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-   -   Emergency power backup via power inverter (http://www.diychatroom.com/f18/emergency-power-backup-via-power-inverter-73148/)

upabove 06-08-2010 10:31 AM

Emergency power backup via power inverter
 
Hi everyone,
After the last batch of storms here in Illinois with power outages I plan on installing a 1800w Xantrex freedom hf with 2 gel marine batteries for lighting and small loads. It has a built in 30 amp transfer switch and 40 amp charger and is made to be hardwired. I will install a 30 amp breaker from the AC panel to supply the inverter power and have a sub panel(2 space 4 circuit). I purchased slim line breakers which have 2 20 amp circuits on each breaker. The circuits I plan on powering are necessities such as fridge and kitchen lighting and outlets, no high load items.

I have been planning this out for a while and have most of the items, just waiting on a t-fuse and battery cables to arrive. I have installed many transfer switches with generators and am very comfortable with wiring. The reason for home battery backup is because the wife can't start a generator and to be honest am afraid she would blow up the house. So a built in transfer switch to save the fridge and give her lighting is my main goal. Question I have is about the sub panel since I never wired one. The inverter output will have 10 awg 3 wire(line neutral ground) which I will wire to the sub panel.

The sub panel has two lugs. one supplying each breaker I presume. The inverter has only one output lug(3 wire). Does the sub panel provide power to both breakers if I wire one lug? Are both lugs in there for 240? It doesn't look like they are bonded to each other. I want to supply power to both breakers(120v) but will have the inverter output to one lug. Hope this makes sense. Everything else looks very straight forward. Thanks for any input...

AllanJ 06-08-2010 11:07 AM

You can remove the 240 volt feed from the subpanel lugs and then connect the single 120 volt inverter feed to both hot lugs of the subpanel to power all of the breakers in which case any 240 volt equipment will get zero volts.

The transfer switch cannot be in the inverter unit with just one cable coming out. There must be a transfer switch in the current path from the main panel to the subpanel if you are going to power that subpanel with the inverter.

The common terminals of the (double pole) transfer switch are attached to the subpanel top feed. The side of the transfer switch (two sides; double throw) that accepts the inverter has two lugs for the two hot poles. Connect the hot lugs together (bridge them; bond them to each other) at this side and attach the single hot for the inverter feed. (Connect the inverter neutral to the neutral lug. Connect the original feed from the main panel to the subpanel to the other side of the transfer switch.)

upabove 06-08-2010 11:18 AM

Hey Allan,
Thanks for the reply. After reading your response, can you tell me why the transfer switch will not work in that configuration? The main panel is not attached per say because the built in transfer switch sees there is no power coming from the main ac panel. Once power is restored, then the transfer switch cuts off the battery side and the main ac panel is now supplying power to those circuits as well as charging the batteries. That is the intended use per Xantrex for hardwiring the inverter? All other Xantrex inverters have the same configuration. This transfer switch is the same as generator transfer switches,but automatic and both prevent backfeeding to the ac panel.(i.e no injured workers) At no time is power coming from both sources.(batteries versus ac grid) I understand both lugs providing 240 volt and each side is 120 volt, my question is if I wire one lug, are both sides receiving 120volt? Could you please explain? Thanks!

Thanks Allan,
You must have been editing your post while I was typing. I think we are both on the same page now. I am going to wire one lug of the sub panel with the inverter output so both sides of the sub panel are getting 120 volts. there will be no need for 240 volts. The transfer switch is between the sub panel and the main ac panel, there will be no backfeeding of power to the ac panel. Thanks again for taking the time!

brric 06-08-2010 12:11 PM

You will be backfeeding the entire electrical system with 120 volts by doing what you are proposing.
The power from the inverter will be going into the subpanel and on through it via the conductors and te breaker feeding ths sub panel from the main panel.

A transfer switch is required to isolate the utility power from the emergency power. The inverter transfer switch does not do this.

upabove 06-08-2010 12:17 PM

Thanks for pointing that out b. What is the easiest way to fix that? Another transfer switch or something between the main breaker and inverter?

AllanJ 06-08-2010 12:34 PM

Quote:

Originally Posted by upabove (Post 452937)
The transfer switch is between the sub panel and the main ac panel, !

Perfect, that's all you need to prevent backfeeding the utility lines. The inverter is also connected to this transfer switch (and not directly to the subpanel) in a manner that the subpanel can get power from the inverter or from the main panel but that the main panel and inverter can never be connected together.

You may not connect the inverter output directly to the subpanel without first unscrewing and removing the wires going to the main panel.

Exception: The National Electric Code does allow separate breakers or switches to connect the main panel feed or the inverter feed to the subpanel but with cams and/or levers and/or sliders that prevent both sets of breakers/switches from being on at the same time.

brric 06-08-2010 12:37 PM

One option would be a GE TM830CUGEN manual transfer switch. About $100.00. You install it as a sub panel with the circuits you want to have for back-up power. The inverter is also fed to the switch. It prevents utility power and back-up power from being connected at the same time.

upabove 06-08-2010 12:48 PM

Hey Br, Allan
Thanks again for the time to answer these questions! Let me take another stab at this..Brr,after looking at the manual again I don't see how this will backfeed into the main ac panel. The 30 amp breaker from main ac goes to power inverter for power. internal transfer switch looks for power on either side so no backfeeding there. If main ac panel loses power, transfer switch sees this and transfers to batteries and inverter output supplies sub panel(4 circuits). These 4 circuits were removed off the main ac panel and relocated to sub panel. so when power is restored, main breaker supplying power inverter is also supplying those 4 circuits and when power is lost "only" those 4 circuits are being powered by the batteries. There is no tie between those 4 circuits and the rest of the ac panel. Does that make sense? I'm trying to visualize this and type in a coherent manner at the same time:)

brric 06-08-2010 12:58 PM

Quote:

Originally Posted by upabove (Post 452967)
Hey Br, Allan
Thanks again for the time to answer these questions! Let me take another stab at this..Brr,after looking at the manual again I don't see how this will backfeed into the main ac panel. The 30 amp breaker from main ac goes to power inverter for power. internal transfer switch looks for power on either side so no backfeeding there. If main ac panel loses power, transfer switch sees this and transfers to batteries and inverter output supplies sub panel(4 circuits). These 4 circuits were removed off the main ac panel and relocated to sub panel. so when power is restored, main breaker supplying power inverter is also supplying those 4 circuits and when power is lost "only" those 4 circuits are being powered by the batteries. There is no tie between those 4 circuits and the rest of the ac panel. Does that make sense? I'm trying to visualize this and type in a coherent manner at the same time:)

I see what you are saying. I must be unclear about how that unit operates. Are you sure that normal utility power is fed through the unit during normal cicumstances. If so your plan sounds ok. I was thinking the power to the inverter only monotors power and keeps the batteries charged until there is a power failure and then transfers to battery power. If it operates as you say then your plan might be ok. Is there a URL which shows the inverter manual?

AllanJ 06-08-2010 01:12 PM

(several edits made)
Quote:

Originally Posted by upabove (Post 452967)
The 30 amp breaker from main ac goes to power inverter for power. internal transfer switch looks for power on either side so no backfeeding there. If main ac panel loses power, transfer switch sees this and transfers to batteries and inverter output supplies sub panel(4 circuits). These 4 circuits were removed off the main ac panel and relocated to sub panel. so when power is restored, main breaker supplying power inverter is also supplying those 4 circuits and when power is lost "only" those 4 circuits are being powered by the batteries. There is no tie between those 4 circuits and the rest of the ac panel.

Correction on what I said earlier.

Connect the (30?) amp cord from the back of the inverter to the main panel. (You can leave the plug attached to the cord and install a matching receptacle in a box near the main panel connected to a matching (30?) amp breaker in the main panel)

For the side of the external transfer switch (between subpanel and main panel) intended for the inverter, connect this to the receptacle on the front of the inverter using another cord. Connect the hot wire of this cord to both hot lugs on this side of the breaker. Do not bond to each other (bridge) the two hot lugs on the common part of the external transfer switch connected to the subpanel.

Subpanel power capacity: Normal -- whatever the main panel breakers for the subpanel are up to the panel rating. Power outage -- 15 amps.

************************************************** *****
Alternate connection, max 15 amps 120 volts and no 240 volts to the subpanel at all times. (The inverter is 1800 watts as you stated)

Connect the cord from the back of the inverter to the main panel as above.

The only power feed for the subpanel is a cord to be plugged into a receptacle on the front of the inverter (or any other receptacle). Connect the hot wire of this cord to both subpanel hot lugs and connect the neutral of the cord to the neutral lug.

No separate transfer switch needed.

With utility power normally on, the power passes through the inverter directly to the output receptacle and on to the subpanel. The utility power also charges the battery. During a power failure, the transfer switch inside the inverter disconnects the main feed and connects the batteries to the output receptacle.

Only by using this alternate connection will the inverter not backfeed the utility power if you omit the external transfer switch.

upabove 06-08-2010 01:13 PM

Hey Brr,
Yes, here is the link... 3rd document installation instructions page 1-7. When A/C power is up it charges the batteries and passes power to ac loads in sub panel. Sub panel circuits are completely isolated from main breaker panel.

http://www.xantrex.com/web/id/257/p/.../8/product.asp

Hey Allan,
I would take the necessary circuits off the main panel and relocate them to the sub panel via twist nut ties which are approved in my area. So the main panel is still supplying these circuits via the 30 amp breaker to the inverter, but once power is lost the inverter transfer switch only allows power from the batteries and doesn't allow back flow via the 30 amp breaker. Since the 4 circuits were relocated from the main to sub panel, there are no ties to allow back flow.

brric 06-08-2010 01:41 PM

After reading most of the instructions I would say you are correct about the feed-thru. Looks as if you're good to go.

upabove 06-08-2010 01:48 PM

Thanks brr and Alan for taking the time to confirm this. Great forum!

AllanJ 06-08-2010 04:32 PM

Just found the part in the Xantrex manual allowing a hard wired cable from the inverter output as the sole feed to the subpanel, in lieu of cord and plug to the inverter front panel receptacles and allowing a power usage of nearly 30 amps with utility power on. (This is not the same cable that comes out the back of the inverter to go to the main panel)

upabove 06-08-2010 11:18 PM

Thanks for checking it out Allan....


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