How do I determine the fill rating of a conduit?

Question

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.

Answer 1

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.)

Answer 2

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

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

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.}

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

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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

Answer 3

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.

Answer 4

Adding to other answers:

• As mentioned before, it is hard to pull wires in a just-big-enough conduit. Allow for e.g. bending the pulled wires over the fish-tape and duct-taping them together: gets to be quite a large bundle!
• Use wire-pulling lube, water-based to not degrade insulation.
• Make the fish-tape to wire junction smoothly tapered.
• Plan conduit so the bends in each pull are less than 360 degrees; under 270 is better.
• NMT (PVC) glues together. Plan conduit junction directions so the bundle will be sliding into the tapered flares at joints, not hitting square pipe ends.