I have a 200 amp MB Panel in my House. I want to Install a 100 amp MB Panel in my Detached Garage that is 175' from the House. I plan on running the wire thru Conduit in the trench due to rocks. I plan on using around 80% of the 100 amp Sub Panel for a small machine shop. Is voltage drop a concern here?, and would I benefit from Copper Conductors vs. Aluminum

  • 1
    You have to wire for the breakers, not expected usage. There are many charts to be found for voltage drop. Have you consulted any of them? (You'll probably need to upsize one step at that length.)
    – isherwood
    Jun 22 at 16:17
  • I understand, but my concern is the Feeder size. Jun 22 at 16:21
  • I don't know exactly what loads I'll have, but i don't want to have a Feeder Voltage Loss after I size the pipe and wire and close up the Trench. Appreciate everyone's help with this. Jun 22 at 16:23
  • An expert will be along shortly. You've come to the right place. :)
    – isherwood
    Jun 22 at 16:26
  • What matters is your maximum concurrent load, your loaded feeder voltage, and your tolerance for voltage loss from your service. Generally the NEC only recommends maximum 5% loss from service panel, Canada goes beyond just recommending a maximum. Many user here will make recommendation allowing 10% loss, I would never go that far on a job I was getting paid to do. Without the info stated opinions will vary greatly. Jun 22 at 18:50


The major benefit to copper wire is the much larger payment I will get, when you buy the wire from this link using my affiliate code :) Seriously on large feeder like this, there's no real benefit to copper, just a 4x larger invoice and some smugness. The lugs you'll attach to are aluminum (since Al lugs play nice with both Al and Cu wire).

You start with your expected usage. The good news is you don't have to factor for every single load being maxed out at once! If it is a residential load, there is a standard load calculation for that.

You then size the feeder for 125% of the expected usage.

So again if it computes to 70A, then 125% is 87.5A.

Now you go to Table 310.15(B)(16), find the column under Aluminum marked 75C. (you get to use the 75C column because you are going panel to panel). Follow that table down to the next size larger. You can always go larger still, if it pleases you to do so.

There are other tables out there on the Web. If they specify 100A for #2 aluminum, they are WRONG - or to be more precise, they apply to a situation that is not yours.

So that 87.5A becomes #2 aluminum at 90A. (I picked that example because it happily lands on #2 aluminum, which is a pricing "sweet spot" due to high volume of wire sold.)

Voltage drop barely matters at this distance.

You WILL have voltage drop.

You're just on the cusp of distance where I even bother doing the voltage drop calculations. DO NOT go straight to voltage drop calculators, as they make bad assumptions - first 3% is only the wire salesman's suggestion. And second you need to compute voltage drop based on your actual load, and NEVER the "breaker trip value".

For instance if you expect to use 70A, put in 70A and 5% and see what the answer is. Remember to do such calculations on 240V not 120V, and make sure to select aluminum wire, because they default to copper for some reason - I guess they are, oh right, they're wire salesmen!

Conduit all the way, if you can.

Since your wires will be in conduit, I suggest running the conduit the entire way from panel to panel. That way you can use the versatile, easy to pull and slightly cheaper THWN and XHHW type individual wires. For <=100A feeder you need a #6 aluminum safety ground. For >100A <200A feeder you need a #4 aluminum ground. You do need a ground WIRE in the pipe along with neutral.

If you want to switch to "stapled to rafters" inside the house, be VERY careful about your choice of cable. Most cable is either a) illegal indoors stapled to rafters, or b) illegal outdoors in conduit. Conduit is assumed to be 100% full of water 100% of the time. Also, cable in conduit is a nightmare to pull.

You must lay and bury the conduit before pulling ANY wire.

Some novices get it in their heads that they're going to slide each conduit stick over the 4 wires one stick at a time. No way, José! There are several fatal flaws with that idea. Code is concerned with damaging the wire with all that. I am concerned a) with the potential of building a conduit that is un-pullable, and b) how it turns the worksite into a huge ugly mess with high theft risk.

You build the conduit, fully complete, buried and tamped. Every turn has either a broad sweep or a pulling access point that will remain accessible forever (such as an "LB" conduit body). Never more than four 90 bends between access points (if you are DIY pulling, no more than two advised).

This actually works better because it means you only have a small work area at any given time. Need to go home, stuff a rag in the hole to keep dirt out, and come back and continue later.

Once the conduit is complete panel to panel, then for each segment between access points, you vacuum through a plastic grocery bag tied to a string, use the string to pull a stout pulling rope through, and pull the wires through. Repeat at each segment. Have a come-along handy just in case it gets stiff somewhere.

Did we mention BIG sub-panel?

Or... "Spaces are dirt cheap".

Here's a question we never get. "Help! We decided to get a hot tub and I need to add a 50A breaker to this panel. But ten years ago, we listened to StackExchange's advice and spent a pittance extra on a 30-space sub-panel instead of a 6. We still have 12 free spaces, where can I put this breaker?"

I want that to be your (non) question 10 years from now. 100A feeder can support a lot more stuff than you realize. And further, you're laying nice oversized conduit (not least for ease of pulling) so you could increase the size of the feeder later. So having a 30-space panel with 200A busing gives you a lot of future-proofing, dirt cheap... pretty much couch cushion change compared to the cost of the rest of the project.

  • I'm not an electrician which is why I think that this is such a terrific, thorough and comprehensive answer! I can understand it! Well done!
    – HoneyDo
    Jun 22 at 18:32

If your utility gives you a full 240v at full load then the minimum allowed size wire (copper #3 or Aluminum #1) allowed by the NEC will not normally give you a voltage drop problem at less than 200'. If you suspect that your machine shop equipment may have extreme power factor issues then upsizing one size is certainly not a bad idea, plus it would alleviate concern about substandard voltage supply or variation from the utility. (I once had a customer who insisted their parking lights were cycling. During the day I found they all they worked fine, came back at night and found the utility supplied lower voltage.)

Aluminum is much cheaper, even offsetting the larger size conduit that must be used. AA-8000 series aluminum alloy is reliable, and when properly installed does not have the problems that the old AA-1300 series aluminum had that gave aluminum it's bad reputation.

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