Generator interlock kit

Discussion in 'Electrical Forum discussion & Blog' started by Theodore, Aug 5, 2020.

  1. Theodore

    Theodore Member

    Sep 18, 2015
    New York
    I have a 200amp panel with a 6 circuit manual transfer switch immediately adjacent to it. I use the transfer switch during power outages when I run my generator and power the water pump, boiler, refrigerator, water pump, and kitchen lights. We've had this setup for 8 years now and, it works, but becomes frustrating because we can't use the lights in the rest of the house (i.e. to walk down to the basement), or purposefully shed electrical loads to run, say, the dishwasher or a window AC without running extension cords throughout the house.
    And so, we've been thinking of switching to a generator interlock kit so we can have more freedom to purposefully decide what to run or not. I would appreciate advice/guidance:

    1. I've seen some generator interlock kits that claim to be UL Listed and cost ($140) at least 2x those that do not make such a claim ($70). Not really sure how a simple piece of metal can be UL Listed, but anyone know what's the deal with this? Do building inspectors balk at a metal plate without the UL symbol?
    2. Can anyone suggest any downside to changing to an interlock? The only thing I can see is that it's more difficult to figure out when line power returns (right now we leave the lights on in a room that is not run by the transfer switch on the generator). Anything else?

    Many thanks
  2. genmaster

    genmaster New Member

    Sep 25, 2017
    Master Electrician
    I've installed interlock brackets from The interlocks are specific to the brand of breaker panel you have, so you'll need to know that. They attach to your breaker panel cover and provide interlocking between your main breaker and the 2/4 position in the panel so that only one of the two can be on at a time. They also sell a weatherproof box with a male receptacle which mounts outside the house. You can also buy this external box at an electrical supply house for less money. The external box is wired to to the 2/4 position in your panel, along with neutral and ground. You connect your generator to the external box with the standard cord that comes with your generator.
    You're correct about not knowing when power comes back though. You can check for when your neighbor's lights come on, or check your service meter display to see when it comes on.
    Just a note - it's best to have a licensed and insured electrician do this type of work. Improper installation creates a hazard for line workers as well as potential damage or fire to property.
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  4. WorthFlorida

    WorthFlorida The wife is still training me.

    Oct 28, 2009
    Orlando, Florida
    You need to consider the entire load that you want to power with the generator. Is it a portable style generator or a stationary whole house? Portable generators usually do not regulate power well and heavy inductive loads such as AC compressors may not work very well. I once ran a refrigerator for days on a portable and it was having trouble keeping things at temperature.

    You may want to look at a whole house stationary generator such as Kholer's. If you want to light up the entire household, a 18-20 KW unit would be needed. Yes it can get expensive but it is far safer and it's all automatic.
  5. Theodore

    Theodore Member

    Sep 18, 2015
    New York
    Answers to your questions: it's a very modest portable generator (5500W/8250 starting watts). And it's only used during substantial (4+hours) power outages, about 1-2 times per year. It's true that the fridge doesn't stay as cold as it should, but (in our opinion) the outages we've had don't warrant investing in a whole house stationary unit. Main reason that we want to change from transfer switch to interlock is so we can leave the dedicated lighting circuit breakers of the house on, and therefore be able to walk into any room we want and just turn on lights.
  6. Bannerman

    Bannerman Well-Known Member

    Mar 19, 2014
    Ontario, Canada
    Can you add an additional circuit for lighting to your existing manual transfer switch? If not, perhaps a portable rechargeable lantern plugged-in at the top of the basement stairs will suffice as it will immediately light up when power is lost.

    A 5500 watt generator is really too small to be feeding a main electrical panel, even with turning off various breakers. In MHO, for 1 or 2 outages per year, your existing manual transfer switch supplying power to those few essential circuits is your safest option.

    Your manual transfer switch should not only disconnect and isolate the hot conductors to those essential circuits from the main panel, but also disconnect and isolate those circuit's neutral conductors to eliminate all potential risk of any power from your generator from backfeeding into the grid.

    An interlock will only prevent the main 2-pole breaker from being activated. This relies on someone first actually turning off the main breaker and sliding the interlock in-place prior to connecting the generator to the panel. Unfortunately, an interlock will not ensure the generator cannot supply power to the panel unless the main breaker is first shut-off. An interlock will also not isolate the panel's neutral bus from the grid.

    A portable generator's neutral conductor voltage will often 'float' above 0 volts since there is typically no bond to ground. To ensure Neutral will be 0 volts, the portable gen's neutral conductor may be bonded to the generator's metal case which should be connected to a reliable grounding point such as a driven grounding rod.

    When connecting a portable generator to a home's main panel, the generator's neutral connection should be bonded at the exact same point as utilized for the home's electrical system which will likely require removal of the bonding connection within the generator if one exists. That may not be required while utilizing a transfer switch that will isolate the powered circuit neutrals from the main panel while also grounding the generator's case to the same grounding point as utilized for the home's main panel.
    Last edited: Aug 6, 2020
  7. wwhitney

    wwhitney Well-Known Member

    Mar 17, 2019
    Berkeley, CA
    I don't believe that's required. Different sources can share a single conductor, that's not enough for a circuit to be created for power flow.

    Isn't that precisely what an interlock does? The generator backfeeds a breaker that is mechanically interlocked with the main breaker. So only one of the two breakers can be on at a time.

    For a standalone generator supplying a couple extension cords to a few isolated devices, I don't think there's any benefit to earthing the system.

    Yes, a 4-wire inlet with a 2-conductor transfer switch should have signage stating that the generator to be attached should not have neutral-ground bond. An effective alternative that may not be NEC compliant is to use a 3-wire inlet with the 2-conductor transfer switch, omit the EGC between the generator and the inlet, and bond the generator case to the neutral. That is effectively how a service works.

    A 3-conductor transfer switch, that switches the neutral, with a 4-wire inlet conversely should have signage requiring a neutral-ground bond at the generator. Without it, there is no effective fault current path. The service neutral-ground bond (on the grid side of the transfer switch) does not provide a fault current path, or allow neutral current onto the EGCs, because the transfer switch is breaking the neutral.

    Cheers, Wayne
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