Wiring – Should Oversize Conduit and Grounding Wire Size be Used?

code-complianceconduitgroundingsubpanelwiring

I'm getting ready to place my materials order and have a few last details/checks I could use help with. This is for installing a subpanel in a detached pump house. I'm putting a 60 amp two-pole breaker in the main panel (rated 225 amps) and a 60 amp main breaker in the subpanel (rated 125 amps). The distance between panels is 265 feet. I’m planning to run 3 wires (2 hots & neutral) of #2 Al XHHW-2 and 1 wire of #4 Al XHHW (grounding wire), all upsized for voltage drop.

Calculations for proportional increase in grounding wire size are right on the cusp of #6 and #4. Depending on whether I use mm2 or Cmils, NEC Table 8-Chapter 9 suggests different sizes. Since 2-2-2-4 MHF is common I thought I would follow that sizing configuration. But each change (wire size, conduit size etc) seems to add $100 which is adding up. :/

I am using EMT inside buildings and schedule 40 pvc conduit buried in a 30" deep trench. The pvc conduit run will have 3-90 degree sweeps. Two 90's are for turns into and out of trench. The third 90 is where the trench makes a turn toward the pump house. The rest of the run is pretty straight. I’ll have someone feeding at the LB that starts the run at the first sweep. My plan was to use 1-1/4" conduit with schedule 80 sweeps coming out of the ground.

Questions

  1. Would #6 grounding wire be sufficient to be code-compliant?
    (calculations say I need 26248 Cmils and NEC table lists #6 as 26240 Cmils)

  2. Should I upsize my conduit? Should I use plain or 2' radius sweeps?
    Conduit fill calculations suggest 1-1/4” conduit. Even where I use schedule 80 (just for two sweeps), that comes back as 34.3% fill. But I’ve only done short indoor pulls before and want this to go smoothly.

  3. What size bare, solid copper wire for EGC running to ground rods from sub?
    I thought I read that this is sized off breaker not feeder conductors. I also read this needs to be equivalently sized as the EGC running back to the main. It’s only running about 12’ out to ground rods.

  4. Color coding just the terminal ends of the grounded conductor with white tape is code compliant as long it is at least 4 AWG, right?

Best Answer

Calculations for proportional increase in grounding wire size are right on the cusp of #6 and #4. Depending on whether I use mm2 or Cmils, NEC Table 8-Chapter 9 suggests different sizes.

You don't need to work that hard. AWG sizes are on a log/exponential curve, which makes proportions stupid easy. You compute the minimum legal wire sizes. If there is a 2 AWG size difference between them... then when you upscale wires, simply maintain that 2 AWG size difference.

If you don't understand why this works, look into how slide rules work.

The distance between panels is 265 feet.

Well, right off the bat, you don't need to compute voltage drop based on breaker trip. That is foolish and wasteful, since voltage drop can't read :) Even it could read, it can't see the number on the breaker handle from its vantage inside the conduit LOL. You compute voltage drop based on the actual loads. Or at worst, 80% of breaker trip because you shouldn't plan to load a circuit more than that.

and a 60 amp main breaker in the subpanel (rated 125 amps)

The main breaker in the sub is not necessary unless you have more than 6 hand throws in the subpanel.

Note that 120V circuits can be handle-tied 2-3 at a time using approved handle-ties, making them count as 1 hand throw.

If you insist on having it, remember a tie-down kit is mandatory.

What size bare, solid copper wire for EGC running to ground rods from sub? I thought I read that this is sized off breaker not feeder conductors. I also read this needs to be equivalently sized as the EGC running back to the main. It’s only running about 12’ out to ground rods.

Pump house? For a well? Gosh, it sure would be nice if there was like a 300-foot-long steel thing going straight down into the ground, right there... that would make a pretty good ground rod!

And yeah, you're entitled to use that if it's metallic. Thanks ThreePhaseEel.

Color coding just the terminal ends of the grounded conductor with white tape is code compliant as long it is at least 4 AWG, right?

If by "grounded conductor" you mean neutral, yes.

I don't recommend internalizing the NEC lawyer-speak for that word, it's too darned confusing. Someone should issue a Zach Snyder's NEC with neutral called neutral. It's what King James would do.

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