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#94793 - 08/14/05 01:49 PM A really long mainfeeder
Rhino Offline
Member
Registered: 06/28/04
Posts: 24
Loc: San Diego,California,USA
I'm up against a situation where a client needs to get 33Kw,120/240 that is roughly 900'away from his house to a new single phase service from a meterpole.Because of voltage drop considerations i am considering the installation of 2 transformers; 1 at the beginning and 1 at end of the line.Going first from 240-480at the meterpole and then from 480-120/240 at the house.Are there any code related problems with this scenario?Thanks in advance for any input you might have
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#94794 - 08/14/05 01:55 PM Re: A really long mainfeeder
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
Sounds like a good plan and you may find you can go to 600 volts for the same money to further reduce the wire size.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#94795 - 08/14/05 02:05 PM Re: A really long mainfeeder
Rhino Offline
Member
Registered: 06/28/04
Posts: 24
Loc: San Diego,California,USA
Thanks iwire,i'll look into the possibility of 600Volt run even so i have to admit that 600Volt on a residential property kinda scare me
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#94796 - 08/14/05 02:10 PM Re: A really long mainfeeder
Bob Offline
Member
Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
VD using 500 kcm is 3% or 7 volts L to L.
Check using 2 37.7 kva 240/7200 volt using
#2 URD cable. Probably be much cheaper.No reason to be afraid. Installed correctly it is safe.

[This message has been edited by Bob (edited 08-14-2005).]

[This message has been edited by Bob (edited 08-14-2005).]
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#94797 - 08/14/05 02:17 PM Re: A really long mainfeeder
Rhino Offline
Member
Registered: 06/28/04
Posts: 24
Loc: San Diego,California,USA
Wow
Talk about upping the ante!
Just for the purpose of study i'll check into this scenario as well-not sure what the county inspection department would think of such an installation,even if properly done!!
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#94798 - 08/14/05 02:34 PM Re: A really long mainfeeder
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
The only problem I see with going above 600 volts is in the terminating of the underground cables.

Only a small percentage of electricians know the procedures or products needed above 600 volts.

I would not worry about the safety aspects, the power company regularly runs 13.8 KV under ground to pad mount transformers on peoples front yards. Installed correctly it is safe.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#94799 - 08/14/05 02:36 PM Re: A really long mainfeeder
winnie Offline
Member
Registered: 09/15/03
Posts: 649
Loc: boston, ma
Stepping up 240V to 480 or 600V, running 900 feet, and then stepping down to 240V strikes me as a very bad idea.

What you save on voltage drop in the conductors, you lose on the impedance of the two additional transformers in the circuit. Say you size the transformers right to load, and they each have a 2.5% impedance. This would be a 5% voltage drop without even considering the voltage drop in the service (feeder) conductors. Once you factor in the losses associated with oversizing the transformers, and the cost of the transformers, simply running fat conductors starts looking much more attractive.

Unless I totally misunderstand transformer impedance (won't be the first time).

Seems to me that the 'right' way to do this is to bring the primary distribution voltage underground onto the property, and place the transformer near the house as Bob (not iwire, who is also a Bob) suggests. Not taking 240V and stepping up to 7200V, but simply starting with whatever the local distribution voltage is, and stepping that down to 240V near the point of load.

Or simply design for the start for the voltage drop, eg. dividing loads up onto separate feeders, one with large allowed voltage drop, one with small allowed voltage drop.

-Jon
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#94800 - 08/14/05 02:48 PM Re: A really long mainfeeder
Rhino Offline
Member
Registered: 06/28/04
Posts: 24
Loc: San Diego,California,USA
It seems that if the power companies all over the world transport energy this way it should really not be that bad an idea.
I have not got the data to know how long it would take my client to recover the savings in energy losses through transformers as opposed to energy losses and added cost of conductors to be used on a 240volt system.This would be good for a really long calculation
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#94801 - 08/14/05 03:10 PM Re: A really long mainfeeder
Rhino Offline
Member
Registered: 06/28/04
Posts: 24
Loc: San Diego,California,USA
I'm getting really tempted at doing the 7200V method.
One of my contacts here does the Hi Voltage splicing here and knows all the ins and outs of making it work safely.
Problem remains to see if the AHJ would go along with this
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#94802 - 08/14/05 03:24 PM Re: A really long mainfeeder
winnie Offline
Member
Registered: 09/15/03
Posts: 649
Loc: boston, ma
Fair enough. Not a bad idea, but one which I belive, when you do the necessary calculations, will prove to be more expensive, and less effective than simply running oversized conductors.

You are quite correct, power companies do this all the time; it is the standard method of moving power around. But this is not without a price, the power companies are always trading off one factor for another to find the most effective (usually cheapest) method of delivering the necessary power.

I would really appreciate some teaching on all of the issues that would go into this calculation. I can make guesses, but it is over my head in terms of experience.

I know transformers have impedance, and that this will result in the secondary voltage dropping as load increases, so the transformers themselves introduce 'voltage drop'. But transformers have taps, so that you can set the no load voltage to be a touch higher, giving the target voltage under load. So with a transformer set you can to some extent 'zero out' your voltage drop. You will still see a dynamic change in voltage with load, but you've now placed that range of voltages _around_ your target voltage, rather than simply below it. I also don't know what sort of impedance to expect for single phase transformers of this size.

Transformer impedance is different from resistive voltage drop because inductive impedance doesn't actually consume real power...so if you are consuming 30kVA with a 5% inductive 'voltage drop', your power costs will be less than if you were consuming 30kVA with a 5% resistive voltage drop. But on the other side of this energy savings a totally unloaded transformer will still consume some power. This means that even when you client turns off _everything_, they will still be consuming (and paying for) electricity. Figuring out the overall energy use over time will not be a trivial calculation.

Curious to see what the 'optimal' design here is.

-Jon
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