I know you can't mark/id anything smaller than 4AWG when phasing..

However, for single phasing, can I just buy a spool of 500 feet....and mark ID red and white , on 8 AWG wire ?

 

No. See Article 200.4(B) with respect to Identification of Grounded Conductors.

 

No, you may not remark wire for this circuit either.

You need real white wire for neutral, real colored (not green) wire for hot, bare or real green wire for ground.

 

“200.6 Means of Identifying Grounded Conductors.
(A) Sizes 6 AWG or Smaller. An insulated grounded conductor of 6 AWG or smaller shall be identified by one of the following means:
(1)
A continuous white outer finish.
(2)
A continuous gray outer finish.
(3)
Three continuous white or gray stripes along the conductor's entire length on other than green insulation.


Excerpt From: National Fire Protection Association (NFPA). “NFPA 70®, National Electrical Code® (NEC®), 2014 Edition.” NFPA. iBooks.
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You do not need to mark one hot red. (You can if you want) It doesn't matter as far as the code goes. You can have two blacks for your hots.

 

I never like to tell anyone they can't do something, but if you do, just be aware that it's a code violation. Tolyn is right, the hot wires are not the issue because they can both be black. The issue is the neutral and grounding conductors.

 

Best to ask your LOCAL inspector. Some inspectors will absolutely demand that you use different colors while some will allow a HOMEOWNER to re-identify smaller conductors. No inspector will allow a professional electrician to re-identify smaller than #4 conductors.

The reason for the break at #4 conductors is that the code-making panel decided that it was not a hardship to require an electrical contractor to use the proper colors since they would be able to use the remainder of the spool on a different job and also because wire of #4 and larger is generally not available with different insulation colors. It IS important to re-identify the conductors at ALL points of access such as conduit bodies and pull boxes regardless of any actual connections in these areas.