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Copper vs. Aluminum Service Wire

derekr | Posted in General Questions on

I’m not installing this, electrician is
he said I can use either one of these and I wanted some opinions on which I should use

i was looking at the specifications of the #2/0 copper and it says it’s good for 195 amps but my breaker panel is 200 amps?

the aluminum says it’s good for 205 amps

i was thinking copper would be better since it’s going in a crawl space and has more corrosion resistance than aluminum but wasn’t sure about the 195 amps

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    There is a special rule in the electric code (Table 310.51(B)(7) that allows 2/0 copper, or 4/0 aluminum, to be used for 200A residential services. Your electrician is correct with this wire sizing in this case.

    Note that when you mention 2/0 copper is good for 195A, and 4/0 aluminum is good for 205 amps, you are reading those ampacities from the 90*C column. You need to use the 75*C column, which is 175A and 180A for those two wires, respectively. The reason you have to use the 75*C column is that while the wire itself may be rated for 90*C, the connections on the circuit breakers are only rated for 75*C, so you are limited to using the 75*C column in the ampacity table for that reason.

    There won't be any difference between copped and aluminum wire in a crawlspace, since the insulation will protect both from corrosion. The only issue is typically at the terminations. I prefer to run these service conductors in EMT conduit for additional protection, but you can use the larger composite cables (such as SER) here. If the run is fairly long, you'll have a seperate disconnect/circuit breaker out at the meter, so the cable will have overcurrent protection. I am most concerned running inside conduit for cables that do NOT have the remote disconnect, which mean short (less than 10 feet or so) runs per code. I would probably just use aluminum wire here, since it's likely to be cheaper. There is no difference between the two in terms of volt drop, either, since the reason the aluminum wire is a larger size for the same ampacity is to cancel out the difference in resistance between the two.

    Bill

    1. derekr | | #2

      It’s about a 30-40 foot run

      You said the copper vs aluminum could make a difference at the connection points?

      I don’t mind the copper costing a little more as long as theres some benefit, if there isn’t I’ll just get aluminum

      1. Expert Member
        BILL WICHERS | | #3

        Copper is physically smaller for the same ampacity, and it's a little heavier too. Copper is less prone to issues at the terminations (the lugs where connections are made), due to less issues with oxidation. Copper is considered to be longer lived, but you're never going to see any difference in that regard.

        To deal with the connection issues, I like to spec no-ox filled compression lugs, but those are very rarely seen on residential projects. The regular mechanical lugs (the kind with an allen screw to clamp down on the wire) are rated for use with aluminum wire, and are rarely a problem as long as they are secured tightly at install time.

        The biggest issue is cost, with copper being up about three times compared to about two years ago. Aluminum is up some too, but not as much. I would probably go with aluminum here for cost savings, but it's really up to you -- you'll see no difference in the functionality of your system either way. In my own home I used copper wire in EMT conduit between the meter and the panel, but it was cheaper when I did the installation, and I get a good price since I use so much wire at work.

        Bill

        1. Expert Member
          Michael Maines | | #4

          Bill, I've seen electricians coat the terminations with a white goo when using aluminum conductors. Is that common practice and/or advised? Do you know what the goo is? I assume a lithium grease but I'm not sure.

          1. Patrick_OSullivan | | #6

            It's Noalox or similar (https://www.idealind.com/us/en/shop/30-026.html).

            It's used almost universally on aluminum conductors even though the code doesn't stipulate its use. Inspectors often look for it because earlier alloys used were more prone to corrosion. Technically you only need it when the manufacturer specifies its use.

          2. Expert Member
            BILL WICHERS | | #7

            The "goo" you saw is usually called "no-ox" or "noalox" in the trades. It's also usually gray, but there are other variants. It's intended to prevent corrosion on terminations, but it's not required if the terminals are rated for aluminum wire (which most of the big ones are). It won't hurt to use it though, and I often spec it as some extra insurance.

            Note that most people don't apply the no-ox paste correctly. Most people just smear some on the exterior of the conductors of the wire prior to shoving the wire into the lug. The correct way to apply it is to coat the conductors, but to also work the no-ox paste BETWEEN the strands of the conductors too (usually using a small brush). Ideally, you want an even coating over the entire surface of each strand of the wire prior to inserting the wire into the lug. Keep in mind that the no-ox paste is usually optional though with modern lugs.

            Bill

    2. charlie_sullivan | | #14

      Just for fun, it's interesting to note that the 4/0 aluminum has slightly more resistance per unit length (82 milliohms/1000 feet vs. 80 milliohms/1000 feet), and yet it's rated for more current than the copper (by a few percent). Neither of those is enough to matter in choosing between them, but it seems backwards. The reason is the the bigger diameter aluminum can dissipate heat a little better, given its larger surface area.

  2. derekr | | #5

    Is the oxidizing of the aluminum something I should be worried about or is it something that can take 100 years or to occur?

    What’s the worse that could happen in this scenario? The breaker just pops and I get a new cable? Or could it start a fire

    1. Expert Member
      BILL WICHERS | | #8

      The oxidation happens instantaneously when aluminum is exposed to the oxygen in the air. That's the reason aluminum doesn't appear to "rust" -- the oxide layer forms immediately, and it stays attached to the exterior of the aluminum (unlink rust on iron/steel that flakes off), forming a protective barrier that prevents further oxidation.

      The problem is that the oxide formed is an insulator, so it increases resistance in the contact area of the termination, and that means more heat when current flows. In practice though, the lug will bite through enough of this that it doesn't matter much. The big problem is if you use dissimilar metals, such as aluminum wire and a brass lug. That's where problems occured, and that's also how aluminum got a bad name. All the big lugs are actually MADE of aluminum now though, so it's really not a problem anymore with modern aluminum alloys and proper termination methodology.

      If you want to save money and use aluminum wire, just spec no-ox be used on the terminations, which is easy (the no-ox paste is cheap), and pretty much eliminates any potential issues in the future. Make sure the no-ox paste is applied correctly as I detailed earlier in the thread, and make sure the lugs are properly tightened.

      Note that loose lugs are just as much of a problem with copper wire too, so regardless of the type of wire you use, make sure the lugs are properly tight (which is different than "superman tight", BTW :-).

      Bill

      1. derekr | | #9

        What’s the difference between these? Can I use either one? It says they are both service entrance cables

        Any advantages or disadvantage over one or the other

        The one with 2 cables is a good bit cheaper than the one with 4

        I have a Leviton meter box and Leviton breaker box if that makes a difference

        Also thanks for all the information

        1. Patrick_OSullivan | | #10

          This is a question for your electrician. You will basically never see the SEU (2 conductors with additional concentric conductor) indoors because it would imply that the cable has not reached the service disconnect yet and therefore has no overcurrent protection. That cable would be incredibly vulnerable inside because a fault on it would be limited only to the fault current of the street, typically 10,000 amps in residential areas.

        2. Expert Member
          BILL WICHERS | | #11

          The one on the left is SE cable, two hots and a concentric neutral (the wires wrapped around the outside of the inner conductors). The one on the right is SER cable, which has two hots and a neutral (the three insulated conductors), and a bare ground.

          SE cable is used where ground and neutral are the same thing, which is ONLY the case BEFORE the main disconnect. For a typical residential electric service, this means between the meter and the main breaker. The exact limit for the length of this cable varies a bit with local codes, but is usually 5-6 feet inside the structure as a max. Since this cable would be BEFORE the main disconnect, it doesn't have any protection aside from the primary fuses on the high voltage side of the transformer serving your house (more on that later).

          SER cable has an extra neutral wire so that ground and neutral are seperate conductors. This is required by code when you have a main disconnect at the meter feeding your panel, because with this setup, the main disconnect is also where your ground/neutral bonding point (connection) is located. The panel inside your house now has to have the ground and neutral busbars seperate and isolated from each other (which means the neutral busbar will be insulated from the enclosure). This cable can go as far as you want, within voltage drop limits, since it has upstream protection in the form of the remote main breaker located at your meter.

          Aside from those two differences, there is no advantage or disadvantage to either cable type -- they are for different purposes. If you have a decent length run (25, 50 feet, etc.), which I presume you do if you're going through the crawl space, then you have to use a remote disconnect, which means a breaker or fuse at the meter, which also requires you to use SER cable (or four wires in conduit).

          Regarding fault current, the typical RATING of residential breakers is 10kAIC, which means 10,000 Amps Interrupting Capacity. That means the breaker or fuse is rated to be able to interrupt (stop) the flow of current into a fault (like a short circuit) as long as the power source can provide no more than 10,000 amps into that fault. Those limits typically come from what is known as the "source impedance" of the power source, which is usually dominated by the transformer impedance in the case of residential services (and most commercial services too). For a typical residence served by a typical 50KVA transformer, the maximum available fault current is likely to be less than about 5,600 amps. I would be very surprised to ever see more than around 8,000 amps of available fault current in a residential building. There are higher rated breakers (and fuses), but they are only usually used in commerical settings.

          Bill

          1. derekr | | #12

            K sounds like I only should use the SER then

          2. Patrick_OSullivan | | #13

            > For a typical residence served by a typical 50KVA transformer, the maximum available fault current is likely to be less than about 5,600 amps.

            I mean, once you're in the thousands, what's a few thousands between friends? :-)

            My town requires an affidavit submitted with a permit application for electrical service stipulating the fault current provided by the utility. If you contact the utility about this, they will simply tell you 10,000 amps (once you finally get in touch with someone who can/will answer the question).

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