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I have existing 1/2" emt conduit servicing 20 amp receptacles connected to a single pole 20 amp breaker. So there are 3 12G wires, 1 black, 1 white and 1 green in the existing conduit.

I'd like to add a single 240V 20 amp receptacle in a separate box at the end of this conduit run, by running black and red 12g wires back to a separate 20 amp double pole breaker. These wires will pass through unbroken through each box. Can I just mark the 2nd black wire with say, brown tape to keep it identified? Or does the 2nd black wire need to be a different color, say brown? Thanks!

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You don't have to do anything to identify the conductors. However, you'll have to worry about box fill, as these conductors will have to be counted in the fill calculation.

National Electrical Code 2014

Chapter 3 Wiring Methods

Article 314 Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings, and Handhole Enclosures

314.16 Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies.

(B) Box Fill Calculations.

(1) Conductor Fill. Each conductor that originates outside the box and terminates or is spliced within the box shall be counted once, and each conductor that passes through the box without splice or termination shall be counted once. Each loop or coil of unbroken conductor not less than twice the minimum length required for free conductors in 300.14 shall be counted twice. The conductor fill shall be calculated using Table 314.16(B). A conductor, no part of which leaves the box, shall not be counted.

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