Volatge Drop

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laketime

laketime

Senior Member
Let me see if I am doing this right. I am using the MH Voltage drop calculator. I have a 20 amp circuit that will end up being 310' at its furthest point, after a couple other stops. The calculator (I think) is telling me it needs to be #8. Does that sound right? Would held if I spelled voltage right in the title!
 
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This is the voltage drop I am coming up with for the following conductor size. Hope this helps.

#12- 23.932v
#10- 15.033v
#8- 9.647v
#6- 6.088v
 
I am saying 20 amp circuit but the load on it at most is (2) receps and (2) 150 watt metal halide light poles. This is for a marina. The receps are right at the end of the gang way not specific to each slip. So the distance would be to the second light pole.
 
laketime said:
So I just read this:

http://forums.mikeholt.com/showthread.php/132806-Voltage-drop?highlight=voltage+drop

Since there are no "electrical drawings" and the only specs are the 150 watt metal halide light poles. What would be the minimum you would go for a run 300'? 200'?
100'?
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It would depend if you wanted to have extra capacity or not. Use this online calc. If you run a larger 4 wire cable you can add a sub panel for future use. If it were just the 2 150 watt fixtures certainly a #8 is enough however I would seriously consider larger conductors.

Also without a panel you would need to consider 250.122(B)
 
laketime said:
I am saying 20 amp circuit but the load on it at most is (2) receps and (2) 150 watt metal halide light poles. This is for a marina. The receps are right at the end of the gang way not specific to each slip. So the distance would be to the second light pole.
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Your possible load could be 5.5a, this includes a projected 180va per recept and only 150w per lamp (supposed to use ballast label load rather than lamp). If this load was at the end it would require #8 to remain 3% or less with VD but it's not at the end so you'd need to calc it progressively in increments and size each length accordingly. Code wise you can have hundreds of receptacles on a 20a circuit with #12 but to use some projected load is wise and for calculation purposes 180va is used so it's a good default for voltage drop calcs unless you know what will be plugged in. The supply voltage is your problem 120v is generally good for #12 up to 120?+/-, 240v is generally good for #12 up to 240?+/-; maybe a transformer for recepts can help?
 
tryinghard said:
Your possible load could be 5.5a, this includes a projected 180va per recept and only 150w per lamp (supposed to use ballast label load rather than lamp). If this load was at the end it would require #8 to remain 3% or less with VD but it's not at the end so you'd need to calc it progressively in increments and size each length accordingly. Code wise you can have hundreds of receptacles on a 20a circuit with #12 but to use some projected load is wise and for calculation purposes 180va is used so it's a good default for voltage drop calcs unless you know what will be plugged in. The supply voltage is your problem 120v is generally good for #12 up to 120?+/-, 240v is generally good for #12 up to 240?+/-; maybe a transformer for recepts can help?
Click to expand...

The supply is set where it will be so the feed will be 120v. Would it be safe to go down to #10's?
 
I am also thinking about #8's to the recep location and then 10's out to the lights. It would be about 200' to the receps and then 100' to the last light. What do you guys think?
 
laketime said:
The supply is set where it will be so the feed will be 120v. Would it be safe to go down to #10's?
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Maybe?
If you have a 3A load at 200' you'll have just about 2%VD
6A load at 200' you'll have around 2.5%VD
7A load at 200' you'll have around 2.9%VD
from the 200' out you'd only need 12's
 
Single phase
VD= 2.K.I.L %CM

K= omhs per 1000 foot ( Cooper =12.9)(Alluminum =21.2)

L= length of conductor in feet

I= Current in conductors ( Amps)

CM = Circular mil of the conductor
 
I am saying 20 amp circuit but the load on it at most is (2) receps and (2) 150 watt metal halide light poles.
Click to expand...

The problem I see is that you have no way of knowing what will be plugged into the receptacles. You have to figure your VD based on the current. Consider what may be plugged into the receptacles. Wet / dry vac, air compressor, refrigerator....pretty easy to make it close to 20 amps.

If you calculate VD on say, ten amps and the project is built, anything approaching 20 will cause a very noticeable voltage drop for everyone on that circuit. Also, be advised that the rating of the breaker is NOT the max it will hold before it trips. Loads connected for as long as a half hour can approach 30 amps on a 20 amp breaker before it trips.
 
K8MHZ said:
The problem I see is that you have no way of knowing what will be plugged into the receptacles. You have to figure your VD based on the current. Consider what may be plugged into the receptacles. Wet / dry vac, air compressor, refrigerator....pretty easy to make it close to 20 amps.

If you calculate VD on say, ten amps and the project is built, anything approaching 20 will cause a very noticeable voltage drop for everyone on that circuit. Also, be advised that the rating of the breaker is NOT the max it will hold before it trips. Loads connected for as long as a half hour can approach 30 amps on a 20 amp breaker before it trips.
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Yes I agree. The problem is I am used to working with engineered drawings on large projects that are designed to the last wire size. Now, in this area, specs and electrical drawings seem to be a frivolous extra that no one cares about. The problem is with no requirements everyone designs the job to the minimum. The good news is I got the job!
 
haskindm said:
That may not be good news. If you got it by buidding the minimum, and the minimum results in unsatisfactory performance, you may end up losing a ton of money in upgrading.....
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I bid it with #8's all the way to the end of the runs. So I am thinking I am ok.
 
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