Apr 19 '16 at 12:56
  • @Kris you mean the voltage drop formula? It's the same for any voltage. – Tester101 Apr 19 '16 at 13:43
  • curious... Would it be the same for S.W.E.R. ? – Kris Apr 19 '16 at 16:52
  • @Kris Sure. But instead of simply multiplying by 2, you'd have to figure in the resistance of the ground path instead. – Tester101 Apr 19 '16 at 17:14
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    Tester101 . . . this is a simply amazing, complete and useful answer to a complex problem. Thanks for taking the time to share your expertise. It is truly appreciated as I prepare to run a 60 AMP sub panel myself. – dscarr Jan 2 '18 at 19:50
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If you are installing a 50 amp sub-panel normally you could use #8 wire and use 3/4" pipe. You can fit 4 - #8 wires in a 3/4" pipe so there is room for a ground wire with your two hots and a neutral.

If you are installing a 60 amp sub-panel #6 wire is good for 65 amps at 75°C. (You have to use the 75°C rating since no one makes a 90°C rated breaker.)

Using schedule 80 PVC you can fit 5 - #6 wires in a 1" pipe. You can fit 3 in a 3/4" but you need to pull a ground wire too so you will have to upsize to 1".

With the distance to your sub-panel it would be advisable to upsize your wire one size further.

So, #6 for a 50 amp #4 for the 60 amp. This would necessitate a corresponding increase in pipe size to 1" and 1 1/4" respectively,

Shop for your sub-panel first and decide how big of a breaker you want to feed it with.

Here are the tables: http://www.jhlarson.com/resources/electrical/sch40-sch80-pvc.html http://www.barr-thorp.com/wp-content/uploads/2011/04/Digest-176-NEC-Tables.pdf

Good luck!

  • Note: All wire sizes are for copper wires only. – Tester101 Apr 18 '16 at 12:29

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