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I am looking at a 20amp wall socket from Leviton (5380). It has surge protection at the socket. I noticed in the description that it says "Do not install this device if there is not at least 10 meters (30 feet) or more of wire between the electrical outlet and the electrical service panel."

I'm curious what the reason is for that?

My use of the socket is to be able to connect to a 120V, 30amp portable generator. Rather than running extension cables in through a window or door, I want to put a 120V 30amp inlet box outside to connect the generator, and wire it directly from the recepticle to two 20amp (Leviton 5380) wall sockets on the inside.

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    My guess would be transit time isolation to prevent weird feedback to the surge protection circuit.
    – Jon Custer
    Commented Jul 9 at 14:16
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    "My use if the socket is to be able to connect to a 120V, 30amp portable generator. ... put a 120V 30amp receptacle outside to connect the generator" No, you would not want to use sockets or receptacles to attach a generator. These induce use of suicide cords (aptly named) with two male plugs, one end in the generator and the other end killing you. Use inlets instead. Commented Jul 9 at 15:00
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    Everything about your plan is dangerous. As for the design question, VtC because it belongs, and already is answered, in [electronics](electronics.stackexchange.com/a/278763/275434). Briefly, the design depends on the cable resistance for its suppression function, and probably saves some cost by doing so.
    – jay613
    Commented Jul 9 at 15:06
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    If you already have the generator, it has 30A capacity, you already want to address the cable-through-window problem, already want multiple outlets inside .... now would be a good time to think about installing a 30A inlet and a panel interlock. A little more cost and effort, a lot more capability. You can run almost anything in your home, if managed properly. Especially, you can have all the bathroom lights on, the fridge, and all the chargers you want. Everywhere, in every room. What else do you really need?
    – jay613
    Commented Jul 9 at 17:50
  • @jay613, "If you already have the generator, it has 30A capacity..." This is not practical with 120V generator output. If my generator output 240, this is exactly what I would do.
    – MC1219
    Commented Jul 10 at 15:39

3 Answers 3

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I don't recall the reason as far as the surge protection distance. But that is, by far, the least of your problems.

DO NOT RUN 20A CIRCUITS ON A 30A SOURCE.

First of all, to clarify, the outside "receptacle" must be an inlet, not an outlet. Otherwise known as "male instead of female". With a properly sized source (generator), wires and load (receptacles), an inlet provides safety because any male (prongs) are always "dead". If you use an outlet to plug in the generator together with a male-male cord (also known as a suicide cord...) then you could end up with live prongs and a really bad situation.

The only way to properly downsize from 30A (or 40A or 50A or whatever) to 20A (or 15A or whatever) is with a breaker panel or with a cord that includes fuses or breakers. My generator actually came with such a plug - it takes 240V 30A and splits it into 2 x 120V 20A, each with its own circuit breaker. 120V vs. 240V is not the problem (that's just a question of hot/neutral or hot/hot, and your generator only has 120V anyway) but the maximum current is a real concern.

If you had a 240V x 30A generator then I would highly recommend installing an inlet that leads to an interlocked breaker in your main panel. Then you could power anything at all, subject to the maximum capacity of the generator. But since you only have a 120V generator that is not so practical - you would be very limited in total capacity and unable to run any 240V loads.

That leaves a couple of options:

  • A small panel. Feed from a 30A inlet from the generator to both hots. Install a 15A or 20A breaker/circuit/receptacle for each thing you want to run on the generator. A lot of work but a very professional (if limited) result.
  • Find a UL or ETL Listed 30A to 2 x 20A cable with circuit breakers. Unfortunately, except for the one I have sitting in a box in my basement (unused because I have an inlet and interlock) I am having trouble finding one of these. And the one I have wouldn't help you anyway because it is for 240V -> 2 x 120V and you need 120V -> 2 x 120V.
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    Use the 20A outlets on the generator, they are already current and GF protected. With a 20A inlet to one dedicated indoor socket, $15 and done.
    – jay613
    Commented Jul 9 at 14:53
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    @jay613 And, due to economies of scale, two 20A extension cords might very well cost less than 1 30A extension cord. Commented Jul 9 at 15:05
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    @MikeCogan: It's not. Fuses don't have to be very big; you could have a cable splitter in that form factor that's good. But the listing doesn't mention fuses, so you should not buy that one (while omission from the listing is not strong evidence that there are no fuses, it is strong evidence of incompetence and you should stay away from that manufacturer)
    – Ben Voigt
    Commented Jul 10 at 15:44
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    @MikeCogan that splitter is safe enough for certain uses. For example, a generator with only a 30A output, used outdoors, and you want to plug in a bunch of small 120V devices, say some phone chargers. Everything is outside, right next to the generator. That thing will do the trick and does not introduce any grave danger. But it's not for feeding wires through the walls of your home.
    – jay613
    Commented Jul 10 at 16:05
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    Using a pair of 120V inlets or plugs would only be safe if there were no 240V loads. If a 240V load were switched on, and one plug or inlet was connected while the other was exposed, the exposed connector would have live 120VAC on it.
    – supercat
    Commented Jul 10 at 18:11
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I'm curious what the reason is for that?

The socket will contain surge arrestors, probably metal oxide varistors (MOVs). Given the size of the socket, the MOVs won't be that big. So they can only handle relatively modest surges.

If your supply voltage is within normal limits, an MOV does nothing. But when faced with a voltage surge of a few kilovolts, the MOV turns into a short circuit, momentarily shorting out the supply and protecting your appliances. Once the voltage goes back to normal, it resets. (Until it's taken one too many surges and fails permanently. That's why there is usually an indicator lamp to tell you it's still good.)

If you wired the socket right next to your main breaker panel, the MOVs would end up trying to protect the entire house. 10 meters of cable run has a small, but not zero, resistance. That's enough to limit the current that can flow to that socket during a voltage spike, and the MOVs have a better chance of handling it.

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    I would add that it might be not just about resistance, but also inductance - reactance in general would help the MOV to properly operate. Commented Jul 10 at 7:44
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The surge suppressor is depending on the length of the wires to add some resistance so the surge suppression circuitry doesn't have to be bigger than will physically fit inside a receptacle. I don't think it's going to matter on a generator, that is a limited source and will have a lot of internal resistance of its own, which will serve the same purpose.

Your plan has flaws, though.

Your idea of having a "second electrical system" in the house with separate receptacles lit up only by the generator, that's fine. Now if your logic is "It's cheaper than those 6/8/10 circuit transfer switches I see on the market", that's true, but those things are a terrible value. You should know about "sliding plate interlocks", which operate between the main breaker and a breaker in the panel and force you to turn off one before turning on the other. They are matched to a particular panel make - there are no universal fits. But they cost between $30 and $80 typically. Add a breaker and inlet and Bob's your uncle.

If you just can't find an interlock to fit your panel, another solution far less costly than transfer switches is a "critical loads subpanel". This is simply "STILL doing the sliding-plate interlock solution, just in a new panel that has interlocks available". And then you permanently move the circuits you want on generator into that subpanel. That's it. There is no limit to the number of circuits in the panel, the limit is on combined amp load of appliances active at once. If you're not in Canada, the Romex can still go into the original panel and be bridged over, leaving ground in the original panel and bringing over hot and neutral.

But if you want to stay with the "inlet to isolated sockets connected only to that inlet" thing, again that's fine in principle, but a few things apply.

  • The thing the generator wires into must be an inlet (weird socket with prongs), not an outlet. No suicide cords! (A suicide cord is an extension cord or dogbone/cheater with prongs on both ends).
  • You can't put 20A sockets on a 30A circuit, NEC 210.21. Now you could downgrade your project to 20A and that would take care of that. But if you NEED 30 amps you'll need to feed a little breaker panel with 20A breakers feeding the outlets. And that may be necessary because...
  • Since you are adding new sockets, all the GFCI and AFCI rules apply. You'll need AFCI protection on the wire homeruns unless they are metal-clad somehow (MC cable, metal conduit) in which case you can do an AFCI receptacle at the first receptacle. GFCI is cheapest done at the receptacles. So unless they make GFCI/AFCI surge-suppressing receptacles, you may be forced out of the "surge receptacle" solution or be forced into a breaker panel with G/AFCI breakers. This "adding new sockets" problem would not occur with a generator interlock or critical loads subpanel.

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