Can I split a 120 circuit from a 240 circuit when there is a neutral wire available?

Question

I have a 240 volt circuit feeding a water well and want to "split off" a 110 circuit for a light and small heater. The well has a safety break of 13.2 amps max.  The wire is apx. 100 feet long, 8/4 with a 20 amp breaker. Can I add a load center at the well, use a 20amp,240 volt breaker for the well, 15 amp breaker/120 volt outlet (or 2, 1 on each leg) utilizing the neutral wire back to the original load center.  Currently it is protected by a 20 amp breaker, will a 30 amp breaker be too too much for the 8/4 wire?
Thank you,

NoSparksPlease · Accepted Answer

By NEC you typically could. You would need to add a ground rod if you don't already have one. A single circuit doesn't require a ground rod, a panel will.
I would throw a little caution here, my well isn't too deep, only 200 feet. I have a 1.5 hp 10.6A motor controller says 12A pump max. Well is 200 ft from the service, that's 400 ft of wire. Voltage drop calculators say 4% voltage loss on the wire. So pump and a few cfl or led lights, I'm good to go if I have 8/4 aluminum.
Add 4A of heat, I get 3% voltage loss to the pump house and another 3% down the well. That is outside of the NEC recommendation of max 5% voltage loss, and that's not even considering the inrush on a motor that is fully loaded on startup. Change that 5A of heat to a 12A heater, I'm not good to go any more. Legal, but could still be problematic.

Ecnerwal · Answer

Assuming it's 8/4 copper wire, you could even use a 40A breaker to protect it - if aluminum wire, 30A.
You have a 4-wire feed, and you'll need to isolate the ground and neutral in the sub-panel. If your well casing is metallic, presumably it's already tied into the ground wire - if not, it should be (they make one heck of a ground rod.)

Can I split a 120 circuit from a 240 circuit when there is a neutral wire available?

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