Voltage drop

[stretch12]

I’m trying to figure voltage drop on a resi service 120/240 single phase . Distance is approximately 2k ft from xformer to residence. Figuring on al conductors, the south wire app is showing ridiculous amount of parallel conduits. I haven’t done one of these calculations in like 15 yrs. And am having the largest brainfart of my life trying to get started.
Any assistance or insight, very much appreciated.

Chris

 

[infinity]

What is the current value that you're using for the calculation?

 

[stretch12]

320 for a 400 a service , so worst case senario

 

[Tulsa Electrician]

2 k ( thousand) is a long distance. Did you mean 200 feet

 

[infinity]

320 for a 400 a service , so worst case senario

Wouldn't the worst case be 400 amps? Regardless of that answer for 2000' you're better off looking at adding a transformer at each end is that an option?

 

[electrofelon]

2000' is probably going to need a step up to MV. You could try just going to 600. I used to say dont mess around with 600, just go to MV, but with the cost of CN cable and MV transformers doubling in the last 6 years, sometimes there is a sweet spot for 600V. For budgeting, T&R has 25 kva pad mounts for $6500, and #2 AL CN primary is about $3.75 last time I priced it (you inly need one cable for single phase). Dont forget 2 transformer pads, prolly $800 each

 

[stretch12]

2 k ( thousand) is a long distance. Did you mean 200 feet

No 2000ft utility cost to bring primary closer is stupid $ so I have been told

 

[wwhitney]

T&R has 25 kva pad mounts for $6500

If 25 kVA is a suitable transformer size, which is only 104A @ 240V, wouldn't the comparable voltage drop calculation be based only on 104A? E.g. 6 sets of 250 kcmil Al per Southwire.

Cheers, Wayne

 

[infinity]

You could try just going to 600

I ran the numbers with the Southwire VD calc and it said 3 sets of 750 kcmil aluminum for 3% VD. Switch to copper and it's 3 sets of 500 kcmil.

 

[Deltaforce]

Given 2k f

No 2000ft utility cost to bring primary closer is stupid $ so I have been told

Utility primary extension cost vs parallel conductors 2k ft cost with power loss cost
Make a cost comparison

 

[electrofelon]

I ran the numbers with the Southwire VD calc and it said 3 sets of 750 kcmil aluminum for 3% VD. Switch to copper and it's 3 sets of 500 kcmil.

If 25 kVA is a suitable transformer size, which is only 104A @ 240V, wouldn't the comparable voltage drop calculation be based only on 104A? E.g. 6 sets of 250 kcmil Al per Southwire.

Cheers, Wayne

Rob for 600v? I get one set of 3/0 AL for 18kw at 600V (75A at 240). Bump up to 250 and you get 100A under 3%.

I get $25,000 in conductor, 3 sets of 350 AL for 75 amps at 240.

 

[electrofelon]

If 25 kVA is a suitable transformer size, which is only 104A @ 240V, wouldn't the comparable voltage drop calculation be based only on 104A? E.g. 6 sets of 250 kcmil Al per Southwire.

Cheers, Wayne

In general i would use 100 amps or less for a VD calc for a dwelling. A 25kva transformer with around 75A load for VD purposes is usually about right.

 

[infinity]

Rob for 600v? I get one set of 3/0 AL for 18kw at 600V (75A at 240). Bump up to 250 and you get 100A under 3%.

I get $25,000 in conductor, 3 sets of 350 AL for 75 amps at 240.

Let me go back and check. I might have kept the 320 amp current value.

 

[solarken]

320 for a 400 a service , so worst case senario

Are you sure 200A service is not sufficient? Is this a new home build? If the load calc was conservative, it might be blowing up the cost unnecessarily.

 

[electrofelon]

320 for a 400 a service , so worst case senario

Sizing something like this to worst case scenario is a good way to flush tens of thousands of dollars down the toilet. Besides 320 amps isn't just worst case it's ridiculous and never going to happen.

 

[electrofelon]

Let me go back and check. I might have kept the 320 amp current value.

At first I missed where he said 320A, so my figures were using my typical value of 75 to 100 amps.

 

[Deltaforce]

Besides 320 amps isn't just worst case it's ridiculous and never going to happen.

I bet utility never allow such long secondary service

 

[electrofelon]

I bet utility never allow such long secondary service

Typically they have a limit on the distance to the meter, but after that you can go as far as you want.

 

[stretch12]

I kinda have the feeling , that a service calc then work on vd. We are at the stage jus spitballing #s just know it’s gonna be a 400a se, it’s a big home.

 

[winnie]

Yes. You can't do a voltage drop calculation without first understanding the current that will be drawn.

But what you should do for your 'spitball' calculations is calculate solutions for 400A, 100A, 50A, and 25A, just to get a rough idea of costs. Also get the power company to price out bringing primary to a transformer near the house.

The different costs of these different service values will help you decide how to approach the problem.

Something else to keep in mind is that residential electrical service is very variable; you may have high peaks but a low average. Often the issue with voltage drop is that you don't want apparent effects (lights flickering, UPSs triggering) when some large load kicks in. So sometimes you are better off putting money into reducing the magnitude of large load inrush, for example by using inverter fed HVAC systems or well pumps.

Depending upon the utility tariffs, this might be one of the rare cases with off grid solar makes sense.