Single Phase step up 240 to 480 to feed step down 480 to 240 xfrm Bonding?

Status
Not open for further replies.
O

onewireatatime

Member
Location
California
Occupation
Electrician
Hello, I have a residence that is being supplied 120/240 single phase. The actual home will be located 1,100' away from the meter, so we will be using a step up transformer from 240 to 480 and then step back down to from 480 to 120/240 to feed the home. My question is, will i be needing to bond the secondary of the step up transformer that feeds the step down? i will not be using a neutral to feed the step down. I'm concerned of not having a ground fault path in that raceway.

Also, the supply house offered a 240/480 - 120/240 and the manufacture said this type of transformer can be wired backwards to become a step up, if that makes any sense. not sure if any of you guys have done anything similar to this. The Transformer is a ACME TP530233S
 
Welcome to the forum.

First, the higher the voltage you transmit, the less you'll spend on conductor and losses, like 600v or more.

Second, there is a provision in the NEC that allows a two-wire feeder with one grounded line conductor.

Third, yes, a transformer should be designated by the manufacturer to be used in reverse of its normal use.
 
LarryFine said:
Welcome to the forum.

First, the higher the voltage you transmit, the less you'll spend on conductor and losses, like 600v or more.

Second, there is a provision in the NEC that allows a two-wire feeder with one grounded line conductor.

Third, yes, a transformer should be designated by the manufacturer to be used in reverse of its normal use.
Click to expand...
Hello LarryFine, Thank you for your response.
I was planning on running Triplex USE cable direct burial for my 1,100' run. Center tap Bonding the secondary side of the step up xfrm which would give me 240v to ground on each leg and terminating the EGC at the Step down xfrm case.
What you mentioned sounds cheaper due to less wire, I've tried looking for that provision and cant seem to find it. would you mind directing me, please.
Thank you.
 
Do NOT do a step up step down for only 1100 feet. Just put in bigger wire. Please. Run a set of 500 Ryder URD and be done with it.
Make sure to calculate in transfer losses forever and the poor voltage regulation of having three transformers (POCO, step up, step down....each transformer will lose about three volts under load in my experience). Trust me, just run bigger wire.

If you are looking for that exception that allows the grounded conductor of an SDS to also be the EGC, it's 250.30(A)(1) exception #2.
 
I tend to agree with electrofelon, but the transformer you are looking at is rated 167kVA. Some 700A at 240A.

Any residence that needs a service that large has money to look at options.

Before hacking a step up/ step down, or burying huge conductors, check with the power company about running primary to a padmount at the house.

Jon
 
electrofelon said:
Do NOT do a step up step down for only 1100 feet. Just put in bigger wire. Please. Run a set of 500 Ryder URD and be done with it.
Make sure to calculate in transfer losses forever and the poor voltage regulation of having three transformers (POCO, step up, step down....each transformer will lose about three volts under load in my experience). Trust me, just run bigger wire.

If you are looking for that exception that allows the grounded conductor of an SDS to also be the EGC, it's 250.30(A)(1) exception #2.
Click to expand...
Better yet try to get POCO to supply you with 480 volt service, then you only need one transformer or even bring primary line and POCO transformer to a closer location. I know that sometimes is impossible with some POCO's though. They will do it here, and at a fair enough cost.
 
Well i feel that i should of described the whole job...
An electrical engineer drew up a single line diagram (which im going to try to post here shortly) Having 400amp 1p 120/240 meter/Main to a 150KVA step up 240v to 480v, to feed two step down transformers. The first step down is 25KVA would feed a property 440' away to 100amp panel.
the second step up transformer, a 75KVA and would feed 1 200amp panel and one 100A panel for a Well.
At first glance i saw some errors on the 150KVA Transformer.
The way i was planning on doing this is the following: Tap off the 1 single 400 amp meter/main to feed the 440' panel with 4/0 direct burial wire. the second tap would feed a 75KVA step up xfrm to feed a secondary step down xfrm 75KVA (At 1,100' away with direct burial #350 AL triplex ). the step down will feed the 200 and 100 panels.
To eliminate all transformers im looking at 1250 kcmil in wire for my 1,100' run. i believe that will put me at around 4.5% VD.
 
kwired said:
Better yet try to get POCO to supply you with 480 volt service, then you only need one transformer or even bring primary line and POCO transformer to a closer location. I know that sometimes is impossible with some POCO's though. They will do it here, and at a fair enough cost.
Click to expand...
The general is telling me Edison (utility) is only willing to supply 120/240 400amp
 
electrofelon said:
Do NOT do a step up step down for only 1100 feet. Just put in bigger wire. Please. Run a set of 500 Ryder URD and be done with it.
Make sure to calculate in transfer losses forever and the poor voltage regulation of having three transformers (POCO, step up, step down....each transformer will lose about three volts under load in my experience). Trust me, just run bigger wire.

If you are looking for that exception that allows the grounded conductor of an SDS to also be the EGC, it's 250.30(A)(1) exception #2.
Click to expand...
That something i honestly did not take into consideration. I would say there would be a cost on just having the transformers connected and live correct?
 
onewireatatime said:
That something i honestly did not take into consideration. I would say there would be a cost on just having the transformers connected and live correct?
Click to expand...
Yes transformers have losses. I would price it both ways and see which is more economical. I think that EF may be on to something. You'll still have to trench and lay in the cable regardless of what size conductors are used so going with larger conductors might still save you money.
 
infinity said:
Yes transformers have losses. I would price it both ways and see which is more economical. I think that EF may be on to something. You'll still have to trench and lay in the cable regardless of what size conductors are used so going with larger conductors might still save you money.
Click to expand...
Yeah DOE 2016 transformers will be around .35% no load losses

Granted I don't know details of this project but the transformers seem way oversized.

To get an accurate comparison you need a realistic load number, which is going to be easily half or more of what the NEC says. Then when you look at the no transformer option, give yourself a free extra 6-8 volts because that's what the voltage drop thru the transformers will be.
 
400A at 240V is less than 100 kVA.

It is possible that the larger than needed transformer was selected to decrease the transformer impedance.

Transformers have a built in 'voltage drop' caused by the resistance and leakage inductance of the transformer coils. Transformer impedance is _rated_ as a percentage relative to full load current. This means that a 25kVA 5% impedance transformer has 2x the impedance of a 50kVA 5% impedance transformer.

This certainly has a trade-off vs price; using a 167kVA transformer at 96kVA means you pay a big price premium. There is probably also a loss trade-off, likely higher no-load losses but lower load dependent losses (but I don't know enough about transformers to do more than guess on that point.)

-Jon
 
Status
Not open for further replies.
Top