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I am out of spaces in my main 200amp service panel.

I have a double pole 40 amp breaker that is available (or I can change it). 5(3 subpanel is 33 feet away indoors. I will need 5 poles (spaces) in subpanel, and the total amps used from all items will be 39/22. I am Using 12/2 to go to all outlets from subpanel.

What wire should I be using from main panel to subpanel?

Do I need a ground rod at the subpanel?

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  • A 40 amp breaker needs a minimum of 8 gauge, larger gauge won't hurt. If using 39 amps(actual) at the same time, then you might need to use a 50 amp breaker with 6 gauge wire minimum. If sub panel is in the same building then I think you only need the ground wire back to main.
    – crip659
    Jul 12 at 23:08
  • @crip659 Write that up as an answer. One tidbit to add: double-breaker amps cover both legs. If (as likely based on 12/2) the connected devices are 120V then that's really 20A in use (if split appropriately across the two hot legs) and not 40A, so a 40A breaker may be plenty. But for 33 feet, go with 2 AWG Aluminum and up to 90A breaker and save money over even 8 AWG copper. Jul 12 at 23:18
  • @manassehkatz-Moving2Codidact My comment might be a partial answer, but even your comment has more information, than what I could or should give.
    – crip659
    Jul 12 at 23:25
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    OK. Then I'll write it up. Jul 12 at 23:32

2 Answers 2

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There are a few different issues here, but basically:

  • Breaker is sized to protect wire
  • Subpanel can provide any amount up to the feed breaker, though it can have (for convenience if bundled) a larger main breaker.
  • Nameplate values matter, but with plug-in receptacles anything is possible and you should plan for that (within reason).
  • 120V vs. 240V makes a huge difference.

And now the details:

Generally speaking, 10 AWG will get you 30A, 8 AWG 40A, 6 AWG 55A. However, if you are using individual conductors in conduit (instead of cables) and 75 C terminations (which, as I understand it, is generally the case for most standard circuit breakers and panels) then you get more - 8 AWG 50A, 6 AWG 65A. The breaker in the main panel connected to the feed wires to the subpanel has to match the wire or be a lower value (with certain exceptions).

However, you can also normally use aluminum wire instead of copper wire when connecting panels, and that can save a lot of money even though you need to use larger wire. For example, 2 AWG aluminum will get you 90A, enough for a huge subpanel, and cost (typically) close to the price of 8 AWG or even 10 AWG copper.

If you need "5 poles" in the subpanel, presumably that means 5 single breakers for 120V loads. Get at least a 12 space panel (larger is fine) so that you have room future expansion.

A subpanel in the same building does not, as I understand it, need a shutoff or main breaker. However, it can have one, and sometimes a "main panel" that includes a main breaker makes sense. If so, that main breaker can be any size equal to or larger than the feed breaker in the actual main panel. But you need to make sure ground and neutral are kept separate in the subpanel, even if they are mixed together in the actual main panel.

You do not need a ground rod at the subpanel if it is in the same building. If it is in a different building then you probably need two ground rods, and a shutoff/disconnect as well.

40A (39.22 rounded up) of 120V loads is really only 20A of 240V load if perfectly balanced. Most of the time things are not perfectly balanced, but it is reasonable to use a 40A double-breaker to feed a subpanel with those sorts of loads. That being said, upsizing will allow flexibility and future expansion.

Also keep in mind that if the subpanel includes any ordinary receptacle circuits (with 15A or 20A 120V receptacles), while you may think "3A for xyz tool", the next user may plug in a 12A space heater. So plan to allow for more load than you need right now.

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    A lot more information than what I could supply, and I learn something more.
    – crip659
    Jul 13 at 0:22
  • You sure took me to school, Thanks but I'm just a laborer, I put the wires in and hook them up. So let me simplify it for me. Subpanel will have 5 circuits of 120V each with 20A fuses, the most draw on any one circuit will be 12.5A. Does it mean I need a 100A subpanel ? Also if I use 6AWG, the main breaker will need to be 50A? This is a small room and there will be no room for any expansion.
    – Randall
    Jul 13 at 1:34
  • You put 2 20A breakers on one leg and 3 on the other (just stack them all up in a column and will do it, because the rows alternate). So that's an absolute max. of 60A per leg. An expected max. of 37.5A, but you have to allow some extra, particularly if they'll all really be in use at once. So 50A is the minimum I'd recommend for the feed, and 60A makes a lot of sense. Yes, you could use 6 AWG copper and 50A, but it will cost less to use 2 AWG aluminum and 60A (and by changing that feed breaker you could go as high as 90A with the same wire). Jul 13 at 2:47
  • Will this breaker box work - Square D Homeline 125 Amp 12-Space 24-Circuit Indoor Main Lug Plug-On Neutral Load Center with Cover, Ground Bar - Value Pack ? Also will be pulling feed wire thru wall and floor before getting to crawl space, may be a tight pull so need wire that will pull easily, not sure about 2awg aluminum being able to make the bends. I will be removing 3-8awg wires from this pathway and pull new feed wire thru with last 8awg to be removed hopefully.
    – Randall
    Jul 13 at 16:20
  • Yes, that panel sounds find. As far as pulling feed wire, we need clarification. If the existing wire is a cable then you will replace one cable with another. If the existing wire is individual wires inside conduit then we need to know what size/type of conduit in order to determine the maximum size individual wires that can be run through that conduit. Jul 13 at 16:24
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Planning for the smallest possible is a bad mistake. You will only be in trouble soon after, when you run out of spaces again!

Cable prices are weird, because aluminum wire is considered bad at small sizes but good at large szies. As such, the best value for wiring is actually #2 aluminum, such as 2-2-2-4 SER. You can use a 60A to 90A breaker with it. The advantage to 60A is that breaker is cheap.

You need 4-wire cable. Neutral must be separate from ground in the subpanel. You will need to buy an accessory ground bar. (A few panels come with it).

On the subpanel, your best bet is to Think BIG. The fact is, panel spaces are cheap. Running out of panel spaces is expensive (I hardly need to tell you that, do I?) So The cheapest thing to do is actually get a nice big panel with PLENTY of spaces - 24 or 30 spaces is not excessive.

It does not need a ground rod if it's in the same building, or if the buildings share a wall or are connected by a hall or breezeway. In the same circumstances, you do not need a main breaker in the subpanel (but hey, if it's free, take it). The size (amps) of the subpanel's main breaker is not important.

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  • IIUC, the sub panel breaker can actually be larger than the one supplying it from the main panel because if there is a draw greater than the breaker in the main panel, that breaker will trip, even if the one in the sub doesn't. The sub panel's main breaker must simply be larger than the expected (or calculated code required) concurrent load. Correct?
    – FreeMan
    Jul 13 at 14:35
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    @FreeMan Yeah, it's not allowed to be smaller than the calculated load. It could be, but then it would just trip a lot lol. Jul 13 at 18:08

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