Conductor Resistances and Reactances Dilemna

[nightfox1925]

Hi to all. I am looking into NEC table 9.1.1.9 and gives me the value of insulated conductor or cable resistances, reactances and impedances in terms of "Line-To-Neutral". Does line-to-neutral impedance synonimous to "Impedance per Phase" or Single Phase Impedance?

It is confusing that the Table is entitled"Alternating Current Resistance and Reactance for 600volts Cables, 3phase, 60Hz, 75 deg C-Three Single Conductor in Conduit" while the values are in Ohms to neutral.

 

[davidbeach]

For a three phase circuit supplying a balanced load there would be no current in a neutral, whether present or not. No current = no voltage drop. So the only voltage drop occurs between phase and neutral. Add to that the fact that nearly all circuit analysis is done on a per phase basis using line to neutral voltages.

 

[nightfox1925]

So does this mean that say I have a 3 phase, 3 wire supply lines and I have line-to-line single phase loads connected on each phase and is not balanced. Then, is it correct that I will just simply take the highest load current among the three phases and use it in the formula below using line-to-neutral impedance and multiplying it by 1.732 say:

Vdrop line-to-line = 1.732 * highest current on phase *
Z(line-to-neutral).

the 1.732 multiplier, although the line current is single phase will account on the assumption of a balanced three phase to simplify the calculation. Is my understanding correct.

If I do have my 3phase load current, single phase line-to-line load current and single phase line-to-neutral load current, using a Ohms to neutral impedances from Table 9.1.1.9, are these correct for me to calculate the watts losses on these cables?

1. Cond. Watts loss 1phase, L-L = 2*[I(line-line)]^2 * R

2. Cond. Watts loss 1phase, L-N = [I(line-line)]^2 * R

3. Cond. Watts loss 3phase, = 3*[I(line-line)]^2 * R

 

[nightfox1925]

Oops sorry the ABOVE FORMULA could have been:

1. Cond. Watts loss 1phase, L-L = 2*[I(line-line)]^2 * R

2. Cond. Watts loss 1phase, L-N = [I(line-neutral)]^2 * R

3. Cond. Watts loss 3phase, = 3*[I(3phase)]^2 * R

where R = Ohms to neutral

Any comments. Thanks

 

[davidbeach]

Unless you have a very good reason for doing phase by phase voltage drop calculations on a three phase circuit, and those reasons are generally associated with homework problems which are outside the scope of this forum, you should figure out your highest phase load and then assume a balanced three phase load with that much load on all three phases.

If you are really looking for conductor losses, rather than voltage drop, they aren't the same thing, just figure out the number of amps in the conductor, square that, and multiply by the resistance of the conductor. Calculate the amps in each phase conductor plus the neutral and calculate the loss in each.

Line to neutral ohms would be the one-way impedance of the conductor.

 

[nightfox1925]

Thanks davidbeach, sorry I got mixed up with the formula. Ohms to neutral wil be the same on each conductor regardless of what connection they are in terms of supply and load.