Transformer secondary grounding.

[KD4UPL]

I install a lot of solar and battery back-up systems. A common situation for me is running a 240v submersible well pump motor from a 120v inverter. I use a 3kVA transformer to step up the voltage. Most well pumps I encounter draw 7 to 10 amps at 240v. I typically use a 30 amp SP breaker and 10-2 romex to feed the trans. primary. I then connect 10-2 romex from the secondary to the pump wires going to the well.
My question is how do I handle the grounding on the secondary side. There is no neutral wire on the secondary. I think I need to connect one of the secondary wires to the equipment grounding conductor. Does it matter which wire? Do I also connect this to the equipment grounding conductor in the romex feeding the primary side of the transformer?

I know this is really basic but I can't get my head around connecting a "hot" wire to a ground wire. It seems like it will be a direct short.

Thanks for any help you can give.

 

[don_resqcapt19]

First 250.20 does not require that secondary of that transformer to be grounded as long as it is a two wire secondary. If the transformer can be wired to supply 120/240, then 250.20(B)(1) would require it to be grounded. If you leave it ungrounded, then 250.21(B) would require the use of a ground detector. Second, if you make it a grounded system, you cannot use the supply equipment grounding conductor to ground it. You need a grounding electrode and a grounding electrode conductor. See 250.30.
As far as connecting a "hot" wire to ground, that is not an issue...any conductor of an ungrounded system can be connected to earth to make a grounded system. There is no current path to cause a short. Current is not trying to get back to the earth, it only returns to its source.

 

[KD4UPL]

Thanks Don,
I had missed the section where it said I didn't have to ground the secondary. I will look into ground detectors. I haven't got much experience with those.

 

[tkb]

First 250.20 does not require that secondary of that transformer to be grounded as long as it is a two wire secondary. If the transformer can be wired to supply 120/240, then 250.20(B)(1) would require it to be grounded. If you leave it ungrounded, then 250.21(B) would require the use of a ground detector. Second, if you make it a grounded system, you cannot use the supply equipment grounding conductor to ground it. You need a grounding electrode and a grounding electrode conductor. See 250.30.
As far as connecting a "hot" wire to ground, that is not an issue...any conductor of an ungrounded system can be connected to earth to make a grounded system. There is no current path to cause a short. Current is not trying to get back to the earth, it only returns to its source.

Wouldn't grounding a two wire secondary that is 240 volt be a violation of 250.20(B)(1)?
The voltage would end up at 240 volts to ground.

(B) Alternating-Current Systems of 50 Volts to 1000 Volts.
Alternating-current systems of 50 volts to less than 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:

(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts

Since it is not three phase, 250.20(B)(2) and 250.20(B)(3) do not apply.

 

[tkb]

Don, I reread you post where you stated that IF it were 120/240 then 250.20(B)(1) would require the transformer to be grounded.

 

[hillbilly1]

First 250.20 does not require that secondary of that transformer to be grounded as long as it is a two wire secondary. If the transformer can be wired to supply 120/240, then 250.20(B)(1) would require it to be grounded. If you leave it ungrounded, then 250.21(B) would require the use of a ground detector. Second, if you make it a grounded system, you cannot use the supply equipment grounding conductor to ground it. You need a grounding electrode and a grounding electrode conductor. See 250.30.
As far as connecting a "hot" wire to ground, that is not an issue...any conductor of an ungrounded system can be connected to earth to make a grounded system. There is no current path to cause a short. Current is not trying to get back to the earth, it only returns to its source.

I know what you mean, but I don't want the OP to think that the equipment ground is isolated from the grounding electrode connection, when it is also connected to XO.

 

[don_resqcapt19]

Wouldn't grounding a two wire secondary that is 240 volt be a violation of 250.20(B)(1)?
The voltage would end up at 240 volts to ground. ....

Tim,
While there is no requirement to ground a 2 wire 240 volt system, there is also no rule that says you can't. 480 volt corner grounded systems are permitted by the code rules. Remember that the rules in 250.20 are telling you what systems must be grounded. 250.22 tells you the systems that are not permitted to be grounded.

 

[don_resqcapt19]

I know what you mean, but I don't want the OP to think that the equipment ground is isolated from the grounding electrode connection, when it is also connected to XO.

That is why I said:

You need a grounding electrode and a grounding electrode conductor.

Maybe I should have added that you will also need a system bonding jumper.

 

[KD4UPL]

Thanks for all the replies fellows. I think I have it straight now. I've been doing it correctly by not grounding the secondary, I just need to install ground detectors to comply with the code. That seems like the simplest approach for my situation.

 

[hillbilly1]

That is why I said:

Maybe I should have added that you will also need a system bonding jumper.

I've seen too many connected to the grounding electrode only, just wanted to make sure the OP connected it to the equipment ground also, so there would not be a possible potential between the two. You only mentioned the grounding electrode, and the grounding ellectrode conductor. Nothing about the equipment ground.