I've got an old dryer outlet in a 1 gang box with 10/3 Romex cable running to a 2-pole 30amp circuit breaker. I've converted the old laundry room into a server room. I want to get a bigger UPS that requires an L5-30R outlet. Since there is already 10/3 Romex run I have a few options.

1) Install an L14-30R and use a Y-adapter that breaks the two poles into two L5-30R outlets.

2) Install two L5-30R outlets, pigtailing the neutrals and grounds and split the two hot legs from the 10/3. This requires a new box and extra work but might be worth it long term.

3) Something smarter?


  • You can't do any of those because a NEMA 10 doesn't have any ground wires. You can't just blow off ground for safety reasons, and though a GFCI can mitigate that, it can't mitigate the equipment protection reasons you need ground. – Harper - Reinstate Monica Nov 12 '19 at 3:08
  • 1
    The old outlet is a NEMA 14-30R and it has a ground. The cable to the outlet has 4 wires. 10/3 with ground, to clarify. – umhelp Nov 12 '19 at 3:19

Convert to a multi-wire branch circuit

It might be possible to simply have a "splitter cable" that splits a NEMA 14-30P to two L5-30R's. There is nothing wrong with that. However we do permanent wiring, so let's explore that.

You would need to either push the box out to a 2-gang box (at least), or extend the box using a box extension (or surface conduit starter box) then conduit to 1-2 other boxes. You can put a NEMA L5-30R receptacles in each of these boxes.

Run THWN-2 wire down the conduit - you will need white for neutral and black for the hots. I'm a big fan of stranded wire when you are attaching to lugs or wire nuts rather than screw terminals. (I do screw terminals too, but they're tricky for novices).

You can also run 10/2 AWG NM cables, but they must be physically protected unless they run in the walls.


I think option two is better, and not that hard. Use a RACO 187 extension box https://platt.com/platt-electric-supply/4-Square-Boxes-Accessories-Extensions/Hubbell-Raco/187/product.aspx?zpid=0052108 . Option 3: You could have a qualified person replace the two pole breaker with one 30 and one 20 amp breaker with handle ties and use one L5-30 and a 5-15 or 5-20 with the above 187.

  • 1
    That sort of asymmetrical MWBC is actually a rather clever way to work things, and one of the few cases where handle-tied MWBCs make sense. +1 :) – ThreePhaseEel Nov 12 '19 at 3:05
  • Very clever! I already have a different 20amp circuit running that I'm using currently to power my 20amp UPS. Although a 5-20 outlet would be easier to use than a second L5-30. – umhelp Nov 12 '19 at 3:35
  • Good note on getting qualified person to do the main panel work.+ – JACK Nov 12 '19 at 12:59
  • This is a great answer, both in the installation and the asymmetrical MWBC. – Harper - Reinstate Monica Nov 12 '19 at 17:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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