DIY interlock?


Old 01-30-07, 09:13 AM
Thread Starter
Join Date: Feb 2005
Location: Frozen Tundra, WI
Posts: 304
DIY interlock?

I know a few others recently have asked basically the same thing, I just want to take it one step further to make sure I do not violate any codes (while at the same time, being safe). I know there are many ways to legally and safely wire a generator. I am planning on backfeeding through a 220 V breaker, interlocked with the main so both cannot be closed at the same time (I believe this is minimum code).

I have seen the attachable interlocks sold at the box stores, while they claim to meet code, I hate to say it, they look flimsy. What I have seen may make you verify the main is open, they don’t seem built (or attached) well enough to actually prevent dual closure. I have also seen some for sale on the net and, while they look like they would work well, they do not seem worth the price tag ($150.00 plus).

My idea (feel free to shoot me down if I am screwed up) is to make a plexiglass interlock. I want to epoxy lugs to the panel (so I do not have to drill and tap screw holes into the panel, but I could do it if that would make it safer). The plexiglass will have 2 sets of holes drilled in it. When in one set of holes, the main can be closed only if the generator is open and vice versa. Would this be acceptable?


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Old 01-30-07, 09:25 AM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,530
> feel free to shoot me down if I am screwed up

I strongly advise that you buy a manufactured interlock with UL certification. Part of the higher price tag is the extensive testing these products go through to prove a level of safety.
Old 01-30-07, 09:34 AM
Join Date: Jul 2006
Location: Southeast MI
Posts: 77
From a lockout standpoint, yes, it would. Would this safety interlocking action remain in place whenever the cover is removed? If not, then it would fail to meet the 4th condition as listed in the NEC:

4) The safety interlock protection must remain in place if the dead-man cover is removed.

What kind of panel do you have? I used a Siemens 100A 16-24 space (16 circuits if usinf full-size breakers, and 24 if using combination full-size and twin-mini breakers), and did the following:

1) I removed the 100A main breaker from it place atop of both rows.
2) I inserted a twin 100A breaker at the two top-most slots on the right side.
3) I inserted a twin 50A breaker at the two top-most slots on the left side.
4) I inserted a slide mechanism in between the two breaker's handles that allows both breakers to be off simultaneously, but only allows one to be on at a time.
5) I installed permanent mount kits for both breakers. The "kit" is actually the red-tab and a screw sold at the Box stores.

There is a Siemens panel available at Lowe's that feeds 100A from the utility and 60A from the Generator (this breaker can be down-sized appropriately), that also functions as a main panel. It has 12 full-size slots that can be expanded up to 24 circuits using twin-mini breakers, if your local code allows it. This is essentially the same panel as my existing one, and I was able to use the parts out of it to make my DIY safety interlock. Next time I see it I will post the model number.
Old 01-30-07, 02:32 PM
Thread Starter
Join Date: Feb 2005
Location: Frozen Tundra, WI
Posts: 304
Thank you for the info on the requirement for the interlock to function with the cover off. I do not think (at least I can’t see how) any of the interlocks I have seen would meet that requirement (the device is removed when the cover is removed).

I have an obsolete Westinghouse 200A panel with 40 breaker capability. The main is at the top of the panel. If I could replace the main (well, because one side is hot, I would have to have an electrician do it… since I can’t turn off the power from the street) with a transfer switch (3 position, generator/trip/utility), that would be optimal.

Right now I am just having the electrician run the cable from the garage into the house and leaving it to be hooked up later (when I decide how to do it). I know my idea would work as well, if not better than those on the market, however, I see the need to “next owner proof” it so any future owner will not kill themselves as a result of removing the interlock as a matter of convenience.

Thanks again
Old 01-30-07, 07:57 PM
Banned. Rule And/Or Policy Violation
Join Date: Feb 2006
Location: North of Boston, MA.
Posts: 2,113
Build it, get it approved then market it. Good stuff sells.

As for price, With the interlock (flimsy or not) $ 150. to power your panel and give you the SELECTION of any ckt is GREAT, if you overload the gen,It trips or stalls. Considering some of the transfer panels for 10 ckts can be upwards of $200. But then again, You spent the $ on the generator,why not do the whole job?

Some of us here take the aux power issue seriously, AS WE all should.

Not just for the linemen/(person)(but they do keep us in bussiness). But for your neighbors and yourself. Electricity can go anywhere, you never know untill it finds you.

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