Voltage Drop
- Thread starter gardiner
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Re: Voltage Drop
NEC does not specify voltage drop, only minimum conductor size vs the OCPD. If you are referring to a 20-amp breaker the smallest wire you can use is a 12 AWG.
If voltage drop is the concern you have to use some sort of formula or calculator. Here is the standard formula:
CM=(25.8 * I * L)/VD
Where
CM = circular mills of wire/cable
I = load current
L = one way wire distance
VD = maximum voltage drop
NEC does not specify voltage drop, only minimum conductor size vs the OCPD. If you are referring to a 20-amp breaker the smallest wire you can use is a 12 AWG.
If voltage drop is the concern you have to use some sort of formula or calculator. Here is the standard formula:
CM=(25.8 * I * L)/VD
Where
CM = circular mills of wire/cable
I = load current
L = one way wire distance
VD = maximum voltage drop
electricman2
Senior Member
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- North Carolina
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- Retired Electrical Contractor
Re: Voltage Drop
Circuit voltage affects determination of wire size to compensate for voltage drop. A 5V drop on a 480V circuit is only 1% but on a 120V circuit it is 4.2%.
Circuit voltage affects determination of wire size to compensate for voltage drop. A 5V drop on a 480V circuit is only 1% but on a 120V circuit it is 4.2%.
- Location
- Lockport, IL
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Re: Voltage Drop
In a 120 volt system, the suggested limit of 3% voltage drop (for a branch circuit) equates to 3.6 volts. Plugging this value into Dereck?s formula, I get a result of 31,500 CM. The smallest conductor that is at least this big is a #4 (41,740 CM).
But what exactly do you mean by calling this a ?20 amp load?? Do you mean that you intend to use a 20 amp breaker, or that the devices connected to this load will really draw every bit of 20 amps? The difference is that for a 20 amp breaker, you should limit the actual load to under 16 amps (80%). Using an actual load of 16 amps in Dereck?s formula gives a result of 25,227 CM. In that case, a #6 is sufficient (26,240 CM).
In a 120 volt system, the suggested limit of 3% voltage drop (for a branch circuit) equates to 3.6 volts. Plugging this value into Dereck?s formula, I get a result of 31,500 CM. The smallest conductor that is at least this big is a #4 (41,740 CM).
But what exactly do you mean by calling this a ?20 amp load?? Do you mean that you intend to use a 20 amp breaker, or that the devices connected to this load will really draw every bit of 20 amps? The difference is that for a 20 amp breaker, you should limit the actual load to under 16 amps (80%). Using an actual load of 16 amps in Dereck?s formula gives a result of 25,227 CM. In that case, a #6 is sufficient (26,240 CM).
Re: Voltage Drop
By using the resistance from Table 8 I calculated that you could go with number 8 coated or non coated and only have a 3.56 voltage drop. Did I do my calculation wrong or is it just a difference in the formula used. VD = I x R. R = value from table/1000 (using 1000 ft column) x distance. I = max current.
Don McLain
By using the resistance from Table 8 I calculated that you could go with number 8 coated or non coated and only have a 3.56 voltage drop. Did I do my calculation wrong or is it just a difference in the formula used. VD = I x R. R = value from table/1000 (using 1000 ft column) x distance. I = max current.
Don McLain
Re: Voltage Drop
Don did you include inductance or just use resistance?
Personally I do not use 12.9 for K that is listed in the NEC. It is to conservative IMO. I use 10.8 for DC and 11.1 for AC.
[ January 13, 2004, 01:43 PM: Message edited by: dereckbc ]
Don did you include inductance or just use resistance?
Personally I do not use 12.9 for K that is listed in the NEC. It is to conservative IMO. I use 10.8 for DC and 11.1 for AC.
[ January 13, 2004, 01:43 PM: Message edited by: dereckbc ]
Re: Voltage Drop
Thanks very much it seems I'm getting the answers I expected. The voltage is 120v and the breaker size is 20 although the actual load should be less then 16 amps. The question ended up in front of me because someone wants to use #10 to handle this load and I was trying to find a diplomatic way of saying something larger is needed. Along with having some proof my math was still good.
Thanks very much it seems I'm getting the answers I expected. The voltage is 120v and the breaker size is 20 although the actual load should be less then 16 amps. The question ended up in front of me because someone wants to use #10 to handle this load and I was trying to find a diplomatic way of saying something larger is needed. Along with having some proof my math was still good.
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Re: Voltage Drop
120 volt 16 amp load @ 220' I come up with 6 AWG for less than 3% drop.
Using the 10 AWG on this circuit would result in about 7.4% or 8.5 volts lost.
This using a voltage drop calculator.
120 volt 16 amp load @ 220' I come up with 6 AWG for less than 3% drop.
Using the 10 AWG on this circuit would result in about 7.4% or 8.5 volts lost.
This using a voltage drop calculator.
jimwalker
Senior Member
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- TAMPA FLORIDA
Re: Voltage Drop
What exactly is the load ?If just incandesent bulbs and they dont mind them looking dim it might work,but it's really a poor way to go
What exactly is the load ?If just incandesent bulbs and they dont mind them looking dim it might work,but it's really a poor way to go
Re: Voltage Drop
Dereckbc
I looked at the table and took the highest resistance. The DC resistance for the stranded coated #8 was higher than the AC resistance listed. I think I made a mistake on my formula. As soon as I left the office I realized I should have doubled the distance to allow for voltage drop on both legs of the circuit. (or 1.73 for three phase)
Don McLain
[ January 14, 2004, 03:43 AM: Message edited by: mclain ]
Dereckbc
I looked at the table and took the highest resistance. The DC resistance for the stranded coated #8 was higher than the AC resistance listed. I think I made a mistake on my formula. As soon as I left the office I realized I should have doubled the distance to allow for voltage drop on both legs of the circuit. (or 1.73 for three phase)
Don McLain
[ January 14, 2004, 03:43 AM: Message edited by: mclain ]
Re: Voltage Drop
I am not really sure what all will be on the circuit, at least standard desk units computers, monitors, some task lighting. Personal objects of person at the desk, (this could be anything).
They seem to be non committal about telling anyone what exactly would be used. I was just trying to tell them for the size of breaker and for the distance and the fact the exact load at the other end is really unknown a #10 wire is too small and being a larger wire is impossible to use some more panels should be installed in the building to lessen the distance.
I am not really sure what all will be on the circuit, at least standard desk units computers, monitors, some task lighting. Personal objects of person at the desk, (this could be anything).
They seem to be non committal about telling anyone what exactly would be used. I was just trying to tell them for the size of breaker and for the distance and the fact the exact load at the other end is really unknown a #10 wire is too small and being a larger wire is impossible to use some more panels should be installed in the building to lessen the distance.
Re: Voltage Drop
gardiner, if this is for GP receptacle, I wouldn't worry much about the voltage drop. If it is for a continious load of say, more than say 10-amps to sensitive equipment, then consider upsizing from a 12 AWG.
Is there a spec calling for a max voltage drop?
gardiner, if this is for GP receptacle, I wouldn't worry much about the voltage drop. If it is for a continious load of say, more than say 10-amps to sensitive equipment, then consider upsizing from a 12 AWG.
Is there a spec calling for a max voltage drop?
jimwalker
Senior Member
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- TAMPA FLORIDA
Re: Voltage Drop
With that kind of load i would not risk more than 5% drop.Computers could get upset easy should it drop even briefly
With that kind of load i would not risk more than 5% drop.Computers could get upset easy should it drop even briefly
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