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A power distribution box would be nice for connecting different loads to a generator at an off-grid job or camp site. But commercial units are big, heavy, and expensive, and they offer some features I don't need, while lacking some of the connections I'd like.

Here's an example of a commercial product.

Could I build a safe, simpler, cheaper version of this myself?

My generator is rated for 40 A peak, 30 A continuous. Its 240 V output is on a NEMA 14-50 receptacle with a 40 A breaker. Like the commercial products, my box would have a suitable generator inlet, and I'd connect the generator to the box with an extension cord.

I'd like a distribution box with another 14-50, a duplex 5-20, a 6-50, and maybe a 6-20. I anticipate using the box with other equipment that needs to be sheltered from weather, so while a weatherproof enclosure might be nice to have, I don't require it.

What would make a suitable enclosure?

What should I use as connectors internally? I've seen terminal blocks but not with insulated jumpers rated for this current. I've also seen set-screw connectors with a cover that looks like a wire nut.

My understanding is that the generator breaker will protect my box from overcurrent, but should I use GFCI outlets for the 5-20s? Is there other protection I should incorporate? Any opinions about the usefulness of a power switch at the box?

Should the components be rated for 50 A like the generator outlet, even though it's on a 40 A breaker? Is there some standard derating applied here?

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    Be mindful of weatherproofing when making a DIY spider box. Also, depending on what kind of setting you're using it in, there may be special requirements for the cabling as well. For example, I sometimes use a small spider box to provide power for events (bandstand, speakers, lighting, etc.) and I use cable called SOOW to meet the requirements for this type of use. Dec 1 '20 at 10:37
  • @JeffWheeler Thanks for the terms! It can be tough to search for info when you don't know the jargon.
    – erickson
    Dec 1 '20 at 15:28
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    Note to self: 50 A service uses CS6375 inlet which accepts a CS6364C connector. See e.g. StayOnline.
    – erickson
    Dec 2 '20 at 16:54
  • CS connectors are durable but also expensive. I use the 50A 125/250V one as the inlet on my own spider box. I don't necessarily count on that type of connector being available as "shore power" though, and have a cable (6/4 SOOW) with bare ends for connecting to panels or whatever. Dec 2 '20 at 18:17
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You absolutely need breakers for the 5-20 and 6-20 receptacles - can't connect those directly to a 40A breaker with any hint of safety. And all the outputs would benefit from GFCI protection, given that you plan outside use; clearly required on the 5-20s and highly advisable on the other outlets.

So, you need a subpanel, at least.

Given a 50A input connector, "your intended use with a 40A generator" is moot - someone could plug this thing into a 50A receptacle, so your provisioning, wire size and protection scheme needs to be based on that case. If (common, but can't be assumed, has to be checked) your 50A devices have connections rated for 75°C you can wire them with 8 Ga copper or 6Ga aluminum and suitable wirenuts or Polaris connectors, or split nuts insulated with tape if you want to go old-school (you probably don't.)

Your 20A stuff, on the far side of the 20A breakers feeding it, can be 12Ga copper.

Depending on your choice of subpanel, you might be able to simply attach a few 4x4 boxes to it to hold the inlet and receptacles. Otherwise you'll end up mounting them all (subpanel and boxes for receptacles/inlet) to a suitable substrate, such as a hunk of plywood, or a plywood box. Size the plywood box right and your inlet cord will fit into it.

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  • Exactly what I was thinking, except that I don't know enough to say "Yes, this is good" and to spit out the wire sizes like that! Dec 1 '20 at 3:10
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    Professionally I'm on the low-voltage side, but at home I'm "everything after getting the main panels put in" and I try to stay on top of things enough to keep it right (also frequently "better than code" or "NYC/Chicago-level in the boonies" since I grasp that my boonie fire department will take some time to arrive, so not catching on fire is my best option...)
    – Ecnerwal
    Dec 1 '20 at 3:42
  • You'd think that not catching fire in the first place would be on most people's minds... ;)
    – FreeMan
    Dec 1 '20 at 14:44
  • Oh no I wouldn't. I've met most people...saving a nickle .vs. decreasing the odds of fire almost always goes to save that nickel, or cut corners and save a dime while increasing the odds of fire.
    – Ecnerwal
    Dec 1 '20 at 15:34
  • Thank you! I should have realized the need for protecting the 20 A circuits myself. That makes me worry about other dumb mistakes I'd make. But, wiring this something like a detached garage makes a lot of sense. Are there rules about leaving some space in a cavity around a panel or boxes? I figure, done right, there should be zero heat and therefore no need for ventilation. But many code provisions seem be there as a backup for other failures. Also, do generator inlets tend to use a standard plug depending on amperage? Or are they proprietary by manufacturer?
    – erickson
    Dec 1 '20 at 15:39
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Subpanel on a post

J random NEMA 3 subpanel, 1" EMT conduit nipples coming out of subpanel in every direction to drawn steel 4x4 or 4-11/16" square boxes, containing outlets and one inlet. All screwed to a 3/4" piece of plywood, that's then attached to a post. Done and dusted.

Even though it's EMT, still run the ground wires so in case the thing gets bashed in handling.

Done for the day, unscrew it off the post and throw it in the truck. Hence the use of drawn not welded metal boxes.

Square D "QO" and Eaton "CH" both have 8-space subpanels that are on the order of 10"x14", so this thing will be liftable in the end.

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I don't think you'll save much money building your own spider box. It's much better to purchase one that's built correctly and known to be safe

Using a power distribution device that does not meet local codes or have UL or CS approval can void your home owners or business insurance coverage. It's jut not worth the risk to save $50 to $100.

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    Most of the 50 A boxes I've seen are $400 and go up (way up) from there, but I am not sure I'm looking in the right places. I'm not sure if specific product references are "on topic" for this site, but if you have examples, it would help my search.
    – erickson
    Dec 1 '20 at 16:22
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    Oddly, @erickson, asking for product recommendations is OT. However, offering them in an answer seems to be perfectly acceptable!
    – FreeMan
    Dec 1 '20 at 16:49
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    I think the problem is that the prefab spider boxes don't provide a receptacle configuration that works for the OP Dec 2 '20 at 0:28

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