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Comment by zibby8 | Hacker News Reader

Back feeding seems insane. Most residential circuits are rated for 15A. A 5000 watt generator can produce over 40 amps at 120V. Seems like a good way to start an electrical fire.

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There's not a big risk of overcurrent here, since the 15A breaker will trip on overcurrent even when it's being backfed. The main hazard with these suicide cables is of electrocution, both inside your house due to the exposed blades being energized, and of linemen working to restore the power.

Isn’t the usual advice to turn off your main breaker? Both to protect a lineworker and to avoid stalling your generator by trying to power up the neighbourhood?

I’m not sure I want to see the Venn diagram of people who would buy this cable, and people who would know they need to do that.

Keep in mind that backfeeding your house won't actually work unless you disconnect from the grid. Your little generator cannot power the whole grid. You'll pop a breaker immediately. So even a total idiot isn't going to electrify the neighborhood (though they could cause serious momentary risk).

I know a couple people who use this approach. They're some of the most electrically savvy people I know. I don't get worked up about it.

They're out there.

Do you think the people using this sort of cable acknowledge things like "the usual advice"?

if you do this you absolutely need to turn off your main breaker.

Better advice is to use a proper inlet, with either a real transfer switch, or a mechanical lockout if allowed in your jurisdiction.

If you're using a cord like this, you almost certainly don't have anything but your brain preventing you from using it wrong. Brains aren't always enough.

The breaker isn't part of the circuit, though. Generator connects to in-wall distribution which connects to appliances. You only go across the circuit breaker on the way to the mains line, which is (hopefully) disconnected. Or else it's not, and indeed you'll get overcurrent protection for any attempt to power your neighbors appliances.

If you make a cable to plug a 15A outlet in your home into a 15A outlet on the generator, then hopefully the generator has its own breaker that would trip if too much current was drawn.

If you plug your house directly into a 40A outlet (which has a different shape) then, yeah, you're screwed.

If you plug your generator directly into a dryer/oven outlet, it should be on a dedicated branch circuit, and everything else you load in the house will actually have two breakers between it and the generator... one for the branch you're feeding from to the bus, and one from the bus to each branch circuit. And the generator probably has a third.

The only real difference between this setup and a proper generator hookup is a mechanical interlock so that the main and feeder branch breakers can't be on at the same time, and the gender of the generator hookup outlet.

Most houses are wired with master breaker(s) connecting the house to mains lines, and various slave breakers connecting the various lines around the house to the master breaker(s).

If you plug a generator into a wall outlet, that power will pass through the slave breaker protecting that particular circuit as the power flows around to the rest of the house.

The breaker is the part of the circuit for the other circuits

you'd have something like

    gen -> circuit A -> main bus -> circuit b-> load
                           | 
                        main fuse/switch
                           |
                         outside

So loads on circuit A would not be protected by any fuse but ones at circuit B would.

Also circuit A would allow full generator amps to the devices there, no RCD protection either.

This is generally why Solar installs don't work with the power out. They just shut off to prevent back feeding into the grid. (That and I think the dc->ac converter syncs with the 60hz of the US grid)

We don't have a battery but if you install a backup battery I believe there is circuity to cut the power off from the grid when powering your house.

Battery backups and generators definitely include an automatic cutoff switch (by law).

I wonder what happens when the grid comes back up at a mismatched phase. If only mythbusters were still around. :-)

The fuse would blow, because you have mains ->main fuse -> circuit fuse -> generator

Maybe? What if it's a slow-blow fuse / breaker, and the phase is just a little bit off?

If it was alternator driven generator it would most likely drift into phase with the network, as the network would either break, or allow it to spin faster depending on in which direction the difference is.

That's actually how the big generators are synced, if there is less load you will see frequency of network increase slightly until the amount of generation drops, similarly in other way.

Inverter based one, hard to tell, entirely depending on code driving it.

But taking into consideration even 20V over low resistance wire could be 20+ amps it would probably trip pretty quickly.

Isn't it more common to use a dryer outlet with a 240v input?

I'm not sure what is most common. Most people with a generator are only wanting to run the fridge, furnace and a couple pumps, all this can be arranged to run from a single 120v phase, and finding a 120v outlet one the right phase near the generator is easy enough. In particular no need to move the dryer when doing this. However larger generators have 240v and it is easy enough to plug them in. The only people I know who have done this used a 240v welder outlet, for whatever that is worth.

Personally I'm glad my house as an automatic transfer switch. power goes off, generator starts, and lights go back on. Last time the power went it was planned so I had the generator running and the transfer happened fast enough for my servers to stay up.

In my experience, yes.

I've seen more "suicide cables" with a male 14-30R (dryer plug) and male L6-30R (twist and lock type, usually on generators) than I can count.

Residential circuits are also protected by 15A breakers. You could generate a lot more than 40 amps of draw by shorting a (traditionally) energized circuit.

Yep. There are some edge cases where it is convenient (eg trip all circuits except lighting) to ensure load is safe but kinda dicey that make random consumers understand and implement that

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