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I'm trying to find the breaker to switch off a light fixture to install ceiling fan. I've tried all but the ones dedicated to ac, furnace, dryer etc. I decided to switch off the main and encountered this. Is this safe? Can I switch the main safely? Thanks!

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    You shouldn't have to shut the main breaker off to work on a light. Switching off the main will not meaningfully 'isolate' the circuit you're working on from the rest of the house. If you find that shutting the 'Main' off cuts power to the light, but shutting off all other breakers does not, then something is very very wrong.
    – Billy left SE for Codidact
    Commented Sep 24, 2017 at 13:31
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    @BillyC. -- he's trying to figure out if something is indeed very very wrong :)
    – ThreePhaseEel
    Commented Sep 24, 2017 at 16:59

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Yes, that screw needs to be there. No, it won't mess up switching off the main.

Your electrical panel uses a backfed main breaker configuration. This works fine, considering the main breaker is always going to be a regular breaker and thus rated for backfeed (GFCIs and AFCIs have LINE and LOAD terminals as they are not backfeedable). However, this invokes NEC 408.36(D) in order to keep the main breaker from coming out and exposing someone to live mains when they pull the deadfront off:

(D) Back-Fed Devices. Plug-in-type overcurrent protection devices or plug-in type main lug assemblies that are backfed and used to terminate field-installed ungrounded supply conductors shall be secured in place by an additional fastener that requires other than a pull to release the device from the mounting means on the panel.

The screw you see is that "additional fastener" the Code requires. It won't mess up switching off the main breaker, either, as the handle OFF position is a mirror image of the handle ON position, roughly.

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  • Wow! Thank you for your responses! I will try switching off the main in the morning to isolate the light fixture. Thanks again!
    – SLK
    Commented Sep 24, 2017 at 0:09
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    @SLK -- we thank people with upvotes and answer accepts around these parts :)
    – ThreePhaseEel
    Commented Sep 24, 2017 at 0:11
  • You one upped my answer with style :D
    – noybman
    Commented Sep 24, 2017 at 6:10
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For a backfed breaker set, yes. This keeps it secured in the panel. The reason is because the mains are running to the breaker.

So imagine you pulled the cover off the box to add a line, and the breaker was not screwed in place, it could come off - quite the hazard. This allows you to work inside the box, and know the main's will stay put.

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  • I don't know a lot about US breaker installations, but isn't that then true for all the other breakers too? Would it not be a hazard if they come off?
    – PlasmaHH
    Commented Sep 24, 2017 at 15:55
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    @PlasmaHH -- no, because they aren't backfed. if they come off, then the breaker/wire assembly is not live.
    – ThreePhaseEel
    Commented Sep 24, 2017 at 17:00
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    As @ThreePhaseEel says, its about the live danger vs. the stationary known danger. it's really the same as keeping live power from being exposed (plugs/sockets). Imagine if all of our outlets & extension cords were plugs, with exposed live voltage! When working in an electrical panel an electrician knows if the meter is in place so the mains are live. They cannot main feed safely unless the main meter is pulled. So INSIDE the box, the main is hot. They'd control the main feed to the bus hot and it cant move. If it's not backfed, the breaker still interrupts the main bus from the panel bus.
    – noybman
    Commented Sep 24, 2017 at 17:13
  • @threephaseeel: ah, because they sit all on a bus? Around here we don't have that kind of breaker boxes, thus backfeeding is not a thing
    – PlasmaHH
    Commented Sep 24, 2017 at 17:51
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    @noybman: I don't know good search terms for UK boxes, but for here in germany, you should be fine with "Sicherungskasten". Keep in mind that here all houses get 3 phase, thus the majority of boxes you will see are 3 phase. Also keep in mind that a lot of the images you find via google are posted in forums as bad examples ^^ Generally also here we have 2 or 3 sets of main fuses, some before and some after the meter. In my house I have one set before the meter (63A), one after (I think 50A), then an rcd/gfci and then a main switch. If you like UK stuff, look at the John Ward youtube channel.
    – PlasmaHH
    Commented Sep 25, 2017 at 9:20

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