 SwedishRiderRider on the StormPremium
join:2006-01-11
Connecticut
kudos:1 | Voltage Drop Tolerances Anyone know what's an acceptable amount of voltage drop in a portable generator installation?
Here's my calculation info: »www.nooutage.com/vdrop.htm
240V
10 awg wire
30 amps
AC single phase 60hz
copper conductor
75C operating temperature
no conduit
single set of conductors
~80 feet cable distance one way (about 30' from generator to inlet, and about 50' from inlet to main panel).
That yields about a 2.4% voltage drop. And all the critical services I'd want to run (furnace, well pump, water heater) are located in the same small utility room as the main panel. |
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BobAccount deleted
join:2012-07-22
New Jersey | Less than 3 volts out of 120? Seems fine to me. But let nunya be the authoritative source... |
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SwedishRiderRider on the StormPremium
join:2006-01-11
Connecticut
kudos:1
1 edit | said by Bob:Less than 3 volts out of 120? Seems fine to me. But let nunya be the authoritative source...
It seems to be within specs... but just wanted to confirm that with the gang here. I've test run my setup a few times, and everything seemed to run fine.
I would have made the run from generator to panel shorter, but since I wanted the availability of propane, I had to run my inlet to the opposite side of the house from the main panel... but even still, total run isn't out of tolerance as far as I can tell.
EDIT: I ran that website calculator again, and used all the same specs as before except I played with the one-way wire distance number. To go over 5% voltage drop, I would have to go over 168'. I realize that number is a calculator estimation and might not match up exactly with real-world application, but it appears that there is still a significant amount of headroom given my current wire distances. |
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| reply to SwedishRider
Where are you getting 30A from?
And what do you mean by "single set of conductors"??? |
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SwedishRiderRider on the StormPremium
join:2006-01-11
Connecticut
kudos:1 | said by Speedy Petey:Where are you getting 30A from?
And what do you mean by "single set of conductors"???
I have a 30A breaker backfed (with an interlock) for the generator, which is 30A, 120V/240V. Wire is 10 gauge. I'm assuming maximum capacity of load on that wire.
Single set of conductors was one of the choices in that calculator... click the link I posted and check it out. I think my situation is "single set of conductors". Something has to be selected in that spot for the calculator to work. |
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| said by SwedishRider:Wire is 10 gauge. I'm assuming maximum capacity of load on that wire. Not at all. What is the output of the genny?
said by SwedishRider:Single set of conductors was one of the choices in that calculator... click the link I posted and check it out. I think my situation is "single set of conductors". Something has to be selected in that spot for the calculator to work.
You should have used "raceway cable or earth" for the calculation.
What kind of wire did you run? |
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 nunyaWho is John Galt?Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:8
 ·Charter
 ·surpasshosting
 ·voip.ms
| reply to SwedishRider
That only get's you VD to the panel. There's more VD in the circuits that are distributed from the panel.
What is the output voltage of your generator under load?
Ideally the maximum VD for feeder and branch should be less than 5%.
With the generator being that far from the house, and the VD of your branch circuits being an unknown and variable (each will be different), I would consider an upsize to 8 AWG. The price difference would be minimal. If you've already run #10, I wouldn't sweat it.
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If someone refers to herself / himself as a "guru", they probably aren't. |
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SwedishRiderRider on the StormPremium
join:2006-01-11
Connecticut
kudos:1 | reply to Speedy Petey
said by Speedy Petey:Not at all. What is the output of the genny?
Generator is 6000 running watts, 7000 surge. I was told that I could use a different generator up to 7500 watts given my setup (30A breaker with 10 gauge wire).
You should have used "raceway cable or earth" for the calculation.
What kind of wire did you run?
-single set of conductors
-2 conductors per phase in parallel
-3 conductors per phase in parallel
-4 conductors per phase in parallel
And they give this note: Examples for parallel runs: 120/240V single phase system with single black-red-white conductors (installed in single conduit) select "single set of conductors", 120/208V 3phase system with 2 conductors per phase and neutral (installed in 2 parallel conduits) select "2 conductors per phase in parallel", dc system with 3 positive and 3 negative conductors select "3 conductors per phase in parallel". |
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SwedishRiderRider on the StormPremium
join:2006-01-11
Connecticut
kudos:1 | reply to nunya
said by nunya:That only get's you VD to the panel. There's more VD in the circuits that are distributed from the panel.
What is the output voltage of your generator under load?
I don't know what it is under load. I have an inexpensive meter and it was saying the generator was pushing 125 volts with no load at the 120v outlets on the genny. I hooked it to my house and while it had a few items running, an outlet in the utility room was measuring 120ish volts. The run from the panel to that outlet was short... but pretty much all the critical stuff I'd need to run during an outage is in that room.
Ideally the maximum VD for feeder and branch should be less than 5%.
With the generator being that far from the house, and the VD of your branch circuits being an unknown and variable (each will be different), I would consider an upsize to 8 AWG. The price difference would be minimal. If you've already run #10, I wouldn't sweat it.
EDIT: added wire pics. |
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nunyaWho is John Galt?Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:8 Reviews:
·Charter
·surpasshosting
·voip.ms
| What's the difference in price for 8/4 SO vs 10/4 SO? I'm showing 8/4 @ $2.60 / ft, and 10/4 @ $1.60 / ft. So it would be about an extra dollar per foot.
A Leviton 2711 L14-30P will accept #8.
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If someone refers to herself / himself as a "guru", they probably aren't. |
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SwedishRiderRider on the StormPremium
join:2006-01-11
Connecticut
kudos:1 | said by nunya:What's the difference in price for 8/4 SO vs 10/4 SO? I'm showing 8/4 @ $2.60 / ft, and 10/4 @ $1.60 / ft. So it would be about an extra dollar per foot.
A Leviton 2711 L14-30P will accept #8.
I'll have to check with the electrician that I do business with... I don't know local pricing.
At your pricing, is it worth the $30 extra bucks? Or is this chasing a problem that doesn't exist? |
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nunyaWho is John Galt?Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:8 | If you are going to make a new cord anyway, I would upsize it. It's $30 bucks.
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If someone refers to herself / himself as a "guru", they probably aren't. |
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SwedishRiderRider on the StormPremium
join:2006-01-11
Connecticut
kudos:1 | said by nunya:If you are going to make a new cord anyway, I would upsize it. It's $30 bucks.
Sounds like a plan. 
Just out of curiosity, is there a practical way to measure the actual voltage drop of my system? I have a Greenlee AM-6 Multimeter. |
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 alkizmo
join:2007-06-25
Pierrefonds, QC
kudos:1 | said by SwedishRider:Sounds like a plan.
Just out of curiosity, is there a practical way to measure the actual voltage drop of my system? I have a Greenlee AM-6 Multimeter.
Good luck.
You'd need to measure before its under load, then put it at max load (30A) to see the maximum drop.
As Nunya said, if it's too late, don't sweat it.
BTW Nunya, I see a lot of people talk about their 10/4 SO extension cord on 30A receptacles. Aren't 10/4 SO's rated for 25A? What's the dealio? |
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SwedishRiderRider on the StormPremium
join:2006-01-11
Connecticut
kudos:1 | said by alkizmo:Good luck.
You'd need to measure before its under load, then put it at max load (30A) to see the maximum drop.
As Nunya said, if it's too late, don't sweat it.
BTW Nunya, I see a lot of people talk about their 10/4 SO extension cord on 30A receptacles. Aren't 10/4 SO's rated for 25A? What's the dealio?
I'm more curious than anything else. I think the 10 gauge between the inlet and the panel is fine... and I'm not going through the hassle of ripping that out and going with bigger gauge. But as long as it's not a ridiculous amount, nunya is right, it makes sense to go with 8 gauge for the longer cord I need from generator to inlet. If I wasn't making up a new one, I wouldn't spend the money, but since I need a longer length anyway, why not.. |
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| reply to alkizmo
said by alkizmo:BTW Nunya, I see a lot of people talk about their 10/4 SO extension cord on 30A receptacles. Aren't 10/4 SO's rated for 25A? What's the dealio?
If I may.
10/4 cord (wh, bk, red, grn) is allowable to use Column B in T400.5(A)(1) if the white only carries the current imbalance, as in this situation.
Column B lists #10 @ 30A ampacity. |
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 Coma Thanks StevePremium
join:2001-12-30
NirvanaLand | reply to SwedishRider
said by SwedishRider:it makes sense to go with 8 gauge for the longer cord I need from generator to inlet. If I wasn't making up a new one, I wouldn't spend the money, but since I need a longer length anyway, why not..
I have 80' 8ga run to my shop where I keep the generator but it only outputs 30A so a 10ga suicide cord is fine.
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PSWired
join:2006-03-26
Annapolis, MD | reply to SwedishRider
Pull the cable off the inlet breaker back at the panel and short the hot and neutral together. Plug in the cord and measure the resistance between hot and neutral with your meter back at the end if the cordset. Use ohm's law (V=IR) to calculate voltage drop with the resistance you measured (in ohms) and your worst case current (30 A).
Edit: 240v circuit, short the two hots, ignore the neutral. |
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| reply to SwedishRider
The issue isn't the voltage drop when running normally, the issue is the voltage drop when a power-hungry motor starts.
I ran #8 for my setup, 60ft. |
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 leiboldPremium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:6
 ·SONIC.NET
| reply to alkizmo
said by alkizmo:said by SwedishRider:Just out of curiosity, is there a practical way to measure the actual voltage drop of my system? You'd need to measure before its under load, then put it at max load (30A) to see the maximum drop. Actually the idle voltage of the generator is not a useful number. To measure the voltage drop you need to measure the voltage while the generator is under load. The first measurement is done at the generator and the second measurement is done near the biggest load. The difference between the two measurements is the voltage drop.
The resistance measurement suggested by PSWired  is technically correct but in real life faces the problem that most multimeters aren't very precise for low ohm readings and even a small difference in value has a large effect on the resulting calculation.
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