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power a sub-panel from either 2-pole main or single pole generator

power a sub-panel from either 2-pole main or single pole generator

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  #1  
Old 11-02-12, 09:24 AM
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power a sub-panel from either 2-pole main or single pole generator

hi there.

110v generator with L5-30R and TT-30R receptacles.
220v 60amp breaker in main panel that feeds to a sub-panel with all single-pole breakers.

I'd like to reroute the wires from the 60amp breaker so that I can choose either the 220v power from the main panel or the 110v power from the generator to power the subpanel. I figure this makes the most sense over buying a dedicated transfer switch kit since most of what I want to power is already in the subpanel.

What switch/box is best for this scenario?

I was thinking this could do but I want to make sure I know what I am talking about before purchase:
TCA1006D Panel/Link | Product Details | Reliance Controls Corporation
 

Last edited by Capslock; 11-02-12 at 10:02 AM.
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  #2  
Old 11-02-12, 11:01 AM
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This looks to be a nice switch and being three-pole gives you the option of switching the neutral as well.

USA, Universal Changeover Switch|Manual Generator|3PDT Center OFF|Rotary Cam| RV Transfer Swith| Pedestal Power Supply | Boat Panel | Power Source| Utility | Line | Shore| Back-up Power| Solar Energy| Battery Charger| Rectifier | Transfer Switch| (I have no connection whatsoever with this company.)

You would need to run the feeder from the circuit breaker in the service (main) panel to the contacts on one side of the switch, the center contacts would run to the input lugs of the sub-panel and the contacts on the other side of the switch would need to be paralleled and then run to a generator inlet connector.

Using such an arrangement it is imperative that the sub-panel has NO multi-wire branch circuits whatsoever. Using this arrangement on MWBCs leaves the possibility of severely overloading the neutral conductors on the branch circuit with no overload protection.
 
  #3  
Old 11-02-12, 11:20 AM
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i've been reading about neutral overload and MWBCs but i'm left a bit confused about that part.

first, unless I am misunderstanding what an MWBC is, i don't see how MWBCs wouldn't exist in my home. The main line is 2 legs plus neutral right? so, every circuit is connected to the same neutral. Therefore, the entire house is basically an MWBC no? In this specific regard (ignoring 220v vs 110v), what's the difference between a theoretical single-pole circuit panel and a double-pole circuit panel being powered by a pig-tail single pole power source?

well, see, the fact that I don't fully understand that leans my away from dealing with powering both legs of the sub-panel.

how about this - can I get a switch that basically does what I just said, but for just one of the legs in the sub-panel? I'm basically looking for something I can pick up tonight/tomorrow since we are still without power from Sandy. so what would that look like exactly?

If I understand this right, it should still be safe in this situation:
-60amp breaker 2-pole -> 1-pole right to sub-panel, 1-pole redirected to a switch which then goes to the sub-panel. If I left the 60amp breaker on while the generator is being used, accidentally, that should still be OK because the main panel would only be powering half the panel, the half that the generator is not powering.

thoughts on that?

perhaps something like this:
100 Amp 240-Volt Non-Fused Emergency Power Transfer Switch-TC10323R at The Home Depot

and I would re-route both poles from main, and only power one of the poles from the generator. (there were others similar to this that were something like $20-$50 but they were only rated for 30amp while my subpanel is served 60amp from the main panel)
 

Last edited by Capslock; 11-02-12 at 12:12 PM.
  #4  
Old 11-02-12, 12:22 PM
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Oh, man, I feel for you. I've been the "urban camping" route in freezing weather and it gets old really fast.

Unfortunately there is no safe way to get power to your panel without a transfer switch. If you want to completely disconnect the connections from the service panel to the sub-panel I can help you but ONLY under those conditions. That means removing the circuit breaker from the service panel and disconnecting the wires from that circuit breaker in the sub-panel. If you are willing to do this then post back.

No, the entire house is NOT a MWBC. Contrary to popular opinion homes are serviced by two 120 volt circuits but one 240 volt circuit that has a "center tap" that allows for using one "hot" lead plus the center tap, popularly called a "neutral" but more precisely called the "grounded conductor" for 120 volt loads. Either of the two hot leads can be used with the center tap to provide 120 volts and the two hot leads alone will provide 240 volts.

A MWBC uses all three leads to provide a 240/120 volt supply to two circuits. If the load on the two circuits is equal, say a 100 watt light bulb on each circuit, it is "seen" as a series circuit of two 100 watt bulbs across the 240 volt supply of the two hot wires and the neutral wire carries no current at all. In fact, in this example you can disconnect the neutral wire and the two lights will be the same as with the neutral. However, if you remove the neutral and then turn off one lamp the other will go out as well as you no longer have a complete circuit. In a properly wired MWBC the neutral carries only the UNBALANCED current of the entire circuit. With the same load on each circuit there is no unbalanced current hence no current on the neutral.

Now replace one of the lamps with a 200 watt bulb. With the neutral connected each bulb will receive 120 volts across its terminals. Disconnect the neutral and the voltage across each bulb will change with one being significantly higher than 120 volts and the other significantly lower. Reconnect the neutral and you will be able to measure the unbalanced current caused by one bulb being twice the size of the other. This ONLY works this way in a properly wired MWBC where the two hot leads have the higher voltage.

Now let's reconnect the circuit so that there is only one hot lead and the neutral. There is no longer a series circuit from one hot lead through the first bulb, through the second bulb and on to the second hot lead. The circuit now is from the hot lead through the first bulb to the neutral and from the same hot lead through the second bulb to the neutral. Since circuit breakers are ALWAYS in the hot lead and NEVER in the neutral lead the protection is on the hot lead. Since both bulbs are being supplied by the hot lead BEFORE the circuit breaker it can be easily seen that measuring the current flow AFTER the circuit breaker shows ONLY the current flow for that particular circuit. But if you were to measure the current in the neutral wire you would find it was equal to both of the hot leads.

So here is the rule. When a multi wire branch circuit is properly connected with the hots on opposite sides the current in the neutral is equal to the DIFFERENCE between the current flows in the two hot leads. If the two hot leads are on the SAME side the current flow in the neutral is equal to the SUM of the two hot leads.

Now to determine if your sub-panel has any MWBCs look at the circuit breakers. If any of them have a red wire follow that red wire to the point where it enters the enclosure and see if there is also a black wire along with the white wire coming in on the same cable. If it does this is a MWBC.

Post back if you want more instruction on getting power to your sub-panel.
 
  #5  
Old 11-02-12, 12:41 PM
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hmm...then no, i don't have any MEBCs as far as I can tell.

The only place I see all three wires, black/red/white is for 240v applications i.e. cooktop/oven/air conditioners - and of course the sub-panel.

I'm not trying to skip away from using a transfer switch by the way (h-e double hockey sticks no, far from it, I have probably an un-healthy sense of paranoia about safety - overbuild I say!).

I'm just trying to figure out if there are cheaper transfer switch alternatives than one of those 6-switch kits you see at ace hardware and home depot like this: Reliance Controls Transfer Switch Kit 6 Circuit, Model# 31406CRK | Transfer Switches| Northern Tool + Equipment

as you can see that's intended for a 240v generator so I could only use half of it (1-pole); seems like a waste and way more stuff than necessary.

I originally assumed I could just get a transfer panel that just has a simple switch between main and generator (generator being a line that goes to the proper l5-30R receptical), and that feeds the entire sub-panel - or now that I know a little bit more - 1 leg of the sub-panel.

The link i sent in my previous post was for an on-off-on switch so I had thought something like this would be, essentially, the same thing? (again, both poles from the 60amp breaker would go to this, and the generator would energize one out of the two poles) but $200 less and less 'stuff' making the whole setup just simpler. is this line of thinking just wrong?
 
  #6  
Old 11-02-12, 01:02 PM
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Sure, you can use that switch and you can power both sides of the sub-panel with your 120 volt generator to boot. You simply need to parallel both of the lugs on the generator side with the hot lead from the generator. When in the utility position you will have 240/120 to the sub-panel and when in the generator position you will have 120 volts to both sides.

I have a similar switch on my sub-panel and I had it wired that way until I changed the generator outlet to a 240/120 model.
 
  #7  
Old 11-02-12, 01:11 PM
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oh, ok then glad we are in agreement

You simply need to parallel both of the lugs on the generator side with the hot lead from the generator.
so my generator has L5-30R and TT-30R recepticals.

when you say to parallel both of the lugs, i'm not quite sure what you mean. There is a positive, negative, and ground on these recepticals as far as I can tell. are you talking about some kind of re-wiring of something internal to the generator?

this might help. here is a photo of the generator plugs:
 
  #8  
Old 11-02-12, 01:25 PM
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Nope, you don't have to change a thing on the generator. Run the interconnect cable to the switch and split out the wires. Connect the white to the sub-panel neutral bus. Connect the green to the sub-panel equipment grounding bus. Take a short piece of #10 type THHN/THWN wire (available at the homecenter) and connect it to one of the lugs on the generator side of the switch and connect a similar wire to the other generator side lug. Connect these two wires to the generator cable's black wire with a red wire nut and you are all set.

Of course you need to wire the two conductors from the service (main) panel to the utility side of the transfer switch as well as wire the center lugs of the switch to the sub-panel but you already knew that part. Don't forget the proper cable clamps and flexible conduit as needed.

BTW, you don't have + and - on your generator, you have hot and neutral. + and - denote direct current, like from a battery.

One more thing, does your generator state anywhere on it or in the manual that it : (a) has a bonded neutral or (b) has ground fault protection? If it does there may be some additional things to do.
 
  #9  
Old 11-02-12, 01:55 PM
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ah ok, i was thinking what you said I guess i just thought of it as a 'pig-tail'. I think my route will be to get an outdoor receptical i can plug the generator into, and run that to the transfer switch then do exactly as you say about the 2-lugs to the black lead.

.i don't know if this is an indicator in the manual but check out that picture; on the right side there is a ground terminal for use with a ground rod.

oh - here is what manual says about grounding:

Grounding
The generator system ground connects the
frame to the ground terminals on the power
panel. The system ground is connected to
the AC neutral wire.
Use a ground fault circuit interrupter (GFCI) in
damp areas and areas containing conductive
material such as metal decking.
A ground terminal connected to the frame of
the generator has been provided on the
power panel. For remote grounding, connect
of a length of heavy gauge (12 AWG
minimum) copper wire between the
generator ground terminal and a copper rod
driven into the ground.
 
  #10  
Old 11-02-12, 05:07 PM
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You have a neutral-to-equipment-ground bond in your generator. It appears that you do NOT have GFCI protection on the generator. You do NOT need to install any wiring on the ground terminal on the generator nor do you need to drive a ground rod. You MAY have trouble with any GFCI protected circuits in the house tripping as a result of having the neutral and equipment ground bond existing at both the generator and the service panel.

You will also have parallel paths between the neutral and the equipment grounding conductor in the generator-to-house interconnect cable. I wrote a fair amount on this subject in another thread today. IF that switch from HD has a third pole for switching the neutral (I don't think it does) then you can alleviate these problems by switching the neutral along with the "hot" lead(s).
 
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