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I have to add an outdoor sub panel rated 60 AMPs and I need at least 8 spaces for breakers. I am not finding this anywhere unless I buy a 125 AMP rated breaker box. Is it acceptable to install a 125 AMP rated panel when the need is only 60 AMPs. I have 200 AMPS service in the house and have space for a 60 AMP breaker in my main panel

  • I presume this breaker box is on a detached structure somewhere? – ThreePhaseEel Jun 4 '20 at 23:54
  • Yes. its 600+ feet away from the house – Anwar Jun 6 '20 at 1:26
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Please install a bigger box!!!

Your box definitely should exceed minimums in this area: Number of spaces. Listen, there's always a future project, there's always another load. The out-of-pocket cost for a larger panel is very small (especially when you consider other factors, as we're about to). But the regrets and unnecessary expense when you run out of spaces is a rather expensive affair.

As such we very strongly recommend you wildly oversize your panels. You need 8 spaces? Consider nothing less than 24 spaces. All of our panels here have plenty of extra space (except an 8-space Pushmatic that's a nightmare) and that's the way we like it. I myself would go 30-space.

I mean "space" not "circuit" - you see a lot of 20 space/40 circuit panels - that's only a 20-space.

You wouldn't drive 85 mph on 85 mph-rated tires, would you?

Suppose you lived in Nevada where posted limits are 80 mph. Yeah, you'll want the 115 or 130 mph tires I think :)

The same applies to service panel amp ratings. You wouldn't run 60A on a 60A box, would you? Of course not, you want some safety headroom. So 125A or even 200A is perfectly reasonable. It's a "limit" thing, not a "mandatory match" thing.

This also ties into the "spaces" problem because I gather you were feeling stifled with only 8 spaces. As well you should! You are at liberty to get as large a panel as you want.

You need a main breaker there!

To be more precise, you need a main disconnect switch. But the cheapest most practical way to get that is to use a box with a main breaker.

Now I happen to know none of the 8-space panels on the market include a main breaker. So setting up one of those "straight" would be a code violation. If there are 6 or fewer throws, "no main breaker" can be legal, but never with 8.

You might skate by the "Rule of Six" if two of your breakers are 2-pole. However that "paints you into a corner" of being unable to add a seventh throw in the future. And you know you will!

Voltage drop - yikes

Obviously, voltage drop will be a big problem. At 2 AWG wire and 48A draw (figuring for 80% of your 60A breaker that you seem happy with), your voltage drop will be 7.04%, which is high even by my standards.

That would not be acceptable in Canada or in a Tim Horton's restaurant. In Canada you'd be required to use 4/0 AL wire, which is ridiculous in my opinion, eh?

I haven't priced 600' of that stuff but I'm sure I will not like the price.

By the way, if you aren't in Canada, you are allowed to breaker the feed breaker for whatever the wire limits are, e.g. 90A for #2 aluminum.

Transformers!

Like I say, I haven't priced the fat wire, but it may be worth looking at transformers for the long haul. The wire is good for 600V (and 600V transformers are readily available in Canada), but around the States 480V is more likely to be findable cheap used. If you're buying new transformers then price 600V.

In this case, you use 2 transformers back to back, 480V-jumpered primaries facing each other. This steps up the transmission voltage to 480V. Now, the wire gets a lot thinner (for the same voltage drop).

If you do it Canada-style, holding voltage drop to a firm 3% at 80% of rated current, then #4 aluminum will suffice, eh? From 4/0 down to #4, that's the power of transformers for 'ya! If you don't mind that 7% drop from your original proposal, are you sitting down? #8 aluminum or #10 copper gets it home.

So transformers let you drop 6-8 numerical sizes. More if they are 600V transformers.

However, I would recommend #6 aluminum at that point, giving a very acceptable 4.29% drop at 48A practical draw (80% of breaker trip).

You probably won't push the wire to its thermal limits. The reason is the transformers are a limiting factor also. I'm imagining you get 15 KVA transformers which will limit you to 60A@240V total throughput and you'd have to breaker for that at the supply side to protect the transformer. You might get a better deal on used 25 KVA transformers, which would let you got 100A @ 240V. If that's the plan, a bump to #4 Al wire might be called for.

Obviously transformers are not free, but they make the wire a lot thinner, so you have to crunch the numbers and see what you get.

  • Hey Harp, Ed B. or 3ph. Does the "rule of 6" apply to sub-panels? I clearly know it applies to primary disconnects (dating back to the old "split bus" panels) as well as other situations, I'm sure. But was curious if it applies to sub-panels. – George Anderson Jun 4 '20 at 19:18
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    @GeorgeAnderson I believe it may still apply to subpanels which require a disconnect owing to their status as an outbuilding/remote location. – Harper - Reinstate Monica Jun 4 '20 at 21:26
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    Harp: Before we go too much over-kill here, we don't know what the OP's power requirements are. What is (s)he planning on running? Lights, some convenience outlets? A "grow" operation? Heavy duty power tools? I'd like to know more before recommending step-up/step-down transformers or overly large wiring. What are the power needs? – George Anderson Jun 4 '20 at 23:49
  • One caveat with the transformers: the OP will need a 480V overcurrent device somewhere in the circuit between the two transformers. Fortunately, this can be done relatively cheaply with a 600V class RK5 fuse in a matching fuseblock parked in the house-end transformer's wiring compartment, since the transformer already has a disconnecting means on the primary side. – ThreePhaseEel Jun 4 '20 at 23:56
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Yes. That's fine. It's always OK to go bigger on a panel and a good idea, as others have stated many times here. What matters is the size of the wires (feed) and proper overcurrent protection in your main panel. So be sure your feed breaker is sized appropriately for the wires (or cable) that feeds your sub-panel. I have a 200 amp sub-panel in my shop that is feed by a 125 amp breaker and correctly sized conductors, passed inspection with no issues.

  • Thanks. This 60 AMP sub Panel is 600+ feet away from the main 200 AMP load center. Because of this distance, I am thinking 2-2-2-4 underground Aluminum feeder with two quazite boxes – Anwar Jun 4 '20 at 17:47
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    Wow at those distances I start looking into step up and down transformers. Bumping the voltage up to 600v then back down allows much smaller wire to be used saving more than the cost of a pair of transformers and a disconnect. – Ed Beal Jun 4 '20 at 18:35
  • Interesting thought Ed. It didn't occur to me probably because I don't know much about transformers. So, what do i need to consider for 600+ feet of distance. I will have 3 Pond pumps all 120 Volt and 8 amps each, 24 Lamp posts with 3 light heads 180 watts each post, 6 additional 20 AMP GFCI outlets and lastly agate opener for which I don't have the specs yet, but I can make some assumptions based on google info. Any help you can offer would be greatly appreciated. I agree heavy gauge wire is very expensive and also hard to work with. Thanks in advance. – Anwar Jun 5 '20 at 16:21

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