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Feeding the service panel.
By jumpingjackflatch, Section Controls Posted on Mon May 16th, 2005 at 02:53:47 AM MST
Will it work?

I have acquired a transformer that is 120 volts on the primary side and has two 120 volt secondaries. It is rated at 4 kilowatts. What I would like to do is connect my 3000 watt MSW inverter to my grid connected service panel through a 30 amp enclosed switch. The idea is if there is a grid feed interruption, I can turn off the main breaker in the service panel, disconnecting the house from the grid, then switch the enclosed switch to connect the inverter, through the isolation transformer, to the service panel for an alternate feed. I have no heavy drain appliances or dryer.
On the transformer, I will connect the two 120 volt secondary windings to form a 240 volt centre tapped winding and will feed the service panel. My question is do I connect the output ground from the inverter to the centre tap of the secondary of the transformer? I think it is unnecessary. The inverter is one of those cheapy types that grenade when you connect ground and neutral.
Randie

Feeding the service panel. | 11 comments (11 topical, 0 editorial)

Re: Feeding the service panel. (3.00 / 0) (#1)
by drdongle (Dr.Dongle1@juno.com) on Mon May 16th, 2005 at 05:44:30 AM MST
(User Info)

I think that you could get away with not grounding the inverter as the transformer provides isolation and hence is intrinsically safe. However I'd suggest using a "transfer switch" which would install between the meter and panel and connect to the transformer. In one position it connects commercial power to the panel and in the other it connects the inverter. This prevents any possibility of connecting the inverter across the live panel.
Carpe Vigor, Dr.D

Re: Feeding the service panel. (3.00 / 0) (#2)
by troy on Mon May 16th, 2005 at 12:35:11 PM MST
(User Info)

I strongly second DrDongle's recommendation to use the appropriate "break before make" transfer switch to guarantee you can't have both your electricity source and the utility hooked up simultaneously, which could lead to your house burning down, or electrocuting some poor lineman trying to fix a downed line somewhere.

Also keep in mind that what you are attempting to do probably won't be inspected or approved by the building code, and as such, probably voids your homeowner's insurance if you have a fire. Not that I wouldn't try the same thing, wink wink. Just so you know the risk you're taking.

Finest regards,

troy

Re: Feeding the service panel. (3.00 / 0) (#3)
by Nando (nando37-at-tx-dot-rr-dot-com Correct theanti-spam) on Mon May 16th, 2005 at 01:20:56 PM MST
(User Info)

You have a common point system.

Connect the transformer input ground to GRID ground and the center tap of the transformer output (120-grd-120) to the Ground of the system ( Grid Ground).
So all grounds are together.

By the way the 4000 watts transformer may be using around 120-150 watts by itself
with no loads.

Be careful with your set up and make sure that you have the proper arrangement to switch over.

Regards

Nando

Re: Feeding the service panel. (3.00 / 0) (#8)
by Ungrounded Lightning Rod on Mon May 16th, 2005 at 06:49:35 PM MST
(User Info)

Connect the transformer input ground to GRID ground and the center tap of the transformer output (120-grd-120) to the Ground of the system ( Grid Ground).
So all grounds are together.

The output center tap should be connected to the panel neutral. The main panel will have the SINGLE bond between the neutral and the ground that is the only one you should have in the entire system (otherwise you may get large currents - or lightning currents - through undersize wires). Even if you feed the panel through a utility/inverter transfer switch the neutral should remain connected, and thus you get your bond.

The two transformer output hot wires feed the two panel hot wires (through a transfer switch).

The inverter hot and neutral feed the transformer input hot and neutral.

If the transformer input has a "ground" connection (such as a third pin on a wall plug), use an ohmmeter to make sure it is NOT connected to any of the windings. (If it is connected to the input winding, connecting it to just about anything is likely to let the magic smoke out of the inverter. If it's connected to the output winding it might let the magic smoke out of the transformer.)

What I'd expect from a "ground" connection on the input side of an isolation transformer is a connection only to the FRAME and to any output ground pins. If that's the case, connect it to your house wiring green-ground. (And you won't need to connect any output "ground" connection because you already got it on the input side.)

If your inverter has a "ground" connection, check for continuity to both the DC inputs and the outputs. If it's not connected to either, connect it to your green ground, too. But if it DOES have continuity (like less than a meghom) to any inverter input or output terminal, you'll have to research the right way to connect it. (Otherwise you might let the magic smoke - and maybe the magic FIRE - out of your inverter or battery wiring, and maybe the magic acid-water and exploding gasses out of your battereis.)

[ Parent ]

Re: Feeding the service panel. (3.00 / 0) (#4)
by Off grid in Tonopah on Mon May 16th, 2005 at 04:21:27 PM MST
(User Info)

Should work as long as you don't need to run any 220 volt loads. As long as you are only runing loads from line to neutral than you will have potential but the two phases will be in line as they come from the same source and you won't have any potential phase to phase.

Re: Feeding the service panel. (3.00 / 0) (#5)
by drdongle (Dr.Dongle1@juno.com) on Mon May 16th, 2005 at 05:49:09 PM MST
(User Info)

No not true. It's not two phases its one phase 240 V( split phase) center taped to ground, you CAN drive a 240 V load no problem. when leg one is + leg two will be -.

Carpe Vigor, Dr.D
[ Parent ]

Re: Feeding the service panel. (3.00 / 0) (#6)
by jumpingjackflatch (rkochals (at) sympatico.ca) on Mon May 16th, 2005 at 06:12:36 PM MST
(User Info)

I guess I'll start looking around for a transfer switch. It seems like the
ONLY wise choice. I tried the transformer on the bench and it delivers 120 and
240 fine. I have no 240 volt loads in the house. Also, the transformer input has only 3 input studs. 0, 120 volts and 130 volts so I'm not sure about Nando's suggestion about the input ground. This is a bare transformer frame, I plan to install it in a listed metal enclosure. I am now curious about the no load draw and
will go back to the bench to test it.

Randie

Re: Feeding the service panel. (3.00 / 0) (#7)
by halfcrazy on Mon May 16th, 2005 at 06:46:14 PM MST
(User Info)

if you have no 240 loads dont waste the time on the transformer just jump between both hot legs on the inverter side of the transfer switch this will simply feed 120 to both sides of the main panel when you are in inverter mode.

[ Parent ]

Re: Feeding the service panel. (3.00 / 0) (#9)
by jumpingjackflatch (rkochals (at) sympatico.ca) on Mon May 16th, 2005 at 08:30:43 PM MST
(User Info)

I was wondering if anyone can tell me if the waveform coming out of the isolation transformer is at all different from the msw at the input?

Re: Feeding the service panel. (3.00 / 0) (#10)
by ghurd on Mon May 16th, 2005 at 08:59:51 PM MST
(User Info)

I think it is a little smoother.
Not great but better.
G-

[ Parent ]

Re: Feeding the service panel. (3.00 / 0) (#11)
by drdongle (Dr.Dongle1@juno.com) on Mon May 16th, 2005 at 09:58:45 PM MST
(User Info)

You can experiment with capacitors across the transformers secondary, use a scope to monitor the wave form. With some experimentation you can get a better approximation of a sine wave, when you have a parallel resonant circuit, It won't be perfect just better. Use 600 volt caps
Carpe Vigor, Dr.D
[ Parent ]

Feeding the service panel. | 11 comments (11 topical, 0 editorial)

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