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1) No, the National Electrical Code does not recognize doubling up on conductor sizes in order to achieve the required single conductor size such as using multiple 12 AWG conductors.
2) The equipment grounding conductor size required for a 50 amp branch circuit would be using either 10 AWG Copper or 8 AWG Stranded aluminum, type THWN insulation.
Per Article 250.122(A) in the 2011 edition of the National Electrical Code as follows:
Copper, aluminum or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in table 250.122
3) If your existing application utilizes an exterior or underground conduit system, then all conductors must be rated as THWN type and not THHN insulation.
THHN is not water and/or moisture proof and is a violation of the National Electrical Code if used on the exterior/outdoors and/or underground.
4) If the 50 amp is a feeder for a sub-panel application, all sub-panels require a 4 wire circuit, ie, 2 hots, 1 neutral and 1 equipment ground. The use of additional conductors (doubling) in order to compensate for the required equipment grounding conductor size is considered an electrical code violation and does not provide for an effective ground-fault current path.
All 4 conductors (2 hots, 1 neutral and 1 equipment ground) feeding a sub-panel must be (1) one contiguous conductor size from the main electrical panel all the way to the sub-panel. Splicing or piercing devices such as split bolting methods are allowed in splice boxes along the path, but not to increase the circular mils of the required individual conductor size.
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