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We recently moved into a older home and there is a garage detached from the house about 100 feet from main breaker box. There is a electrical panel under the meter (outside of home) with a 60 amp breaker that feeds the garage. I was wondering if anyone knew how to identify the electrical wire running from 60 amp breaker to sub panel in barn/garage?

I want to upgrade to 100 amps and thought there might be a chance that the wire already ran the 100 feet would be rated for the boost in amperage. Also, what wire would be ideal, copper and aluminum, for running new 100 amp service line 100 feet to detached barn/garage?

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To identify the wire gauge, the best way might be to take the cover off of the breaker box (advise to do this with power off for safety, but power can be left on - proceed at your own risk) and see if there are any indications on the wire sheathing. If there isn't, use a guide like to identify the gauge of wire.

Wire gauge sizes

As for copper vs. aluminum wire, aluminum might be cheaper in larger sizes. However, aluminum requires some special anti-oxident joint compound when you make connections with aluminum wire (see Ideal no-alox for an example of this stuff). If you don't use the compound, its possible that the connections will fail over time.

No-Alox

EDIT: Using the info you provided in the comments, and the wire voltage drop calculator at http://www.southwire.com/support/voltage-drop-calculator.htm, assuming:
- Single Phase 240V
- Direct Burial
- no more than 3% voltage drop
- 2 AWG aluminum wire
Your wire gauge will allow you to maximum run somewhere in the 75-80 amp range. I wouldn't go all the way to 100 amps, but 60 amp service should work. You'll need 1 AWG wire to run 100 amps safely that distance.

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    Note: the table above applies to copper only, a better chart can be found here: www.usawire-cable.com/pdfs/nec%20ampacities.pdf
    – Tyson
    Commented Nov 21, 2016 at 0:14
  • Thanks for your help, I have discovered that the wire leading to barn/garage is Aluminum 2 AWG AL Type USE-2 600V XLPE (UL). The wire is running 100 feet, How many amps can wire carry? Thanks again, Josh A.
    – Josh A.
    Commented Nov 26, 2016 at 3:49
  • Is this wire direct buried or in conduit for the full run?
    – ThreePhaseEel
    Commented Nov 26, 2016 at 16:14
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The maximum amperage of the wire can be found in NEC Table 310.16, subject to restrictions of NEC 110.14.

310.16 shows Al USE-2 to be 90°C, but 110.14(C)(1)(a)(1) limits assume termination ratings to be 60°C unless 110.14(C)(1)(a)(3) identified for a higher rating, which your circuit breaker and panel lugs probably are and certainly won't exceed 75°C.

So back to the 75°C degree column of 310.16 you will find you must protect #2 USE-2 at no more than 90A.

Voltage loss (Voltage drop) is a factor of load and length, which maximum acceptable is subject to some interpretation, but the feeder at 90A would create about a 2% loss, which by nearly all standards is not a problem. Lower load would produce even less loss.

A 60A breaker may be a result of Section 220 Load Calculation. For instance the existing loads except garage may have used 140A of available 200A.

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At 100 ft you will lose over 5% of your voltage at 100 amps. I would suggest not exceeding 60 amps with your current wiring. If you want to get 100 amp to the building, it would be better to use at least 3 awn copper or 1/0 aluminum.

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Not really sure what these other jokers are talking about, but...

You need a #2awg for a 100a sub panel.

The 60a line out there is a #4awg, which is not rated for 100a.

Aluminum wire is less expensive but copper is a better conductor, don't skimp, you will thank yourself in 30 years when it still works.

I would consider piping out to the garage/barn, the direct burial cables are super expensive. But with a little extra labor you can have a solid run.

Ask your local inspector about burial depth (usually around 18 to 24 inches) and any other things they would like to see.

As it is an out-building, make sure you use 2 ground rods, separated by at least 12 ft.

Also keep in mind that to support a 100a sub panel, you need at least 200a in your house. It is possible to double lug off of your meter but I would hire a pro for that (for liability and insurance mostly)

Don't worry about voltage drop, it's not a problem until you are over 500ft.

DO NOT USE WIRE CHART OR ANDWERS GIVEN IN PREVIOUSLY, IT IS MASSIVELY INCORRECT AND COULD LEAD TO PROPERTY DAMAGE OR DEATH.

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  • 2AWG is only good for 90A as a generic subpanel feeder (the 2AWG = 100A thing is only good when you're feeding an entire dwelling unit)
    – ThreePhaseEel
    Commented Aug 24 at 14:09
  • -1 Lots of little but important errors: Voltage drop can be a problem at a lot less than 500 feet, but not normally at 100 feet; wire size depends on type - for 100A subpanel it is 2 AWG copper but 1 AWG aluminum (aluminum highly recommended); we don't know what the 60A line is - it could be 6 AWG copper or larger, 4 AWG aluminum or larger, or it could be smaller and incorrect; copper is a better conductor - that's why aluminum has to be sized larger, but follow the rules and it is fine;
    – manassehkatz-Moving 2 Codidact
    Commented Aug 25 at 2:26
  • "piping" is the wrong term as it implies "any pipe" - it needs to be proper electrical conduit; burial depth depends on type of cable/conduit and for some conduit in residential applications can be less 18"; ground rod separation minimum is typically 8 feet, not 12; 200A -> 100A may be enough or may not - only way to tell is proper load calculations.
    – manassehkatz-Moving 2 Codidact
    Commented Aug 25 at 2:26
  • NEC 250.53 only requires 6' ground rod separation, but check with AHJ for local amendments.
    – NoSparksPlease
    Commented Aug 25 at 5:50
  • Do the math, voltage drop at 500ft for 100a is within the 3% variance of voltage, so not a problem. It is a 2.81% drop, so at 120v thats just over 3v.
    – Will Kessler
    Commented Aug 25 at 17:39

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