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In sizing underground feeder wire to a detached garage/shop do I calculate voltage drop based on distance from the main service panel on back of house to the shop(175feet)? Or do I include the distance from the meter to the house main service panel(160feet) and the distance to the shop(175) to come up with a total distance of (335feet)?

I want to run power to a new building(24X32). I've trenched approx. 165ft from my main service panel(200amp) on back of the house to the shop. I am thinking of installing 100amp sub-panel in a one-person shop and run; air compressor, small heater, lights, table saw, welder. Thinking of running 2/0 Al MFH cable to sub-panel. The small house was built on sub-divided property, so the meter is actually 160 feet behind the house(main service panel) and the cable coming into the house I believe is 4/0. I am trying to determine cable size to run in my trench, in conduit, with these variables.

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  • This is an XY problem, you're asking about a method to solve a general problem, but not really describing the underlying problem, and there may be factors which influence the matter. Given the distances you are talking about and the significant $$$$, especially in Cu wire, you may want to describe your entire situation so we can look for cost saving alternatives.
    – Harper - Reinstate Monica
    Dec 5, 2017 at 18:56

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For a sub panel fed from your main panel normaly we size base wire size on the distance from the panel to the load or location using distance and the max load or supply breaker size. The conductors are already sized for the panel size from the meter to the panel.

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Here is what the National Electrical Code has to say about voltage drop. It appears a couple times in the Code but it is an Informational Note and not an enforceable part of the Code.

It is however an engineering recommendation that is followed by most designers.

Article 215.2(A)(1)(b) Informational Note No. 2: Conductors for feeders, as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, will provide reasonable efficiency of operation.

Since the Code defines a feeder as:

Feeder. All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.

And the Code defines service conductors as:

Service Conductors. The conductors from the service point to the service disconnecting means.

So, the answer to your question according to the Code is; from the beginning of the feeder at the main panel to the final outlet is how voltage drop is calculated and either the feeder or the branch circuit can be 3% but both combined should not exceed 5%.

Good luck!

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You need to consider both length segments.

The first segment (160ft) was designed to ensure an acceptable voltage at the 200A main panel. You know you have adequate voltage at the main panel: you know there's a voltage drop, but it's within acceptable limits.

When you add the second segment to the 100A sub-panel, you will incur a second voltage drop.

Both voltage drops add up.

Calculate the voltage drop on the 160ft segment, then calculate the voltage drop for the 175ft segment.

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