Upsizing / Downsizing

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onelectrishun

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Please excuse my ignorance on this issue. I probably should know what the answer is from my apprenticeship days back in the 70?s?

We?re feeding a 1Ph, 120/240V, 200A sub panel from a source 1500 ft. away. The wire has to be upsized to 1250 kcmil conductors to compensate for voltage drop over that distance. (I did some quick voltage drop calcs on different calculators on line and came up with varying results, so let?s just assume the 1250 kcmil is correct for now. Let?s say that we set a junction box every 250? along this run.

So?could we run 1250 kcmil for the first 250 ft., a smaller conductor, say 900 kcmil for the next 250 ft., 750 kcmil for the 3rd 250? and so on. The smallest conductor(s) would be 4/0?s for the sake of discussion.

Depending on the answer to my quandary this brings up other questions like:
If so, how do you figure the size of wire at each segment of the run?
If not, how does the above example differ from a distribution system?say you run from a 1200A DB to a 600A DB with 2-350?s, then from the 600A DB to a 400A DB with 2-3/0?s, then from the 400A DB to a 200A PB with a 3/0, etc. Assume no other loads on any of these panels. Isn?t that exactly the same thing in theory?
 
Re: Upsizing / Downsizing

I don't think you can make this work the way you want.

You are trying to spread the VD out in an unequal manner. That is not a problem by itself. What you calculated was that 1250 would give you a VD of 0.33% for each of the six 250 foot sections of the run. You can't get past that 0.33% average value, and still get a total of 2% VD for the entire run. If there is more VD in one section, then there must be less VD in another section. Therefore, if you use smaller cable towards the end, then you will need conductors even larger than your calculated 1250 MCM at the beginning. That is not what you are trying to achieve.

The process you are trying to use would work only if there are loads that branch off at each junction point. That is, if the entire 200 amps runs in the first 250 feet, but at that point you have a 40 amp load, then only 160 amps will travel through the second leg. If at the second junction box you have another 40 amp load connected, then only 120 amps will travel on towards the next box. But in your case, the entire 200 amps will travel the entire 1500 feet.
 
Re: Upsizing / Downsizing

Originally posted by onelectrishun: How does the above example differ from a distribution system?say you run from a 1200A DB to a 600A DB with 2-350?s, then from the 600A DB to a 400A DB with 2-3/0?s, then from the 400A DB to a 200A PB with a 3/0, etc. Assume no other loads on any of these panels. Isn?t that exactly the same thing in theory?
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The difference is that you are not sending the entire 1200 amps all the way to the 200 amp panel at the end of the line. Therefore, you can use smaller conductors for each leg of the journey.
 
Re: Upsizing / Downsizing

What about a transformer on each end or use a higher voltage supply if available and then a transformer to drop it to 120/240.
 
Re: Upsizing / Downsizing

Yeah, I thought about that, but my quess is that by the time you add up all the costs of installing the transformer(s) you'd be up to the same cost as installing the larger wire.

Charlie,
Is my assumption right that all of this has to do with VD correction which is not mandatory as long as the volage serving the load is within the manufacturer's parameters?

Thanks for your replies.
 
Re: Upsizing / Downsizing

Originally posted by onelectrishun:Is my assumption right that all of this has to do with VD correction which is not mandatory as long as the voltage serving the load is within the manufacturer's parameters?
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I've never seen the phrase "VD correction" used before. This isn't about correcting or compensating, in the sense that we say "power factor correction." It's more about prevention. When you have long run, the only reason you use larger wires than are needed for ampacity is to prevent the voltage from getting too low at the far end. And you are right that voltage drop considerations are not enforceable. And you are also right that if you have enough voltage to satisfy the manufacturer's requirements, then you have an acceptable installation.
 
Re: Upsizing / Downsizing

We did an install where the load was 1/2 mile from the source. Source voltage is 208, 1ph. The load voltage is 120. Everything works great. The voltage reading is about 160V at the disconnect by the load. From the disconnect we installed a constant voltage transformer made by SOLA to get our 120V.
Our load fluctuates between 5A, daytime to 24Amp night time. It is for lighting.
Of course the transformer is operating full time, irregardless of the load state.
 
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