Generator "backfeeding" and other Questions

[wh500special 2,249]

I asked this in Another Thread , but no takers. I am betting the electricians here might know.

I am not in any way, shape, or form advocating you hook a generator to your house with anything other than an approved transfer switch. Please keep that in mind.

My question is more theorectical.

IF you were to feed the output of your generator into your panel in some way other than a transfer switch (e.g. thru the dryer plug) AND you forgot to switch off the main breaker - thus leaving you connected to the grid - would:

A. You shock the lineman working on the lines somewhere?

B. You just blow the fuses in your generator since you'd be trying to power up the whole utility grid with your 40 Amp beast?

C. The "backfeed" stop at the nearest transformer but leave everything between you and it energized?

I am betting on the blown fuses thing. But I really don't know. And, again, no matter the answer I think it is foolish to use any method but a transfer switch. The whole negligence thing would come into play if there were an accident.

Finally, when using a portable generator should you bond the ground of the generator's output to a grounding rod, pipe, or your house ground? Seems like if you didn't you'd be making 120/240 volts between wires but you'd be floating with respect to ground. I think this is especially important if you're NOT connected to the house which has grounding provisions already to keep the potentials anchored at 120 V above ground. Or do they usually self-ground by being grounded to their chassis which is then - hopefully - grounded in some way?

Obviouly I'm not an electrical guy and I bet these are easy questions to answer for those who are.

Steve

[HorseFixer 2,013]

Yeah I would say all your questions are correct!

Edited by nylyon:

This post contained copyright material which can not be reproduced without the owners permission. In compliance with copyright laws, this material has been removed. The original document can be found here: http://www.smps.us/transferswitch.html

[whchris 5]

you can also use a mechanical interlock system between the main breaker and usually the #2 and 4 spaces in the panel most panel manufactures sell a kit that requires you to drill 2 holes in the panel cover and bolt this kit to it so only one breaker can be on at any time size breakers appropriately to your generator and always wire to code if you do not know the code talk to some one who does. the books at home depot and lowes for "home wiring" are extremely inaccurate and should not even be allowed to be sold. other than that i think duke pretty much nailed it not bad for a hvac guy :thumbs:

[linen beige 14]

Some (maybe all) of the new homes in this area have been supplied with meters that supposedly prevent backfeeding. Do these have a diode bank of some sort? If so, how do they maintain alternating current?

For those of you who are considering installing a transfer switch with a built in panel. If you are not an electrician or at least competent, have an electrician examine the panel to insure it is wired correctly. When I bought mine I found a 12 guage wire connecting each amp guage to the generator input breaker. This was supposed to be a 30 amp circuit yet 12 guage wire is only rated for 20 amps. I replaced the 12 guage with the proper 10 guage. The neutral bar was also bonded to the box enclosure as if it were intended to be used as a main service panel. The neutral buss bar is to be isolated from ground in any subpanel.

Looks like some manufacturers will submit one design for UL approval and then downgrade materials on the production runs.

[Sparky-(Admin) 22,084]

The voltage coming into your house is 120/240 but out at the street its much higher. Probably around 2300- 7400 volts and much higher if your in a business/commercial zone or on a main street. The higher voltage goes thru a transformer out at the street or on a pole which steps it down to the 120/240 your home needs.

If you were to send power from your generator back out to the street it would be 120/240 up to the transformer but would be coming out of the transformer at 7400 volts or whatever the street power was. It would have the potential to kill a Lineman or anyone else who came in contact with the line.

A transformer will convert voltage both up and down.

Mike..........

[whchris 5]

mike last i heard we were up to 10,000 to 15,000V around here i attended a cl-p seminar about 2 months ago they stepped it up to send it further i know they did this around my house and screwed me over with my voltage in down around 116 per leg noticeable to me with the grinders and what not

[linen beige 14]

Finally, when using a portable generator should you bond the ground of the generator's output to a grounding rod, pipe, or your house ground?

It should be connected to a ground rod. The operator's manuals usually have diagrams/ instructions for grounding them.

[kpinnc 13,735]

Connecting a generator directly into any point of electrical wiring without a transfer switch is illegal and dangerous because of so called backfeeding.

I set portable generators out all the time at work, and we have a manual breaker that MUST be switched to generator power so we can provide AC to our equipment. The catch is that the commercial breaker AC has to be opened to close the generator breaker. We use two types of breakers- One is magnetic and the other is mechanical, but both serve the same function.

There are technically no transfer switches, but the safety feature is there for just the purpose of not energizing the lines back out to the power grid.

I guess these technically are considered transfer switches.

Kevin

[linen beige 14]

I guess these technically are considered transfer switches.

Kevin

Good point.

Technically, most of the products out there that are known as "transfer switches" are actually two breakers tied together so that only one may be on at a time. A true tranfer switch is exactly that, a switch that transfers the incoming power line to the panel from one source to the other. These true switches usually will carry pretty high current loads and are installed between the meter and the panel (electrically if not physically). The alternate power source(s) is/are fed through different leg(s) of this switch.

On the other hand, since breakers are really just on/off switches that automatically turn off when overloaded or can be manually shut off, and these devices use combinations of breakers to tranfer the incoming power sources, calling them transfer switches may not be all that far off. :thumbs:

[wh500special 2,249]

Thanks guys for all the replies. I knew this would be informative.

Still not clear whether it is possible to backfeed to the grid without popping the fuses in the generator or the breakers in the house (thru which you are feeding) but no reason to chance it.

Thanks again,

Steve

[linen beige 14]

Sorry Steve, I didn't realize we didn't answer you. Unless the meter has a backfeeding prevention circuit, if you don't turn off the main breaker you WILL energize the line out to the nearest break in that circuit.

Whether it overloads your generator, fries a lineman, or trips your gens breakers has many variables. It may be connected to a line that is down among fallen trees and can start a fire without tripping it's breakers. If it is connected to a line that has no load and is suddenly repowered by the utility the surge from the utility will likely burn up the generator before it's breakers can trip. If the line it is backfeeding is connected to enough load it will trip the gen's breakers. If that line is connected to a repairman's hands it will trip HIS/HER breakers.

The last one is the one that worries repair crews the most. A generator could be feeding power to a line that has no load. With no load the generator's operator may not even know it is putting potential into the line since there would be no effect on the generator until a load is applied. That load could be the repairman.

[KyBlue 655]

I cant really add anything to whats been said...Except pass on one of those...Here's you Sign Stories..

My Boss at work, lives in a town that got hit pretty hard by the recent storm, so he, and his neighbors, lost power (along with ALOT of others). We got to talking in the office yesterday, I suggested getting a generator and a transfer switch, for the next time this happens.

He Started laughing, and told me about his neighbor, that Tried to hook up a generator by Cutting the female end off a Ext. Cord..Putting on a Second Male Plug, and running it from the generator, to a Wall Outlet... Well you can just imagine how well that went..

Thought you could use a smile, or at least a bit of WTF was he thinkin!!!

[wh500special 2,249]

It may be connected to a line that is down among fallen trees

Ah HA!

That's the scenario I didn't think of..if the line is open between your generator and the next load capable of drawing enough current to exceed the fuse capacity THEN you'd have potential there w/o current flow that would blow the fuses.

I figured I was missing something!!!!

Thanks!

Steve

[linen beige 14]

I cant really add anything to whats been said...Except pass on one of those...Here's you Sign Stories..

My Boss at work, lives in a town that got hit pretty hard by the recent storm, so he, and his neighbors, lost power (along with ALOT of others). We got to talking in the office yesterday, I suggested getting a generator and a transfer switch, for the next time this happens.

He Started laughing, and told me about his neighbor, that Tried to hook up a generator by Cutting the female end off a Ext. Cord..Putting on a Second Male Plug, and running it from the generator, to a Wall Outlet... Well you can just imagine how well that went..

Thought you could use a smile, or at least a bit of WTF was he thinkin!!!

If he got the legs right on the cord in respect to polarity it should have done no more than trip the breaker on the circuit he plugged into when he got past 15 amps of load.

But seeing as how he tried this approach I may be giving him too much credit in assuming he would know why one prong of a plug is wider than the other.

It does sound like a "Hey y'all watch this" moment in the making.