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Long electrical wiring home runs

cnote75 | Posted in General Questions on


I’ve got a new construction home underway. My electrical meter is on the wall of my garage furthest from my house. My electrician ran the numbers both ways and said it was surprisingly cheaper to run all the homeruns from the house all the way back to the panel in the garage at the meter rather than run one or two big wires to a subpanel in the house and bring all the homeruns to it. What we did not discuss is how much electricity (i.e. money) am I loosing in lots of little wires spanning that additional 50′ or so rather than one big one. Frankly, this is now an intellectual exercise as the work has been done, but I’d still like to know if it is a meaningful amount. Thoughts?

Thanks for your time. I appreciate all the help I’ve received on this forum.

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  1. charlie_sullivan | | #1

    For the most part, wiring within a residence has negligible power loss in the wires. That's especially true for GBA readers, because we tend to buy efficient refrigerators and LED lights, such that we are using each circuit at a small fraction of its full capacity. The exception to that is a big load, such as a clothes dryer, oven, or heat pump, where one might use a large fraction of the rated current. Over the year, the minisplit energy consumption is probably more than the other uses (especially for GBA readers who line-dry their clothes). Suppose a mini-split drawing 10 A on a 20 A circuit, run with 14 AWG wire. In the 100 feet of wire (there and back), the wire has a resistance of 0.25 ohms. At 10 amps, that's a 25 W power loss over the length of the wire. That a small percentage of the mini-split's power consumption (a percent or two, depending on its power factor and whether it's a 120 V or 240 V circuit), but 25 W is generally above my threshold for caring about power loss, so it might be worth thinking about.

    One good thing is that because minisplits modulate, they spend a lot of time drawing less than 10 A. When it's drawing 5 A, the loss in cut not just in half, but by a factor of 4, so it's only losing 6 W. So it's certainly not worth losing sleep over, but it might be worth considering in a house with substantial electrical usage.

    The subpanel option would have cut the losses substantially--A 100 A subpanel would be wired with AWG 4 or bigger copper wire. AWG 4 would have a tenth the resistance, so the loss in the AWG 4 would be 2.5 W for a 50' run, when the mini-split was drawing 10 A, and it was the main thing drawing power from that sub-panel. Another option would be simply to up-size the conductors for the large loads that run a lot of the time--e.g. use 10 AWG in the run to the miniplit--which would cut that loss from 25 W at full power to 10 W at full power.

    Then again, if you were to consider the life-cycle cost including mining the copper, I'm not sure you'd want to up-size conductors. You would, however, want to use aluminum conductors where possible, so they can be up-sized at lower financial and environmental cost. Fortunately, if you buy carefully, you can now get Romex and other building wire with lead-free PVC insulation.

  2. cnote75 | | #2

    Thanks for the detailed response. My electrician ran 12 AWG for everything but the high load items which he ran 6 AWG I believe. So it sounds like any resistance losses should be pretty negligible. That's good to know.

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