Maybe I could get some help? I bought a 8.5kW (10kW peak) generator and made a 6ga. cord with L14-30 plugs—one for the 240v on the generator and one for the inlet box. Inlet box is wired to the panel onto a 240v/30a double pole breaker that is off when the main is live and on only when the main is off and generator is on (I have an old box with the main switch in a separate box than the breaker panel, so no interlock switch is possible). I tested it out and did this: 1. Turned off the main. 2. Turned off all breakers 2. Turned on the generator 3. Plugged in the cord gen—>inlet box 4. Turned on the gen breaker 5. One by one, turned on the breakers

And then I sat and wondered why the generator was humming along nicely but roughly 40% of the lights/outlets weren’t on, while the other 60% were?
Then I reversed the process, went back on company power and all the light were on.

So I figure I’m only feeding one bus in the panel somehow, right? And that’s bad. So I need to check the voltage through my “generator system” (thru the new breaker, the wires to the inlet box, the inlet box, and the cord to the generator. And I’ll check it using utility power, not generator power. Presumably I’ll find a loss of voltage at some point as I test out to the end of the cord to the generator. Am I close? I appreciate any help you might have, thanks.

  • @Harper the are listed mechanical interlocks that can do this function. Probably not up to all areas AHJ requirements but some do allow.
    – Ed Beal
    Commented Jan 22, 2018 at 18:30
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    @EdBeal I don't like the interlocks that go per-circuit, 6 wires per circuit x 8-10 circuits. Tangle of wires. I prefer the thing that resembles a proper subpanel, but with the top 4 spaces occupied by two backfed breakers and a bit of metal to interlock them. Clean, simple, legal, cheap. Move each circuit to the subpanel, done. No size constraints, 24, 30, 40 space no problem. Commented Jan 22, 2018 at 18:45
  • Your original comment just sounded a bit rough since it sounded to me like it could have a legal mechanical interlock, may not be the best way but they work and are very cheap.
    – Ed Beal
    Commented Jan 22, 2018 at 18:54
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    @EdBeal He said clearly he has no interlock and has substituted a checklist, which we see too often. Maybe it's not sociopathy, just a lack of confidence to undertake an interlocked subpanel or changeover switch feeding a new subpanel. Commented Jan 22, 2018 at 19:10
  • I’d love to know how to get an interlock on this? Take a look: ibb.co/chFQWw Commented Jan 22, 2018 at 19:12

3 Answers 3


This is a super hard problem, because you are renting and seem to be "at war with" with your landlord and AHJ, rather than working with them. That's a shame, because what you want is reasonable and it should be possible to get all to agree to it.

What you have done so far puts the landlord in a no-win situation: he absolutely must positively end the practice by shutting off the power and evicting you. If he does not, he becomes liable for it just the same as if he did it, for lawsuits if anyone is injured, and if the city finds out, fines and paying a pro to rip it out and search for any other gypsy work. Meanwhile, if the city gets wind, they will condemn the occupancy and again you will be evicted. The landlord might even "drop a dime" so the city is evicting you (not him) and to avoid the fine. Any costs for him would come out of your deposit.

That said, I think a proper generator inter-tie is something most tenants would find valuable. So from the landlord's perspective, he has a more marketable property if it is so equipped. So there is certainly room to work out an agreement. In your shoes that is what I would seek to do.

Of course all work would have to be to Code and inspected. Depending on the jurisdiction, it might have to be hired out unless the AHJ recognizes you as qualified to do the work. I assume that would zorch the deal. But you don't know 'til you ask. I suspect outside the big cities, where everyone is just trying to get along, the AHJ may abide reason.

Do keep in mind that all tool-installed modifications become property of the landlord the moment you install them. You must do good work because it is electrical, but you must also be prepared to leave it (intact) for the next tenant when you move. That is the price of being a tenant, just like having to suddenly drop $2000 on a furnace is not.

Many ways to get a generator interlock when you don't have one:

Convert each load to be cord-and-plug connected

And just run an extension cord to it.

For instance your furnace wiring probably ends in a simple $1 Handy-Box. Stick another one 3" away from it, and use the furnace wiring to fit a receptacle there. Then add a short cord with proper strain relief coming from the furnace which, you guessed it, plugs into that receptacle.

Power failure comes, run an extension cord down there and do the obvious.

This is Code legal because Code allows utilization equipment to be cord and plug connected for special applications like this.

This will set you back about $10 per circuit.

On each circuit, add a DPDT switch

A DPDT (double pole, double throw) switch has 6 screws and has an A side, a B side and a common. It's a mini transfer switch and costs about $10.

find an appropriate location along the circuit where this will be possible. Wire the common on the DPDT switch to the downline circuit, the "A" is wired back to the service panel, and the "B" goes to an inlet. Or alternately, wired to a subpanel exclusively powered by the generator.

Switch both hot and neutral.

The trick to wiring this is you lose about 18" of wire with the 9" of wire that needs to be inside the box so you can work on it... so you need to make that up somehow. You can do this near the service panel or a load and simply replace the now-too-short section. Or you can put two boxes 18" apart, and have a 3' jumper between the two boxes. The second box can be a good place for the inlet.

I know I went real fast by the part where the generator has a cheapie subpanel all its own. This can be a tiny thing, it is just to provide correct overcurrent protection for each branch, e.g. A 14 AWG-wired branch needs 15A breaker protection. Or you can use inlets and extension cords to the sockets on the generator.

Install a subpanel with onboard transfer switch

The general idea here is that you install a new sub-panel near the main panel (or wherever else convenient). You then take each of the circuits you want to power off the generator, and permanently relocate their hot, neutral and ground (the whole cable really) to the subpanel.

This type of subpanel has a "sliding piece of metal" interlock which prevents both the utility supply breaker and a breaker in a very particular location from being on at the same time. Obviously, you put the generator backfeed there.

Where it differs from the "retrofit this" example that I moved from the bottom, is that in this case, the manufacturer has engineered the whole kit-n-kaboodle to work together - subpanel, breakers and interlock - so it costs far less. It fits the breakers and the breaker bolt-down feature (discussed below) is present. Often the top 4 spaces are occupied by two 2-pole breakers; and a simple bit of sliding metal sits between them. It's super simple and easy to understand.

enter image description here

Yeah, but with a lot more spaces.

This is big-box's offering. A real electrical supply house should have much better options.

As always with subpanels, spaces are cheap, so don't scrimp on the spaces. 30 is not excessive, running out of spaces is seriously frustrating. With 4 used, that leaves you 26 spaces to put loads on. I myself would go 42, but I'm crai.

The tie from main breaker to sub should be the biggest wire-breaker match-up you can stand to pay for. The breaker in the main panel must match the wire you are using since it protects the wire. The breaker in the subpanel is really just there to be a cheap high-current switch.

The breaker on the generator side should be sized for the generator, unless the gen has its own onboard overcurrent protection, in which case again it's just a switch.

When moving a circuit, if it doesn't reach, I simply put a large box where it and other circuits can reach - typically a 4-11/16 deep box - and bring the cable into that box, then extend the last few feet to the new sub. I do this all the time for other reasons. Use good wire-nuts (Ideal) and nut quite firmly (it is one place "gorilla tight" is appropriate). I've never had a splice problem.

When using the subpanel, simply cut all the breakers off including the interlocked main breaker, hookup and start the generator, turn the generator-side main breaker on, and throw on circuits one at a time as desired.

A subpanel with an external transfer switch

This option may be more expensive since it isn't using $10 breakers as cheap switches. Or maybe; the above pictured "micro subpanel" would totally work as an external transfer switch. It certainly doesn't work as a subpanel.

This has one big advantage: you can switch neutral. That matters if it's downstream from a GFCI or AFCI, and many generators have integral GFCIs these days.

This is a large DPDT, or 3PDT (double/triple pole, double throw), changeover switch. You can use ones specifically made for generators, or ones made to switch 3-phase power. (neutral as the third "phase".) The point is they are double throw, so the "common" position is connected either to source "A" or source "B", and A never connects to B (hence the interlock).

A and B go to mains and gen; common goes to the new subpanel.

enter image description here

Made for 3-phase.

Again, a real electrical supply house will have much better selection and price. The switch, and the mains to subpanel wiring, should be as heavy as you want to pay for. The switch-to-gen wiring can be sized for the gen.

Here, you use a perfectly ordinary subpanel, with no special features needed. It can have a main breaker, or not. Then as above, you cut over to the new subpanel the circuits you want to have on generator.

An external automatic transfer switch

As the last case, ditto ditto ditto but with an automatic switch that senses mains power and cuts over to it when it's present and stabilized. You can also get switches which will auto-start the generator.

You can guess the huge gotcha: since you wouldn't be there to throw over circuits one by one, it slams all load onto the generator. That requires a higher "tier" of generator product, not a $1000 model with wheels but more like a $5000 job that bolts down to a slab and takes natural gas.

You can't do this, but for others...

Retrofit an interlock on an existing panel

There are several companies that make aftermarket interlocks for almost any panel. These work about like what you're doing now: you retain your existing main breaker, fit your generator backfeed breaker in a very specific location, and then they supply a sliding metal bracket which interlocks so the main and this backfeed breaker cannot both be on at the same time. Done and dusted.

There are two serious hitches with this: First, they don't make such things for every single panel, so it's luck whether your older panel is supported. (cooking up your own [that works] is far better than having no interlock, but it isn't listed, which means it isn't legal.)

The other hitch is a special rule in Code for backfed breakers, which is that they must be "bolted down" so they can't easily snap out (most breakers on most panels snap out) and nail somebody with their exposed metal bits. That requires a design concession in the panel. Or simply a panel where all breakers normally bolt down, such as some Siemens panels. (doing it anyway isn't legal, but again is far better than not having an interlock, can't tell you to break Code.)

What not to do: octopus "transfer switches"

You will see these multiple-circuit "transfer switches". They look like something out of an RV, very impressive with an entire row of 6-10 switches, sometimes breakers too. They are aggressively marketed, and all too easy to buy. However they are a nightmare to wire.

Because for each circuit to be put on generator, they require you intercept the circuit on the way into the service panel. Divert the load side to the box, divert the line side to the box also, so it can be on "A" and common on the switch (we're up to 6 wires so far), then also the wires to the generator. And that again for each of 6-10 circuits. It's crazy bunches of redundant wiring, hard to understand and unnecessary. But like I say, the generator salesman practically won't let you leave the store without one. He gets a commission on those hokey things and not on subpanels.

enter image description here

Not pictured: 52 wires going in and out of it.

The only nice thing these transfer switches do is allow some circuits to be on "line" while others are on "gen". I fail to see the usefulness of that. If utility is available, just use it.

  • Note that some panel cover mounted interlock kits are not UL (or other) listed. My local AHJ said it had to to be labeled listed to be approved on inspection.
    – Tim B
    Commented Jan 22, 2018 at 20:49
  • @TimB ... and that's... why the company I linked wants $150 for one... and why the random Cheese machinists on eBay want somewhat less (but still usurious) and theirs won't be listed. Manufacturer-designed ones made for their own panels are much, much cheaper and do have the required bolt-downs, and obviously are listed. Commented Jan 22, 2018 at 20:53
  • I sure appreciate the thoughts you guys put into my bone-headed question. I could only hope to know as much as you all someday. I know there’s better ways to skin this cat, but I’ll tell you, I still think my plan is better than my neighbor and the suicide cord he showed me he uses to plug into his dryer outlet! Funny thing—he told me he invented the idea of how he uses his “custom” cord! I better make sure I keep the brush cleared between his house and mine!!! Commented Jan 24, 2018 at 0:39
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    You might want to tell your other neighbors to do the same thing lol. No seriously, you're not going to burn your house down. Your risk is of throwing the wrong switch, backfeeding the transformer, putting 2400 on the neighborhood 3-phase feed that the lineman thought he turned off, and next election you lose the senate because he stops voting. You're not rebelling against a cruel rule by New York suits. You're endangering working class heroes who came in on their day off to help you. They are your bros. Commented Jan 24, 2018 at 3:35
  • Harper, you worry too much. Statistically speaking, you, me, or my neighbors are far more likely to die in a car crash this week than because I forgot to flip the switches in the right order. We all have the responsibility to avoid negligent behavior that prevents catastrophic results each time we drive on the freeway. This is as safe as things I do every day. Commented Jan 28, 2018 at 18:56

On your generator system, there is apparently only one hot feeding to the panel. For proper operation of the whole panel, you need 2 hots (one to each buss bar). You should have a ground, a return, and 2 hots the whole way through. If at any point you only have one hot, you'll get the behavior you are describing.

  • I thought this was the easy part of figuring this whole thing out, but doesn’t installing a 240/30 double pole breaker supply both bars? Isn’t that the way 240v is provided to a circuit, engaging each of the 120 bars? When I check a simple voltage tester at the inlet plug while on company power, there are two hots? What am I missing? Thanks!!!! Commented Jan 22, 2018 at 17:45
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    Sounds like the generator is not supplying both hots, then, could be a defect in your cable. Or the generator. Commented Jan 22, 2018 at 17:48
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    The house-end receptacle on the cable (to marry with the inlet plug) would be the place to test whether you have 240 across the two hots from the generator, assuming you tested the correct contacts of the inlet plug.
    – Upnorth
    Commented Jan 22, 2018 at 17:54

In Canada...after you turn off the mains breaker and after turning off all breakers... take out the dryer plug and short the two hots together with an extra dryer plug wired with the 2 hots shorted... You will be connecting both hot rails in the mains fuse box by doing that.. Then feed 120volts into any standard wallplug and only turn on single breakers that are not 240volt breakers and that do not total over 15 amps because the whole house will be energized with 120 volts...put a note on the mains breaker to remove the dryer plug before restoring mains power!!!!!!!!!!

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    There's a reason the Code requires a transfer switch -- do you want to be waking up to the smell of fried utility linesman outside your house? Commented May 21, 2018 at 11:45
  • Thank you everyone for the help. I finally figured out my problem and it works perfectly. The problem was that I plugged the breaker into a position that only contacted one of the poles; when I slid down a spot to one that contacted both, it worked. I was theorizing that I had purchased the wrong breaker model and when I showed the Ace Hardware guy a pic of the breaker (in position in the box),he explained that I needed to move it down a slot. Commented Sep 26, 2018 at 6:12
  • Oh! And as I suspected, after careful inspection of the neighborhood and combing news reports, I have confirmed that I remembered to cut the main connection and in addition to having light and heat during outages, no linemen were harmed by my project. Commented Sep 26, 2018 at 6:21

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