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Wanted to make sure I had the right size wire for the 125A Subpanel since it will be almost 200 ft away, went to buy a 125A breaker for the feed wires and they all only take 2/0 maximum. I have a two Challenger 200A panels with type BR breakers made by Cutler-Hammer and Eaton.
Wondering if there is a way to use 4/0 with the existing main panels? main panel

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    Do you need to have 125 amps at the sub panel? 125 amps is the max you can feed to it, but using 60 or 90 amp feed is perfectly okay. Have you done a load calculation to see if you have extra 125 amps available? The panel in the picture seems to have enough but what about the second one? What is the service size?
    – crip659
    Commented Oct 1, 2023 at 21:34
  • Well yeah I have thought a lot about it and really want 125A since I don't want to push or overload anything. the outbuilding will be lived in as an ADU and will have a mini split heat pump, a hot water heater, a stove, and someday maybe even an EV charger... basically everything a tiny house would have. The main panels are 200A Each but most of the heavy loads are on the other panel (HeatPump, Water heater, Ovens, Sauna). The panel in the picture does have a dryer, but mostly loads that are never used at all (Electric grill, Jaccuzzi tub)
    – Mike L
    Commented Oct 1, 2023 at 22:07
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    I'm not sure why you think you need more than 2/0. Normally 120A gets 1/0, and I don't even bother doing the math on a voltage drop calc til past 150'. So you're on the cusp of even needing one, and you're doing one for sure at 2/0. If it means that much to you, you can switch to copper wire... 2/0 copper is good for 175 amps.
    – Harper - Reinstate Monica
    Commented Oct 2, 2023 at 6:45

2 Answers 2

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TL;DR Stick with 2/0

In the hypothetical case of:

  • You have 125A of capacity to spare on your main panel
  • Your new panel will hit 80% continuous usage on a regular basis (100A)

you still only need 2/0 aluminum wires. Why? Check a voltage drop calculator:

  • 2/0 Aluminum
  • 200 feet
  • 240 volts (in most cases all the large loads will be 240V - dryer, oven, cooktop, water heater, EVSE, etc. - even if those loads have some 120V components, the vast majority of the usage is at 240V)
  • 100A at end of run (80% of 125A - you should not be going past that on a regular basis)

and you get a whopping 2.71% voltage drop. Which is under the 3% general guidance (which itself is not a hard and fast rule in most places, but works as a guideline).

So there is no reason to spend more on wire. Stick with 2/0. Yes, you can adapt 4/0 to 2/0 with various types of connectors - but those connectors, plus the wire itself, are unnecessary expenses.

Now there are a few additional things to consider:

Subpanel Size

Whether you feed the subpanel with 60A or 125A or anything in between, there is no reason to stick with a 125A subpanel. In fact, a 200A "main panel" is often a better deal. The included large main breaker is not an issue because the feed breaker takes care of the safety aspect. You need a disconnect anyway, and the main breaker takes care of that. Extra spaces for branch circuits is always a good thing, and 200A panels are generally larger than 125A panels. All you really have to do is make sure that neutral is not bonded to ground and ground wires are on a ground bar instead of the neutral bar - adding a ground bar is a minimal cost if one is not included. 200A panels are often available at very good prices with a bunch of "bonus breakers" which makes the panel an even better deal.

Feed Size

You must do two NEC Load Calculations:

  • Main service to determine how much capacity is available. With a typical 200A service you may find anywhere from no available capacity at all (unlikely but possible) to over 100A available. That determines how much current you can feed to the subpanel.
  • Subpanel to determine how much you need.

Both of these have to be done properly. It is not "add the breaker sizes". It is not "guess what makes sense/wishful thinking".

There are a number of things that can be done to reduce the load, if necessary, including:

  • Natural gas or propane for water heating, dryer, cooking, space heating
  • Automatic load shedding to reduce one load when another is in use
  • High efficiency heat pumps instead of resistance electric heat

and don't even think about putting in electric tankless water heating.

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A short 6" to 6' section of 2/0 connecting the breaker to the 4/0 will not add any significant resistance to the 200 foot run. A butt splice, split bolt, or insulated polaris connector in the panel or a junction box below the panel would work just fine.

I also suspect you may not need or be able to support a 125A panel, a load calc will likely be required before an electrical permit will be approved.

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  • Thanks NoSparksPlease, I had no idea, nor would I have ever thought you could do that! I always looked at wire size and resistance in a "Lowest common denominator" type of way... thought the maximum would be dictated by the smallest point/wire and you couldn't go up from there. And yes, a load calc will be the next step, I am prepared to eliminate certain loads if need be, mainly by going with a propane water heater and cooktop/oven. I am wondering if a 4/0 solid pin terminal connector with a 2/0 pin would work as well? that should fit in a breaker that takes 2/0 right?
    – Mike L
    Commented Oct 2, 2023 at 0:11
  • Just to confirm, voltage drop is just a result of ohms per foot, a few feet on either or both ends doesn't matter. I have found pins work pretty good when feeding the main breaker, but the space between breaker and side of panel can is pretty close and the radius on the 4/0 may not be workable.
    – NoSparksPlease
    Commented Oct 2, 2023 at 1:18
  • Wires overheating because too thin to carry expected load are a least common denominator phenomenon. 1cm of too-thin wire in 200ft will start a fire. That's how fuses work. But wires safely shedding heat and thus causing power loss because too long, is just an additive phenomenon. A very short segment that is a little thinner than ideal, but not so thin to be dangerous, will only cause marginal additional loss. That's also how fuses work!
    – jay613
    Commented Oct 2, 2023 at 1:35
  • Heat is the result of current through resistance, Extended length of 135A 2/0 Al won't increase current, it will increase resistance and reduce current resulting in less heat. The increased accumulative resistance causes the voltage drop on the wire to be a larger component in the circuit resulting in less voltage at the point of use. Increasing the wire size offsets the accumulative resistance, restoring full voltage to the point of use. 2/0 Al is .000159 Ω/ft (NEC Ch 9 Table 8) and 125A would result in 2.5w heat per foot, not enough heat to damage the wire.
    – NoSparksPlease
    Commented Oct 2, 2023 at 3:31
  • @NoSparksPlease: Increased resistance of the feed wire will have a negligible effect on current (which is controlled by the load) and possibly even increase it (if the load has any sort of feedback such as SMPS, due to the decreased voltage reaching the load). But even without the feedback, the resistive losses (heat) will be higher not lower.
    – Ben Voigt
    Commented Oct 2, 2023 at 14:58

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