Hello Mike and welcome to Just Answer. My name is ***** ***** I will be happy to assist you with your electrical question.
1) Based on the sub-panel loads that you want to connect, I recommend a minimum of a 70 amp double pole feeder breaker will be required. The 70 amp sub-panel feeder circuit will require a 4 wire circuit comprised of 2 hots, 1 neutral and 1 equipment ground. A quantity of (3) three 4 AWG copper wires (type THHN insulated) will be required for the 2 hots and 1 neutral. An 8 AWG copper can be used for the equipment grounding conductor.
2) If the existing 200 amp main panel can sustain a 100 amp feeder circuit, I recommend installing a 100 amp double pole breaker in the main panel as this will obviously take care of the 3 sub-panel loads as well as providing for some future circuits. If using a 100 amp feeder breaker, the 2 hots and 1 neutral will require 4 AWG Copper (type THHN insulated) and an 8 AWG Copper for the equipment ground wire.
3) If connecting the 200 amp main panel to the sub-panel by EMT metal conduit, then the equipment grounding conductor can be eliminated since the EMT metal conduit will act as the equipment grounding conductor.
4) Since the sub-panel will be installed directly next to the main panel, a main lug sub-panel can be installed.
5) At the sub-panel, DO NOT install the green main bonding screw or a main bonding jumper strap to the neutral bus bar. On any sub-panel, the neutral feeder must always be isolated from the sub-panel metal enclosure cabinet. By not installing the main bonding screw or jumper, this is how the neutral remains isolated.
6) At the sub-panel neutral bus bar, only white neutral conductors will land there. Never inter-mix bare copper or green insulated ground wires onto the neutral bus bar. The sub-panel will require a separately installed equipment grounding bar to land any equipment grounding conductors.
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