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100 amp sub panel for detached garage

p_550spyder | Posted in General Questions on

Hi all,

Calling on electricians or anyone with good experience and knowledge with electrical work. I recently just updated my electrical service in my home, to 200 amp service with a new panel box. I’m now looking to install 100 amp sub panel in my detached garage.  From my main panel, situated at the front the house, the distance required to run wire to the garage would approximately run 100′. The length of the actual house is about 45′, & I would need to add another 12-15′ of wire upwards along the side of the building for an overhead application. From the end point of the house to the garage would be approximately another 40′ +, bring the total to 100′ + in distance/length, as mentioned earlier.

My questions: 

1) What size of service wire is needed for 100 amp sub panel, based on the distance/length of wire (100′ +) factoring in voltage drop, for both both copper and aluminum wiring? 

2) In addition to the size of wire, I’m also looking to see if there is an armour cable wire that would house all the wires required (hot, neutral, ground) in one larger size cable jacket for a 100 amp sub service wires that could be used externally, since it would be exposed to the elements, without having to go the PVC conduit method?

Reason for looking at an armour type cable that encompasses all needed wires is it would offer a simpler method to mounting the wire to the side of the house instead of using PVC conduit and feeding the wire through. The challenge with PVC would be bending around two protruding chimney stacks that run the full height of the house, plus another sweep or two that might be needed internally from my main panel box.

Hoping for some great knowledgable folks to help answer these questions and offer any valuable advice or guidance. Sincerely appreciate all the help! 

Thank you,

GBA Prime

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  1. Expert Member
    BILL WICHERS | | #1

    You're OK here with #2 copper or #1 aluminum wire, both are under the recommended 2% volt drop for a feeder. You would need three of those size conductors (hot/hot/neutral), plus a #8 copper ground, or #6 aluminum ground.

    There is armored cable that could work for this, but you really don't want to use it -- it will be hugely more difficult to install compared to conduit. The reason for this is that heavy, armored cable like this is massively heavy, and difficult to bend, so it will fight you every step of the way when you're installing it. It's much easier to run seperate wires inside conduit, since that lets you split things into steps, first running the conduit, then pulling in the wire.

    If you don't need the armor, you could use SER cable, but it needs protection from physical damage. Note that if you have an aerial run (overhead), you should be using triplex type cable, which is the twisted-looking cable commonly used by utility companies for power drops to houses. You would need 3W+G type triplex here, so that you could keep the neutral and ground seperate. You can't run SER overhead. You could run single conductors overhead with an insulator rack on either end, but it won't be as clean of an appearance.

    If at all possible, I would try to run this underground. If you go underground, you can run 1.5" PVC conduit underground, then pull the needed wires into the conduit. I'm not a fan of direct bury cable, and always run conduit for underground runs. With conduit, you install the conduit first, then pull in individual conductors of the correct type for the application (THWN for copper, or XHHW for aluminum in this case, although there are other less common wire types that would also be suitable).


    1. charlie_sullivan | | #3

      Just as an opportunity for me to learn, why do you recommend THWN for copper but XHHW for aluminum? It looks like all four combinations of THWN or XHHW and Cu or Al are available, and I would assume that XHHW is both better and more expensive with either conductor material.

      1. Expert Member
        BILL WICHERS | | #4

        THHN/THWN (most of these two types of wire are dual rated) is the most commonly available standard copper building wire. XHHW is more common for aluminum wire when ordered as single conductors, like you'd need if you were planning to pull it into conduit. THHN/THWN is vinyl insulation with a nylon outer jacket, and that relatively hard and slick nylon outer jacket makes this wire easier to pull into conduit. Note that the "W" in "THWN" is for water resistant, which limits the 90*C THHN to 75*C when used in wet locations. Note also that "THWN-2" is THWN wire that is good all the way to 90*C. Temperature ratings over 75*C are only helpful with derating when used in multicircuit conduits though, since the terminals on the circuit breakers usually limit you to using the 75*C column of the ampacity chart for wire sizing. You need the THWN or THWN-2 rating here because the cable run underground would be considered a "wet" location, even though it's inside conduit. XHHW is cross linked polyethylene, and while it's very tough and can take higher temperatures, it's also grippier, so it's more difficult to pull into conduit. XHHW is acceptable for use in wet locations like THWN. There are some other advantages to XHHW, notably less nasty stuff is produced in a fire with XHHW than with THHN/THWN wire.

        In the case of aluminum wire, the additional protection of the XHHW jacket material is a plus, since aluminum is more prone to oxidation issues if it gets exposed to moisture. The downside is the jacket isn't as slick, so it takes more force to pull XHHW wire into conduit. In the case of copper wire, the somewhat lighter THWN style jacket isn't as much of an issue, and it's also easier to pull into conduit. This is why I recommended those particular two types of wire.

        I have not myself ever seen THHN/THWN aluminum wire, but you certainly can get copper XHHW wire. It would not hurt to use copper XHHW wire here, the only downsides being a more difficult pull to install it in conduit, and likely slightly higher cost and less availability of the material.


    2. p_550spyder | | #5

      Thanks Bill for the feedback. Great advice for suggesting the conduit over the armour cable, a few things there I didn't consider. I will need to price out the difference between the copper vs the aluminum but I suspect copper will be substantially more expensive. It was when I was choosing for my main service panel install. That being said, I'd prefer going with copper in many ways, other than the fact it's harder to bend than aluminum.

      Going underground would be ideal in many ways, it certainly offers a more cleaner approach with no visible wires overhead and easy to feed wires once it's in place. Unfortunately, in my case it would involve cutting some concrete out, excavating part of a patio and trying to navigate around some roots from a few established trees and shrubs, which will certainly make challenging. Is why I'm likely considering the aerial route.

  2. joenorm | | #2

    Not to be a squeaky wheel but if you don't know how to find the answers to these questions in the NEC I think it's best to hire a licensed electrician to at least draw you up a plan and maybe even do the install. You'll need a grounding electrode system for a detached garage on top of what Bill has has mentioned.

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