step up / down transformer grounding

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wickhdm

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We have a 240v 1phase load that is about 1200 ft from the 240v panel. We want to use a 1 phase 240 to 480 transformer at the panel go 2 wire with egc through non metallic conduit and handholes to a 480 to 240 transformer with disconnect all mounted on a common metallic structure with the load.
It seems to me that to make these grounded systems I would have to ground 1 leg of 480 at the up transformer and 1 leg of 240 at the down. If I did that with a gec and bond at each transformer then my egc would create multiple ground paths.

I would prefer to go ungrounded between transformers but don't think it would be legal.

Any suggestions appreciated
 
If it is a 277/480 grounded wye sytem as your primary, you will have a dead short when you ground the 480 volt side. It is OK to ground one side of the 240 volts though. I prefer though to ground the center tap of the 240 volt side if available, giving me 120/240 on the other end. (assuming you are using single phase)
 
hillbilly1 said:
If it is a 277/480 grounded wye sytem as your primary, you will have a dead short when you ground the 480 volt side.
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Hillbilly, the 480 volt side is the secondary not the primary. He's trying to make a 480 volt, 2-wire circuit with one conductor grounded.
 
wickhdm said:
. . . then my egc would create multiple ground paths.

I would prefer to go ungrounded between transformers but don't think it would be legal.
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I don't believe you would have a problem if you grounded to the EGC. Also, you can have an ungrounded system but you would also have to install ground detectors. :smile:
 
wickhdm said:
It seems to me that to make these grounded systems I would have to ground 1 leg of 480 at the up transformer and 1 leg of 240 at the down. If I did that with a gec and bond at each transformer then my egc would create multiple ground paths.
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Standard transformers have 'galvanic isolation' between primary and secondary. This means that there is no electrical connection between primary and secondary. When you add the ground bond you are not creating a closed circuit, and thus not creating the sort of parallel path to ground that we are normally concerned about.

Many common single phase transformers have dual primaries and dual secondaries, which can be arranged in series or parallel to give two different voltages. This means that you can easily access a center tap terminal for grounding. You are probably better off using center tapped runs (240V to ground for the 480V stretch, 120V to ground for the 240V load).

You might be able to do this using transformers in an 'autotransformer' configuration, which will reduce the KVA ratings required of the transformers. A 100KVA 240 to 480 boost can be supplied using a 50KVA 240 to 240 transformer, wired in an autotransformer configuration. I don't know enough about the legalities of such an installation to offer better advice than 'consider it'.

Remember that transformers have impedance and regulation; the voltage drops built into the transformer itself. This impedance is primarily reactive, meaning that it has a greater effect on low power factor loads. When calculating the voltage drop of your installation, include these factors, not just the voltage drop in the wire.

-Jon
 
infinity said:
Hillbilly, the 480 volt side is the secondary not the primary. He's trying to make a 480 volt, 2-wire circuit with one conductor grounded.
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I missed that, he's stepping up, then back down, that's an interesting scenario. Would you want to ground the 480 at both ends or just at one end.
 
I think that article 240.4(F) would apply in this case. The primary ocpd would protect the secondary conductors. The transformer would be protected if kept at 125%.

Rick
 
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