Grounding conductor needed in boost/buck system?

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PetrosA

PetrosA

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Lancaster, Pennsylvania
I'm planning a job for a customer where they need power for a fountain approximately 1200 ft from the house. There is an existing 1.5-inch PVC conduit running there with pole light circuits in it, which we plan to repull as we pull in new conductors for the fountain.

The plan is to boost from 240V to 480V with a 7.5 kVa transformer at the house, then run two #4s THHN to a second transformer at the pond, buck it back to 120/240V and feed a small panel there to feed the control box.

My question is: Do I need to run a #4 ground with the two hots to the second transformer? I've been reading through 250.30 et al and can't figure out what exactly or how it applies to what I'm doing. Any help would be great! Thanks!
 
The way I 'm reading your setup you have a branch circuit feeding the transformer in PVC, that branch circuit will require an EGC run along with the circuit conductors.
 
PetrosA said:
I'm planning a job for a customer where they need power for a fountain approximately 1200 ft from the house. There is an existing 1.5-inch PVC conduit running there with pole light circuits in it, which we plan to repull as we pull in new conductors for the fountain.

The plan is to boost from 240V to 480V with a 7.5 kVa transformer at the house, then run two #4s THHN to a second transformer at the pond, buck it back to 120/240V and feed a small panel there to feed the control box.

My question is: Do I need to run a #4 ground with the two hots to the second transformer? I've been reading through 250.30 et al and can't figure out what exactly or how it applies to what I'm doing. Any help would be great! Thanks!
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Are you really going to 'boost' and 'buck' here, or use isolation transformers? I'm not an expert on this but but boost/buck is a particular type of transformer and I don't think it is likely to be what you want to go from 240 to 480 and back again.

Your answer to whether your transformers are isolation transformers significantly affects questions of grounding and bonding.
 
Yes. In terms of grounding the 2nd transformer is no different from an appliance or motor.
The transformer case would need an equipment ground from the source voltage,
 
infinity said:
The way I 'm reading your setup you have a branch circuit feeding the transformer in PVC, that branch circuit will require an EGC run along with the circuit conductors.
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Yes, the first transformer will be in the basement of the house and the secondaries from that will run in a PVC out the driveway 1200 feet to the second transformer. I'm trying to wrap my head around what the EGC would be able to do with the secondary. Would a dead short from secondary to EGC have low enough impedance to trip the breaker feeding the first transformer?
 
jaggedben said:
Are you really going to 'boost' and 'buck' here, or use isolation transformers? I'm not an expert on this but but boost/buck is a particular type of transformer and I don't think it is likely to be what you want to go from 240 to 480 and back again.

Your answer to whether your transformers are isolation transformers significantly affects questions of grounding and bonding.
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You're right. I had my terminology confused. We'll be stepping up at the house and stepping down at the pond using an isolation transformer. I haven't dealt with this setup in decades, so sorry for the confusion.
 
PetrosA said:
My question is: Do I need to run a #4 ground with the two hots to the second transformer? I've been reading through 250.30 et al and can't figure out what exactly or how it applies to what I'm doing. Any help would be great! Thanks!
Click to expand...


250.122 (B)
Table 250.122
 
PetrosA said:
Yes, the first transformer will be in the basement of the house and the secondaries from that will run in a PVC out the driveway 1200 feet to the second transformer. I'm trying to wrap my head around what the EGC would be able to do with the secondary.
Click to expand...

The EGC is to provide a low impedance path for a ground fault to the transformer enclosure on the primary side of the 480V to 120/240V step down transformer to return to the source. How are you planing on configuring the secondary of the step up transformer at the house? Ground one leg of the 480V making it a grounded power system?
 
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Jamesco said:
The EGC is to provide a low impedance path for a ground fault to the transformer enclosure on the primary side of the 480V to 120/240V step down transformer to return to the source. How are you planing on configuring the secondary of the step up transformer at the house? Ground one leg of the 480V making it a grounded power system?
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The EGC provides a low impedance path for ground fault current to return to the source.
 
Some guys here have mentioned an allowance for no separate EGC if one of the circuit conductors is grounded.
 
LarryFine said:
Some guys here have mentioned an allowance for no separate EGC if one of the circuit conductors is grounded.
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I'm pretty sure that's not the case here.
zbang said:
I think it should for a properly-sized EGC should be able to trip the breaker. How are you planning to bond the secondary of the 240->480 transformer (since it's an SDS)?
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The secondary side of the transformer is two hots at 480V in this configuration, which I think also allows me to not fuse the secondary wiring (OCP for the primary side will be 30A, secondary conductors are #4, so the 30A breaker should be good). At the other end, where it gets stepped down, there's the option to tap the center winding and ground it, creating a neutral.

 
LarryFine said:
Some guys here have mentioned an allowance for no separate EGC if one of the circuit conductors is grounded.
Click to expand...
I didn't see it mentioned in this thread, bit the way to do this is use 250.30(A)(1) exception 2 if possible. So you just have a hot and a grounded conductor that is "rebonded" like we used to be able to do to out buildings. Although to use the exception, it appears the step up must be located outside.
 
Your wire sizes don't make sense to me. A 7.5 KVA transformer would be around 30 amps on the 240V side and 15A on the 480V side. Why the #4 wire? Depending on the true load, #6 or maybe even #8 would be adequate on the 480V side for a 1200' run. The equipment ground on the primary side would be #10. The ground on the 480V side would be the same size as the ungrounded conductors.

If you're mixing 480 and 120V in the same conduit run, I'd pick different colors for the 480 (suggest yellow and brown or at least tape them these colors). Multiple circuits can use a common ground, it just needs to be sized for the largest requirement of any one circuit in that conduit. Or why not just run the lights of this new 120/240-480 circuit since it sounds like you're putting a panel there anyway?
 
I dont know what the actual load is to try and make a judgement, but don't forget to consider transformer cost and losses in your analysis. You will have two transformers with load and no load losses potentially 24 hours a day (depending of course on when this runs and the control scheme if it's not continuous). Two transformers will themselves have non insignificant voltage drop. Sometimes a step up step down doesn't pay for itself. Also for a fixed load like a fountain, perhaps just using a boost transformer if the voltage is too low would be more cost effective.
 
suemarkp said:
Your wire sizes don't make sense to me. A 7.5 KVA transformer would be around 30 amps on the 240V side and 15A on the 480V side. Why the #4 wire? Depending on the true load, #6 or maybe even #8 would be adequate on the 480V side for a 1200' run. The equipment ground on the primary side would be #10. The ground on the 480V side would be the same size as the ungrounded conductors.

If you're mixing 480 and 120V in the same conduit run, I'd pick different colors for the 480 (suggest yellow and brown or at least tape them these colors). Multiple circuits can use a common ground, it just needs to be sized for the largest requirement of any one circuit in that conduit. Or why not just run the lights of this new 120/240-480 circuit since it sounds like you're putting a panel there anyway?
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Every calculator I've used shows that I need a #4 to maintain a max 3% drop. I need 230V at the far end, and #4 gets me to 465V stepped down to 232V. We'll definitely be using different colors for the 480 circuit. The lights are fed via a control box for the fountain, so I think that's internally coming off of one of the legs. If not, we'll have a 6 circuit disconnect there to pull a 120V circuit off of for the 40-50 watts of light it needs.
 
zbang said:
Right, but from where I sit, you still need to ground/bond the 480v secondary; otherwise you have an ungrounded 480v feed.
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So if I understand correctly, I need to run a bonding jumper from the center tap at the source transformer to ground, correct? Do I need a separate ground rod for this size transformer the same as I'd need for a big one? Seems a little strange... Obviously, there will be a rod installed at the far end when we step it back down.
 
PetrosA said:
Every calculator I've used shows that I need a #4 to maintain a max 3% drop. I need 230V at the far end, and #4 gets me to 465V stepped down to 232V. We'll definitely be using different colors for the 480 circuit. The lights are fed via a control box for the fountain, so I think that's internally coming off of one of the legs. If not, we'll have a 6 circuit disconnect there to pull a 120V circuit off of for the 40-50 watts of light it needs.
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How big is this pump? I get only 4.3% drop for a 240v 15A load (no transformers) using #4, and that's a huge pump.
 
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