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Im running two individual single pole 20A circuits though a single 1/2" EMT conduit and hanging a duplex outlet every 6'. They will alternate which circuit (phase) is feeding them (im running black and red 12AWG for these to simplify hook up) Ive also got 2x 12 AWG White neutrals... and everything gets grounded through the metal conduit. Do I need both neutrals (the sketch shows a common neutral)? And if I can run a single neutral do I need to upsize it from 12guage? enter image description here

Update:
I ended up running an additional neutral because all of the outlets needed to be GFCI protected. The First box on the string contained two GFCI outlets, each with separate neutrals and a different hot leg. The following string had one outlet from each GFCI LOAD side (also with labled paired neutrals) Here is the final outlet in the string (the lower 120V one with 2 duplex 120s) we kept the two phases on different color outlets (white and tan) but used a DUAL Pole 20A breaker with handle tie, so that when we shut off the box, both circuits were cut. (The Top outlet, NEMA 6-50 is on its own branch circuit in its own conduit). The the shop now has Three of these 120V utility branch runs, with 2 independent hot and neutrals handle tied in three 20A 2pole breakers ( plus another 3 50A 2poles for the 240v circuits!).
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    If you run both circuits (total four current-carrying conductors) in a single conduit you need to adjust the ampacity of the conductors to 80% of the nominal value. See NEC §310.15.
    – Jeff Wheeler
    Sep 24, 2020 at 2:46
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    @JeffWheeler However you are derating off the 90C column of Table 310.15(B)(16), which means 80% of 30A for #12Cu THHN..
    – Harper - Reinstate Monica
    Sep 24, 2020 at 6:26

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If you have a double pole breaker or 2 handle tied breakers you can run a normal neutral it is called a multiwire branch circuit. Are you going to need arc fault or GFCI protection in the room? If so I would run a second neutral and keep the breakers separate. GFCI’s don’t always play well with multiwire branch circuits. They can still be used but there is the occasional trip of one or the other.

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  • Thanks Ed, I think I will keep the TWO neutrals, to 'futureproof it' in case someone in the future tries to move that DP breaker into a pair of singles or a space saving double on one pole. Its for equipment and tools, and i wasnt planning on running a GFCI.
    – mark f
    Sep 23, 2020 at 22:39
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    I have to agree with Ed Beal. With all the newly required, highly sensitive breakers, I don't like MWBCs except in very specific circumstances. It's probably only a few dollars to keep it simple and run that 2nd neutral. EDIT: Meant to add that, depending upon the location, GFCI might be required. If not, esp. on equipment and tools, it's a good idea to use GFCIs.
    – George Anderson
    Sep 23, 2020 at 22:42
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    @markf anyone who moves a double-pole breaker to a pair of singles or a tandem is too stupid to work in a panel. Especially if the hot wires are black and red. You can’t make anything idiot-proof, they keep coming up with better idiots lol.
    – Harper - Reinstate Monica
    Nov 11, 2020 at 3:48
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To add to what Ed wrote, the neutrals must be clearly marked as to which neutral is with which circuit. Noting that it is THHN individual wires within conduit, you are not allowed to re-mark a white conductor to be a hot. Therefore any marks on white wires are NOT remarking to hot, but simple markings.

You can also just bundle the pairs of wires. Take care that you do this correctly if you're doing it after-the-fact.

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    And what about gray/grey as a 2nd neutral?
    – manassehkatz-Moving 2 Codidact
    Sep 24, 2020 at 15:41
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    @manassehkatz Yup, that is ideal, and that's what gray is for. I just don't assume people are willing to buy another spool of wire, nor hunt it down, since the big box stores do not stock gray.
    – Harper - Reinstate Monica
    Sep 24, 2020 at 17:13
  • Note that striped THHN is a thing, although sadly rather harder to find than plain grey
    – ThreePhaseEel
    Nov 11, 2020 at 2:02

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