Transfer switch vs. interlock

thomas1975November 13, 2012

Hi. I'm looking to get a Generac 7500 (or something similar) and am getting completely different answers from different electricians (at about a 50-50 split). I've had 2 over for estimates with 2 more coming this week, and have spoken/emailed with several others.

Some of them recommend an interlock saying it's much easier and I can use my entire existing panel (of course making sure not to switch on things like central a/c, etc.) and not have to worry about only selecting certain circuits that would go on a transfer switch, but others say don't go with the interlock because it's too risky that someone might flip the central a/c, and it's therefor much safer to have a 30-amp 10 outlet transfer switch, which will be able to power pretty much everything we need w/o worrying about what we can/can't switch on with the interlock. I was also told by a large generator retailer that "An interlock kit does not meet NFPA 37 or NFPA 70 for proper connection of generators to residential electrical systems."

So I'm leaning towards the transfer switch but am curious why several (highly rated on Angie's List) electricians are pushing the interlock.

Thanks for your thoughts.

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Ron Natalie

The interlock is substantially cheaper. It gives you extra flexibility that you can pick and choose on demand which circuits to use (it would behoove you to figure this out ahead of time). Essentially, once the utility power goes off, turn off the main, turn off all the loads you don't want to power with the generator and then turn on the generator feed.

The advantage of the transfer switch is that it can make the switch automatically (or at least very easily). Some have convenient ways to monitor the load and the return of utility service.

The salesman was blowing smoke. First off, NFPA 37 has nothing whatsoever to do with interlocks or transfer switches, it covers installation and use of stationary engines (i.e., the generator itself if it wasn't a portable one). Mostly things like fuel lines and exhaust. NFPA 70 doesn't apply to installations either, it's guidance for electrical safety in the workplace and doesn't dictate any such demands on interlock design.

All the code requires is a device that will prevent the generator and the utility from being connected together. The applicable compliance is listing (i.e., UL) which kits sold for sale should all meet.

    Bookmark   November 13, 2012 at 10:30AM
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You can only use an automatic switch over if the generator is large enough for the whole panel.

The is one advantage the smaller panels with a few selected circuits has over using the whole main.

It CAN be automated.

Detects power out, starts up generator, cross4es over.

The required load shedding is built into the smaller panel.

Unless someone is home (and awake) you will have to manually switch from the dead POCO lines to your generator.

    Bookmark   November 13, 2012 at 1:41PM
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if your looking to get a generCrap, and that is what most of us call them, You HAVE to use their transfer switch.
Others don't work properly. Most of the time the gen will shut down and lock out in 90 seconds if it doesn't have a
generCrap matching transfer switch.

    Bookmark   November 13, 2012 at 3:13PM
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So generac is not so good? What would you recommend (sorry to change the topic of my own post and I'm still interested in everyone's opinions on the interlock vs. transfer switch)?

    Bookmark   November 13, 2012 at 4:01PM
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I had been looking here ( and Generac seemed to rate very well.

Here is a link that might be useful: generators

    Bookmark   November 13, 2012 at 4:03PM
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My first Generac natural gas generator, from 2000, definitely deserved the name GenerCrap. My current one, from a couple of years ago, is far, far better.

    Bookmark   November 14, 2012 at 5:22PM
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Is Briggs & Stratton good?

Here is a link that might be useful: Briggs & Stratton 8,000W

    Bookmark   November 15, 2012 at 12:35AM
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Debbi Branka

We just bought a Generac 20k withe the transfer switch. I can't tell you if it's good or not because we learned during set up that we would need a bigger gas meter (to the tune of $485) which will take 3-4 weeks to be installed by the gas company. Uggh! But I read good reviews on it and our heating and cooling guy said it was a good one.

    Bookmark   November 15, 2012 at 3:19PM
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Ron Natalie

I've had an 80KVA GENERAC with the HTC auto transfer switch for over three years now without a problem. It's amusing how little power I actually use. The wattmeter in the thing shows typically only about 6KVA draw. Damn EnergyStar house.

    Bookmark   November 16, 2012 at 6:32AM
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