0

I have a 30amp pool subpanel wired with 10awg THHN through 3/4" schedule 40 PVC conduit. I'm considering installing a heat pump pool heater which will require a beefier subpanel.

Can I run 2x 4awg hot/hot and 1x 8awg neutral through the conduit and utilize a ground from a nearby junction box to serve as the subpanel ground? This is about 38% fill which would be a beast to pull particularly through old conduit, but seems like it would fit.

Most of the pool loads are 240v, so undersizing the neutral to the grounding conductor size seems appropriate based on calculated load. The only 120v load is the pool light and a receptacle for a powered pool vacuum. This would allow me to have an 80amp breaker to protect the sub panel.

Pool pump is ~10.1 amp 240v load, pool heater is 37amp 240v load. In the future I would like to add a second pump and possibly a second smaller heater so I'm trying to maximize available capacity.

Improve this question
8
  • @Jasen Sorry I should have been more specific - Schedule 40 PVC. Will edit.
    – Travis
    Commented May 22 at 5:15
  • Is it in the same building? Is the conduit accessible? EGC doesn't necessarily need to be inside the conduit.
    – KMJ
    Commented May 22 at 5:37
  • Conduit goes from an interior junction box on one side of the house, down under the slab foundation, up to a junction box on an exterior wall on the opposite side of the house. There is a junction box for my HVAC compressor nearby which I would like to pigtail a ground to serve the subpanel ground bus.
    – Travis
    Commented May 22 at 5:53
  • 1
    Are the pool and compressor fed directly from the same upstream panel? Is the compressor's ground wire back to the panel big enough for an 80A circuit?
    – jay613
    Commented May 22 at 10:04
  • 1
    Ground needs to run with the conductors except in retrofit situations. #8 copper bare ground. Also note #6 is 65A, and you can squeeze 75A out of it if your conduit is 90C and you stop at an intermediate junction box to splice to #4 conductors for final connection to your 75C panels. A real PITA but can squeeze a few extra amps out of limited conduit space.
    – Harper - Reinstate Monica
    Commented May 23 at 22:24

1 Answer 1

Reset to default
5

You can't run ground separate from conductors except in a retrofit scenario when running with no ground was legal at the time the other wires were installed.

Downsizing the neutral is governed by NEC 220.61, and may be done based on the calculated load.

Now there's the 90 degree C cheat. THHN and THWN-2 wire, by itself, is 90°C wire. However, it is constrained by the thermal rating of enclosures it goes through and terminals it lands on. Consumer tier panels are generally 75°C rated, which is what lands us at 65A for 6 AWG.

If you exit a panel with 4 AWG wire, go to an intermediate junction box which is rated 90C and splice (using 90°C rated terminals) to 6 AWG wire.... and then do the same thing again at the other end... then the 6 AWG wire is entirely in a 90°C world and you're good to that amp rating (75 amps).

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.