Hi all,
I'm looking for some information about adding a new sub panel (125 amps) to a addition, which will have a hot water tankless heater (220 vac) and propane furnace (125vac) along with some lights, ref. and oven. Currently, I have a 200 amp service in the main house and this new panel will be mounted about 36' away from the main service. Any advice on wire size and grounding methods.

Thanks
Jeff

I would recommend using #1/0 Aluminum 4 wire SER cable, which is the size allowed for a residential 125 amp feeder.

No special grounding is needed, only keep the neutral bar insulated and isolated at the subpaenl, install a separate equipment ground bar in the subpanel, bolted directly to the metal box using machine screws. Use the neutral bar for white neutrals only, and use the equipmnet ground bar for green or bare equipment ground wires, including the one in the SER cable.

You might want to check out your tankless water heater. Some of those are huge power hogs. (100A or better) Will your existing service support that along with everything else you have in the home?