Electrical questions

[Mike in Ohio]

I have 2 unrelated electrical questions. 1 I am building a shop I need to run about 100' from the panel in the house to a subpanel in the shop. I have 200 amp service in the house I figured on a 100 amp panel in the shop. Welder and compresser are only 220v tools. What size of wire do I need to get there? 2 I got a generator this week, our power was out for 6 days, The generater is 5000 watts 6250 watts startup.All my appliance tags show volts and amps. How do I figure what all I can hook to it. For this week I had the wellpump, the powervent on the water heater, I alternated the refrigerater and freezer, and couple of lamps. This seemed all to work ok but a few more items would have made life easier. I want to have everything ready the next time this happens. Thanks alot, Mike

 

[Brian Jasper co. Ia]

I have the same size generator. I just back feed the house through a welder outlet in the machine shed and turn off the main breakers on the meter pole. I ran the gas furnace, fridge, lights, and TV pretty much all at once. When I noticed that the engine was running near full throttle, we cut back on lights.
Back feeding through a welder outlet is not the "right by the code" way, but it works. The main thing is isolating your system from the high lines. I've heard stories that the line workers will short the lines together and burn out your generator if you're energizing the high lines so they can work on them. I'm pretty sure the correct way involves a transfer switch of some kind. I like having my generator out in the shed. Can't hear it running in the house. Since the meter is on a pole out back, it uses the service supply wires just like the regular service does, just doesn't have full capacity.

 

[dave guest]

Mike, in mich. if your service is 200 amp, outbuilding, garage or shed is limited to 80amp. Generator should have transfer switch. Do it right if you can, or ask insurance company what happens if it is not inspected. May not insure your work. Not saying you are not capable, just facts of the matter. Retired contractor. p.s. since Katrinea, inspectors make the rules, not as previous enforce it. Ask them for a b reak, but be safe.

 

[Anonymous-0]

Your generator is rated in VOLTAMPS. That's volts times amps, not watts. With an incandescent lamp load thats watts. With motors watts are always less.

I've run all but the electric clothes dryer and the electric range on my 5 KVA generator.

The gas engine driven generator's main limitation is the peak load. It takes 5 or 6 times running current to start a motor (sometimes listed as the LR or locked rotor current on the name tag, most often on compressors for refrigeration and air conditioners) and so if you can keep from starting everything at the same time you can get along. I'd think running your refrigerator and freezer at the same time would work well. Monitor the generator output voltage. So long as it recovers after a motor start and doesn't settle below 105 or 107 volts everything on, it can run. Electric dryers and ranges along with electric heat are often loads too great for your generator.

I'm running a subpanel about 80' by wire from my main panel. I had to go to that for starting my radial arm saw and a shop vac simultaneously. The 240 volt circuit is probably #6 copper and I arranged with the subpanel to put the two motors on opposite sides of the 240 volt circuit.

Service is better with #4 copper maybe even #2 IF you can get #4 into the lugs on the main and subpanel breakers. You have to run 3 wire with ground and use an insulated neutral bar in the subpanel to isolate neutral from ground there.

Gerald J.

 

[Bill-KCKs]

If you're backfeeding through a welder outlet and you forget to trip that main breaker, you can kill a lineman.

Transformers are "bi-directional." They don't care which way the current flows. So if you don't trip that breaker, you're putting 240-v into the transformer and it's coming back out at 7,200-up.

Not only that, but you'd be trying to light up your entire neighborhood with your generator. Not good on the generator.

Don't do it.

Put in a legitimate transfer switch and do it right.

IANAL, but I believe you'd likely be held liable in the event of an injury or death.

 

[MarkB_MI]

Brian,

Most welders use a 3-wire outlet. If you're backfeeding through such an outlet you're asking for trouble. Since there is no neutral, you're backfeeding the neutral through ground. Although this SHOULD work, you have no way of knowing if the ground and neutral are properly tied together, or if there's a weak connection in the ground conductor. Worst case, you can get somebody killed. You're also putting your appliances at risk because the 115 voltage can fluctuate due to a bad neutral.

 

[MarkB_MI]

#2 aluminum will work for your application.

Just to clarify what Gerald said, "3 wire with ground" means a total of FOUR conductors. 2 hot, 1 neutral, 1 ground. Ground and neutral MUST be isolated.

Regarding generators, I run pretty much my whole house off a US Army Signal Corps generator rated at 2.5 kW. The trick is that you don't plug everything in at once. I takes a LOT of current to start up a refrigerator, and we have two fridges plus a freezer. Once each fridge has started, they can all run simultaneously no problem. (Of course that old generator has a lot of rotating mass that helps to get the fridges started.)

 

[Anonymous-0]

Feedback cant happen if there is a load on the line.the will be plenty of motors,water heaters etc all turned on.The power co can handle the surge that happens on turn on ,your little generator cant..Putting a short on a 7200 volt line would make a big flash.I was driving under a power line that shorted,it lit up like daylight.In any case linemen treat all lines as hot, they wouldnt last long on the job otherwise...

 

[Anonymous-0]

Just a question, why would he have to run 4 wires to another building. Could he not put rods down for ground at other building..Is this not what he did at the main building, run ground to ground rods..just asking a question...thanks


MAY GOD BLESS THE USA

 

[John T]

Its indeed possible to just run 3 wires (2 hots and Neutral) to the outbuilding BUT if theres any shared phone or other communication lines or conductive water pipes etc then one should use the 4 wire delivery (2 hots, Neutral, Equipment Grounding Conductor) system to the outbuilding and treat the shop panel as a sub panel in which case its Neutral Buss and Ground Busses are ISOLATED AND SEPERATED same as any other sub panle would be in the same structure.

John T

 

[lucas boy]

i used four #8 copper wires to do the same thing 90 ft.used the chart in electric book to decide on what wire. i pulled them thru a plastic grey pipe i buried about a foot. that plastic pipes cheep, id recomend useing it,for saftey that is.lucas

 

[John T]

Yo Mark, I think we have discussed this before, Id also go with 4 wire delivery like you (see my post below) EXCEPT if I were running to a farther remote building with NOOOOOOOOOO PHONE OR COAX OR METALLIC WATER LINES that can serve as a current conductive path to/from either location. In such a case Id treat it just like a regular first service entrance where the Neutral is bonded to the grounding electrode system such as water or gas pipes or made electrodes like driven ground rods.

You have any heartburn with the above?? As you may know I been longgggggggggg retired from electrical engineering practice and rusty as an old nail on current NEC lol

John T

 

[bc]

Just wondering but isn't the ground and neutral tied in together at his house main panel? If so then the house would be ok but running anything in the shed could pose some neutral/ground problems.

 

[bc]

That always seems to be the age old question on the electrical boards. Whether to treat something as a sub panel or a new service.

I think the safeest alternative is the subpanel method but there is always someone trying a save a buck or bucks by not running one more wire which is quite expensive now. You never know when water or gas or phone or computer may be added later connecting the grounds between the buildings.

Same for trying to save a buck on the next smaller wire instead of going with a little bigger. And as always with big wire, you have to make sure the lugs, breakers, and wires all match up.

 

[Anonymous-0]

Dave- just wondering why you said he was limited to 80 amps in the feed to his outbuilding?

 

[Anonymous-0]

NEC does not allow the neutral and the ground to be connected in the sub panel. You can add a ground rod at the sub panel for the ground, but not the neutral. I think that has to do with safety problems with grounded appliance or tool cases should that neutral break open and the tool or appliance insulation fail. Then if the short circuit current is carried totally on the ground there can be enough voltage rise to cause shocks. I had a case where the neutral and ground used the same wire on a construction site. That wire broke from being driven over, killed a carpenter.

Gerald J.

 

[John T]

Thanks, yeppers thats the problem as I see it, so long as theres no other current paths 3 wires will do same as the 3 from pole xfmr to main panel buttttttttttt those other conductors like water or communication lines, if present, are tied to the building ground (or supposed to be) and therein lies the paralell path

John T

 

[John T]

As usual my old friend we agree (hey thats gotta be worth something lol) if the Neutral and Ground are tied at the sub panel and iffffffffff you get the right (albeit unlikely) combination of line breaks/opens coupled with a short, the Equipment Ground Buss (and all the appliance metal frames) can become hot energized and that can KILLLLLLLLLL

John T

 

[Anonymous-0]

Concerning starting loads and generators - I know that the starting load for the refrigerator is much higher than the normal operating load. One of the comments in this thread talked about plugging in the refrigerators one at at time so they don't all start at once. Don't you get the high starting load every time the compressor on the frig kicks in? So theoretically there is a chance that if you have two refrigerators running off your generator, you could have both to coincidentally kick in their compressors at the same time?

 

[Anonymous-0]

It is possible, but since their operations are not coordinated, its not likely. When it happens the generator will get pulled to its knees, might even stall the engine.

Gerald J.

 

[bva]

Local family builds a 3 hundred thousand house. Son installs stand-by generator. Everyone away from home,power goes out and burns house to foundation.When insurance finds out that unqualified son installed generator they say no to paying. The way I look at it you have 3 options. 1 get qualified electrician to do work and stand by it.2 get your local electrical inspector to spell out item for item on how to do it and approve it. Bootleg it yourself and accept the risk. Oh yes there is one more. Untill you learn more about generators, get rid of the one you got.

 

[Charles (in GA)]

2008 NEC no longer gives exceptions on the four wire to outbuilding, etc. feed. Whether there are other metal connections or not, it must be four wire now.

Charles

 

[Anonymous-0]

volts x amps = watts . as for the wire size no 4 tw or uf is good for 105 amps in free air 4 thhw or thwn is good for 125 no4 thhn is good for 140 amps . if youre gonna use 100 amps in the garage you need a seperate service. as for the generator when you hook it into a home you need a way to disconnect the house from the service line i dont think you would want to be found neglegent in the death of linemen trying to restore the power to their customers and make a living to provide for their families.

 

[MarkB_MI]

Hi John,

I always enjoy your posts. I also don't work as a double-E anymore; I just know what it takes to make the local inspector happy.

The NEC doesn't explain the reasons for why things should be done in a certain way, but I like to try to figure out for myself. I'll take a stab in the dark on what the reasoning is behind the four-wire versus three-wire connection to a subpanel in an outbuilding. I see four different ways to make such a connection; here are my thoughts on what is wrong with each one:

1) 3 wire to subpanel with no ground at all: Clearly this wouldn't be acceptable from a safety point of view. And there would be no way to make a subpanel safe with a GFCI (although you could run a branch circuit on a GFCI).

2) 3 wire with ground and neutral tied together at the subpanel, no ground rods at subpanel: The problem here is that if the neutral wire has an open, then the ground becomes hot.

3) 3 wire with ground and neutral tired together at the subpanel plus ground rods at the subpanel: I think the hazard here is the potential for stray currents in the ground. This is a big problem for livestock, but it's not particularly safe for anyone.

4) 3 wire with ground and neutral isolated at the subpanel; ground tied to ground rods at the subpanel: This eliminates the possibility of stray ground currents, but there is a very real risk that the ground won't be good enough to fault the circuit breaker in case of a short.

So that leaves the four-wire hookup as the safest and most reliable way to make the connection. Or at least that's what the authors of the NEC think. And I figure there's about one dead sparky for every rule in the NEC, so I'll stick with what they say! What's your thoughts on this?

 

[MarkB_MI]

Dan,

Yes, in theory it's possible for two or more of my refrigerators to start up at once. But in practice it's almost an impossibility. For two refrigerators to start at the same instant, I have to have at least two refrigerators that are STOPPED. But typically when the generator is running ALL THREE fridges are running.

When I get around to firing up my ancient generator set, power has usually been out for a couple of hours. And I seldom run the generator for more than four or five hours. It takes a long time to cool down the fridges after power has been interrupted. They never start a second time under generator power because they never STOP.

 

[MarkB_MI]

The ground has two purposes: One is to provide a path to ground for any stray voltages. The other is to provide a solid path to ground to ensure that the circuit breaker faults if there's a short circuit. Let's say you have a 30 amp breaker and you get a short in an appliance. If the ground isn't somehow tied back to neutral, the ground rods need to be able to sink 30 amps. This might be doable if the ground is wet, but not necessarily so if it hasn't rained for a while.

 

[MarkB_MI]

bc,

The problem with backfeeding through a three-prong outlet is that you're using the ground wire as a neutral. But you have no reason to believe that the ground is capable of handling the current requirements, because it's never had to. So you start running lots of current over the ground conductor, and it might burn out some place and then you've got an open neutral which is very bad.

 

[John T]

Wow, thanks Mark, gee aint this fun for us sparkies at least huh.

Heres my thoughts, its posted below 2008 NEC requires 4 wires anyway, so this may all be moot?

YOUR SCENARIO

3) 3 wire with ground and neutral tired together at the subpanel plus ground rods at the subpanel: I think the hazard here is the potential for stray currents in the ground. This is a big problem for livestock, but it's not particularly safe for anyone.

RESPONSE I dont see much problem with this scenario (unless 2008 NEC now forbids it) as its the VERY SAME THING done when running the first main service entrance from the utility xfmr. 2 hots, Neutral, (3 wires) and at the serice entrance the Neutral is bonded to a grounding electrode (like a driven rod or water pipes etc) and the N and Ground busses are bonded together. If you think about it, out at the utility xfmr the same things done, the secondary Neutral is tied to a no 4 copper wire down to a ground rod and 3 wires (2 hots and Neutral) are ran to the house where again Neutral is tied to mother earth..... SOOOOOOOOOOOO prior to 2008 NEC it was permissible to go ahead n run to an outbuilding with 3 wires (same as to house from xfmr) and then do as you suggested, tie Neutral to earth once more and bond N and G busses just like at main HOWEVER AS WE BOTH KNOW YOU CAN NOTTTTTTTTTTTTTTTTT DO THAT AT A SUB PANEL IN SAME BUILDING........... In my chats with an inspector when I pointed out how the Neutral had to carry fault current from the main panel back to utility xfmr why didbt they require 4 wires from utility to xfmr (like from main to sub panel in a building) and he said in the UK they do require 4 wires from xfmr to main service entrance

As far as I know the problem with only 3 wires to an outbuilding is iffffffffff theres water or gas pipes or phone or coax etc which is supposed to be (shields at least) bonded to the Ground buss system at both buildings, then you end up with paralell possible ground/fault current paths which is a no no............ But if that condition dont exist I se no prob with only 3 wires to outbuilding which I think was NEC ok until 2008 at least??

YOUR SCENARIO

4) 3 wire with ground and neutral isolated at the subpanel; ground tied to ground rods at the subpanel: This eliminates the possibility of stray ground currents, but there is a very real risk that the ground won't be good enough to fault the circuit breaker in case of a short.

I DONT SEE THIS AS EVER ANY GOOD WHATSOEVER I agree mother earth is wayyyyyyyyy to high resistance to conduct fault current back to the panel so the breaker will trip to clear the fault


BOTTOM LINE if the 2008 NEC thing is true but even if not Ima thnkin from now on at these forum boards I/we ought to recommend 4 wires n be done with it

John T