backfeed 110v generator

Reply

  #1  
Old 07-22-12, 09:09 PM
Member
Thread Starter
Join Date: Jul 2012
Posts: 6
backfeed 110v generator

I have a 3500 w portable generator that I would like to connect to my home wiring.

Here is what I would like to do. install a sub panel fed from my main 200 amp home panel. I would be using a square D generator panel with interlocked breakers rated at 30 amps, my generator has a 30 amp twist lock plug. Can I wire that into one of the 30 amp 220 v breakers by spitting the 110 v to power both phases of the 220v breaker, I would only be powering 110v loads with the sub panel so I don't ever need 220v but would like to have both phases of the sub panel powered so I can use all available places for breakers.

I don't see another way to use interlocked breakers, all the mechanical interlocks I have found seem made for 220v breakers, this is my reasoning for trying to come up with this solution any and all help is appreciated.
 
Sponsored Links
  #2  
Old 07-22-12, 09:17 PM
Member
Thread Starter
Join Date: Jul 2012
Posts: 6
might have left out some details.

Use the appropriate 2 wire plus ground to wire generator to appropriate generator outlet then use 3 wire plus ground to go from outlet to the 220v breaker with both hot wired connecting to the single hot connector at the generator outlet. Then each hot wire going to the opposite poles of the 220v breakers
 
  #3  
Old 07-22-12, 09:30 PM
lawrosa's Avatar
Super Moderator
Join Date: Dec 2010
Location: USA
Posts: 17,607
Can I wire that into one of the 30 amp 220 v breakers by spitting the 110 v to power both phases of the 220v breaker, I would only be powering 110v loads with the sub panel so I don't ever need 220v but would like to have both phases of the sub panel powered so I can use all available places for breakers.

I don't see another way to use interlocked breakers, all the mechanical interlocks I have found seem made for 220v breakers,
Kind of contradicting, no? Why get a sub? What do you want to power? Just install a proper transfer switch.

 
  #4  
Old 07-22-12, 09:36 PM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 33,047
I was going to answer but I got more confused each time I read it. Okay I will just make comments. Correct voltages are 120v and 240v not 110v and 220v. A true 240v (2-wire+ground) receptacle can not be used to provide 120 volts because you need a neutral. For that you need a 4-wire (3-wire+ground) 120/240v receptacle.
 
  #5  
Old 07-22-12, 10:18 PM
Member
Thread Starter
Join Date: Jul 2012
Posts: 6
transfer switch are much more expensive than interlocked breakers from what I've seen. I'll try and explain this more simply. the crux of what I would like to do is take a 120v source use a 4 wire conductor with bot hot wires connected to 120v hot to a 220v breaker. so that both phases see the 120v and can supply separate 120v breakers.
 
  #6  
Old 07-22-12, 10:34 PM
lawrosa's Avatar
Super Moderator
Join Date: Dec 2010
Location: USA
Posts: 17,607
what I would like to do is take a 120v source use a 4 wire conductor with bot hot wires connected to 120v hot to a 220v breaker.
Ahh I see. I am not an electrician but I get it.


What is your 120 source? So really what you want to do is take a 120v gen power and power both sides of the panal?


Dont know if its allowed but in a transfer switch I would assume to tie both hots toghther. But you need to be 120 only from the gen.

Give us more info.



 
  #7  
Old 07-22-12, 10:47 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,399
If I am correctly interpreting what you want to accomplish it CAN be done BUT several things must be done in a very specific manner to make it safe.

You will need to have all the circuits you want powered by the generator in a single circuit breaker panel. That panel MUST be fed from the SERVICE (main) panel through an interlocked circuit breaker and the circuit breaker that is interlocked with the service feeder will be fed from the generator.

You absolutely cannot have any 240 volt circuits in this sub-panel nor can you have any multi-wire branch circuits. You would connect the "neutral" lead from the generator to the neutral bus and the "hot" lead from the generator would connect to both both sides of the 240 volt interlocked generator circuit breaker. Some Square D circuit breakers will allow for two wires under one screw but otherwise you will have to "pigtail" a single lead from each circuit breaker terminal with the lead from the generator.

You will need to be careful not to overload the generator, any generator that has only a 120 volt output is going to be a fairly small generator.

If your generator has a "bonded neutral" you do not want to use an equipment grounding conductor between the generator and the panel. If the generator does NOT have a bonded neutral then you DO want the equipment grounding conductor. The generator circuit breaker in the subpanel should be rated no higher than the circuit breaker on the generator itself. If your generator has GFCI protection then all bets are off.
 
  #8  
Old 07-22-12, 11:00 PM
Member
Join Date: Aug 2010
Location: USA
Posts: 118
Like Furd said, you can do that, but...

The neutral current in the subpanel with 120 volt feed x2 will be L1 + L2. That is much higher than the usual 240V neutral current which is L1 - L2. That means if your neutral wires are undersized you may have problems.
 
  #9  
Old 07-22-12, 11:20 PM
lawrosa's Avatar
Super Moderator
Join Date: Dec 2010
Location: USA
Posts: 17,607
The neutral current in the subpanel with 120 volt feed x2 will be L1 + L2. That is much higher than the usual 240V neutral current which is L1 - L2. That means if your neutral wires are undersized you may have problems.

nor can you have any multi-wire branch circuits.



I may be wrong but I thought the undersized neutral is only in play on MWBC as Furd suggests?

With that said I will leave it to the more experienced to finish this thread.
 
  #10  
Old 07-23-12, 01:47 AM
Member
Thread Starter
Join Date: Jul 2012
Posts: 6
I think we are now all on the same page. As far as undersized neutral, all the wiring is for 15 amp house wiring. I don't plant on running anything large, refrigerator lights etc. Im trying to find an easy solution to operate. This may be much harder to implement but Im hoping in the end it will be pretty much plug in the generator and flip a few breakers. I'm in the Navy and want it usable for my wife if I am gone for the duration of an outage. Week long power outages not unheard of.

Thanks for all of the help so far.
 
  #11  
Old 07-23-12, 01:51 AM
Member
Thread Starter
Join Date: Jul 2012
Posts: 6
Its a 3500 watt generator with only 120v output. That is correct I would like to power sides of a small interlocked sub panel that will only supply other 120v loads
 
  #12  
Old 07-23-12, 07:06 AM
Member
Join Date: Mar 2010
Location: USA
Posts: 3,093
You can use a standard interlock and a standard double wide double breaker that are compatible with your panel. Both hots are wired to a 120 volt male receptacle to backfeed the generator but ...

The only big problem is the presence of multiwire branch circuits somewhere in the panel or a subpanel.

You might look into a transfer switch box with several (typically 6 to 12) small transfer switches instead of one big transfer switch. You install this box near the panel to be fed. For each branch circuit eligible to get generator power you run one wire from the breaker into the transfer switch box (via a large nipple or conduit) and run a jumper (pigtail) from the branch circuit as it enters the panel into the transfer switch box. These two wires connect to one of the small transfer switches. Only one side of an MWBC would be eligible to receive generator power, you choose which. (correction) Neither side of an MWBC and no 240 volt circuit may be run into the transfer switch box and be eligible to receive power this way when you have a 120 volt only generator feeding the entire switch box.

(added later) Since it is a subpanel you are feeding or backfeeding, it would be easy to install a big transfer switch (rated for at least the amperes going to the subpanel) just upstream of the subpanel. But the subpanel must not contain any MWBCs. Note that if the subpanel and its wiring to the main panel are rated for X amps at 240 votls then your 120 volt generator is limited to X amps at 120 volts, not 2X at 120 volts. An additional small box with a breaker can be installed on the generator side of the transfer switch to limit the incoming power if a larger generator got hooked up later.
 

Last edited by AllanJ; 07-23-12 at 07:25 AM.
  #13  
Old 07-23-12, 11:01 AM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,399
I had a sub-panel that I had installed a few years before I got my generator. I bought a used 3-pole, double throw, center off 60 ampere switch and after re-routing all the loads I wanted to run from the generator to the sub-panel I then rerouted the power from the service (main) panel via a 60 ampere circuit breaker through the three-pole switch to the sub-panel. The switch has a mechanical interlock so that pulling it from the UTILITY position it locks in the OFF position until you manually defeat the interlock and then you can pull it the rest of the way to the GENERATOR position. Going back to utility power is the reverse motions, including the mechanical interlock.

I wired the generator side of the switch to an outside power inlet box with a four-wire connection as standard. This allows the use of a 240/120 generator up to 7500 watt capacity in a standard configuration with no wiring changes should such ever be desired or I ever move.

My generator, a Yamaha EF3000iSE is rated at 2800 watts/120 volts continuous with a 3000 watt maximum for 20 minutes. I changed out the original 3-wire twist lock receptacle for a four-wire model and wired both hot sides of of the receptacle to the single hot from the generator. I use a 4-wire interconnect cord so everything but my generator output receptacle is standard 240/120 volt; only the generator receptacle is non-standard. This allows me to run the 120 volt loads in my sub-panel from a 120 volt source. I had to make absolutely certain that there were no 240/120 volt (multi-wire) branch circuits in the sub-panel and that required re-wiring the furnace away from the combined furnace/laundry circuit.

I power my furnace, my kitchen counter top (including refrigerator) receptacles, the receptacle supplying the TV (with DVD player) and my computer and Internet connection along with a few lights. I can live quite comfortably with this set up. I can use the microwave OR countertop convection oven along with the refrigerator and furnace without overloading the generator.

I also converted my generator to use gaseous fuel rather than gasoline. I tested it just a week ago when the power went out for a while. The story is down a few lines here in the electrical forum. I'll state here that it generally takes less than five minutes from the time I decide to start the generator until I have it running and powering my essential loads.
 
  #14  
Old 07-24-12, 04:22 PM
Member
Thread Starter
Join Date: Jul 2012
Posts: 6
Thanks Furd

this is pretty close to exactly what I want to do
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes