Voltage Drop Question

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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I don't fully understand table 9, in particular the heading with "ohms to neutral per 1000 feet".

With a three phase three wire circuit I could care less about any neutral, so what exactly is the values in the table?

With any circuit you have the resistance of the conductors going out to the load and the resistance of the conductors coming back.

Sometimes people will write voltage drop equations that include both sides of the drop, in which case you have a _different_ calculation for single phase and 3 phase circuits.

The 'ohms to neutral' just says that it is giving you the resistance of the single wire out to the load. I know that there are formulas to convert this to a 3 phase value, but I find it simpler to just calculate the line to neutral voltage drop versus the line to neutral voltage for 3 phase loads.

-Jon
 

Adamjamma

Senior Member
Easiest way to think about this is it is written for most applications. Thus, neutral part of the thought but not a part of the actual usage. If you are using a three phase without neutral but with a ground, do you size all three live wires the same? Do you size the ground the same? If you do, then your resistance figure for all four wires should be identical and thus your values all equal... a to ground, b to ground, c to ground.. plus voltage drop should be identical on all three phases as well. So, when selecting the correct wire, you only need to select based on one phase to neutral or phase to ground value. If your voltage drop on one is acceptable then the voltage drop on all three phases should be acceptable.
 

kwired

Electron manager
Location
NE Nebraska
With any circuit you have the resistance of the conductors going out to the load and the resistance of the conductors coming back.

Sometimes people will write voltage drop equations that include both sides of the drop, in which case you have a _different_ calculation for single phase and 3 phase circuits.

The 'ohms to neutral' just says that it is giving you the resistance of the single wire out to the load. I know that there are formulas to convert this to a 3 phase value, but I find it simpler to just calculate the line to neutral voltage drop versus the line to neutral voltage for 3 phase loads.

-Jon
Table title says it is for three phase, so I would assume it means actual voltage drop on a balanced three phase circuit, then they throw in that ohm's to neutral thing which makes no sense.

If using table 8 values or a formula with a k factor in it, you have a multiplication factor of 2 in your calculation for total voltage drop on a two wire circuit, or a factor of 1.732 for a three phase circuit, so I would think according to the title that 1.732 had already been factored in. :huh:
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Table title says it is for three phase, so I would assume it means actual voltage drop on a balanced three phase circuit, then they throw in that ohm's to neutral thing which makes no sense.

If using table 8 values or a formula with a k factor in it, you have a multiplication factor of 2 in your calculation for total voltage drop on a two wire circuit, or a factor of 1.732 for a three phase circuit, so I would think according to the title that 1.732 had already been factored in. :huh:

In the case of a balanced three phase circuit you can just use the one way distance (no current through the neutral, so no voltage drop) and reference Vd to the voltage phase to neutral. The sqrt3 falls out.
 

kwired

Electron manager
Location
NE Nebraska
In the case of a balanced three phase circuit you can just use the one way distance (no current through the neutral, so no voltage drop) and reference Vd to the voltage phase to neutral. The sqrt3 falls out.
:?

If I have a three phase motor, 480 volts, if I want to see no more than 5% VD then 456 line to line is what I am interested in for a minimum volts. I might not have a "neutral" conductor though I will still have a neutral point if drawing the vectors and it would be 277 volts x .95 = 263 volts on each line at a max of 5% VD. Is this what you are saying?

All I am saying is my motor wants to see 480 volts less any VD I allow for as far as I am concerned.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
:?

If I have a three phase motor, 480 volts, if I want to see no more than 5% VD then 456 line to line is what I am interested in for a minimum volts. I might not have a "neutral" conductor though I will still have a neutral point if drawing the vectors and it would be 277 volts x .95 = 263 volts on each line at a max of 5% VD. Is this what you are saying?

All I am saying is my motor wants to see 480 volts less any VD I allow for as far as I am concerned.

Not necessarily; I don't know from motors, but in the earlier post that talked about calculating %Vd in a balanced three phase wye system, it was calculated with respect to phase to phase voltage and there was a sqrt3 in it. The ratio of phase to phase voltage to phase to neutral voltage is also sqrt3, so you can just use the phase to neutral voltage and leave out the sqrt3. The distance is one way only because there is no current on the neutral so therefore no voltage drop.

If it's a delta configuration and balanced it seems to me that %Vd would be the same as if there were a neutral, but I haven't done the math.
 

kwired

Electron manager
Location
NE Nebraska
Not necessarily; I don't know from motors, but in the earlier post that talked about calculating %Vd in a balanced three phase wye system, it was calculated with respect to phase to phase voltage and there was a sqrt3 in it. The ratio of phase to phase voltage to phase to neutral voltage is also sqrt3, so you can just use the phase to neutral voltage and leave out the sqrt3. The distance is one way only because there is no current on the neutral so therefore no voltage drop.

If it's a delta configuration and balanced it seems to me that %Vd would be the same as if there were a neutral, but I haven't done the math.
So it is actually the line to neutral VD that is in the table and you would have to multiply by 1.732 to get actual line to line VD? That is what I am seeing after this explanation.

That would still work with delta system and three phase three wire -you still have a neutral point with such system that is 277 volts between each corner of the delta, you just can't directly connect a 277 volt load to it, connect identical loads in a wye configuration and you autotransform that neutral at the wye point.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So it is actually the line to neutral VD that is in the table and you would have to multiply by 1.732 to get actual line to line VD? That is what I am seeing after this explanation.

What table are you speaking of? The only table I use for this calculation is Table 9 for AC wire resistance. You can figure Vd as the reduction in phase to phase voltage or as the reduction in phase to neutral voltage. Either way the percentage Vd is the same, since the two Vd's and the two voltages both differ by a factor of sqrt3.
 

kwired

Electron manager
Location
NE Nebraska
What table are you speaking of? The only table I use for this calculation is Table 9 for AC wire resistance. You can figure Vd as the reduction in phase to phase voltage or as the reduction in phase to neutral voltage. Either way the percentage Vd is the same, since the two Vd's and the two voltages both differ by a factor of sqrt3.

Too much time between posts and losing track of where I am in the discussion? Plus I just am not familiar with using table 9, just looked at it and it didn't quite make sense to me, ohm's to neutral threw me off - a lot of the time with three phase calculation I'm not even using neutral or at least don't care about it much. When dealing with three phase systems I seem to get more projects with high line to line load and little or no neutral load.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Too much time between posts and losing track of where I am in the discussion? Plus I just am not familiar with using table 9, just looked at it and it didn't quite make sense to me, ohm's to neutral threw me off - a lot of the time with three phase calculation I'm not even using neutral or at least don't care about it much. When dealing with three phase systems I seem to get more projects with high line to line load and little or no neutral load.

Table 9 is just like Table 8 but AC resistance instead of DC resistance, and the numbers are much the same. Ohms to neutral doesn't make much sense to me, either, unless the wire is connected at one end to neutral and the resistance is measured from there to a point 1000' away on the conductor in question.
 
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