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I Will be installing a subpanel in a detached garage. The subpanel will be 60 amp. The garage is about 30' from the main-panel in-house, and I will of course get an exact length before selecting the proper gauge wiring. I'll be using pvc underground for the wiring. There is already existing wiring going underground from the house to the garage, which will no longer be used.

  • I saw a youtube video that mentioned that different sized PVC conduit have different limits on the number of conductors that can be ran through them underground. Is this true, and if so where can I find documentation on this? Specifically, the number of conductors for each size conduit.
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3 Answers 3

up vote 3 down vote accepted

All conduit has a fill rating (which equates to 40 % of the free area full of wire, unless there are only 1 or two wires - in practice, less fill is better - the full-rated fill is VERY hard to pull into conduit. That is slightly different for different type conduits due to the different actual size of the hole in the different types of conduit. There are charts in the NEC, and there are also a LOT of on-line conduit fill calculators (and some charts.)

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If you can settle for a 50amp feeder, not only is it the next smaller size wire, but it could mean the difference between 3/4" and 1" conduit. That's a bigger difference than you might realize right now.

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I was actually thinking about 1-1/2" or 2" conduit for adequate room. As far as amperage goes I'll probably run a high enough gauge to give the possibility of upgrading to a 100A subpanel in the future. –  BigHomie Apr 23 at 18:31
OK! You don't have to tell me twice HaHaHaHa. When it comes time to drive those ground rods in consider using a hammer drill. You can rent a drill. Youtube has some videos on it. –  Rand Apr 23 at 21:25
+1 for that, I was going to get a small sledge to get that done, but I already own a hammerdrill for masonry work, so I'll use that first. –  BigHomie Apr 24 at 2:12

To determine the number of conductors allowed in a conduit, you can use Table 1, 4, and 5 from chapter 9 of the National Electrical Code.

National Electrical Code 2014

Chapter 9 Tables

Table 1

If you're pulling more than 2 conductors, you'll only be able to fill the conduit 40%, according to Table 1.

Table 4

Table 5

To calculate conduit fill:

  1. Get the 40% area value from the table corresponding to the type of conduit you're using (Table 4).
  2. Get the approximate area value for the size conductors you're using (Table 5).
  3. Divide the 40% conduit area by the conductor area, to determine the number of conductors allowed.

NOTE: When calculating conduit fill, if the calculation results in a decimal of 0.8 or larger, you can round to the next whole number.

- 1/2" Schedule 40 PVC 40% fill = 0.114 in².
- 12 AWG THHN conductor = 0.0133 in².
0.114 in² / 0.0133 in² = 8

If you're using all the same size conductors, you can use Table C.9 or C.10 from Annex C

Annex C - Conduit and Tubing Fill Tables for Conductors and Fixture Wires of the Same Size

Table C.9 Table C.10

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