Voltage Drop on a 240 VAC 3-Wire Circuit

[ejerrym]

I do voltage drop calculations regularly, no problem with the math. My question is when I do the calculation for a 3-wire 240 VAC circuit am I computing the VD between the neutral and a phase conductor or between the two outside phase conductors?

Example: 240 VAC, 43 Amps, 220', AWG 4 (uncoated) Copper

VD = (2 x 220' x .308 ohms x 43 amps)/1000 = 5.82 volts

VD% = (5.82/240) x 100 = 2.425 %

I'm thinking the answer is the two outside phase conductors, but I can't visualize it. Any insight is appreciated.

 

[Smart $]

I do voltage drop calculations regularly, no problem with the math. My question is when I do the calculation for a 3-wire 240 VAC circuit am I computing the VD between the neutral and a phase conductor or between the two outside phase conductors?

Example: 240 VAC, 43 Amps, 220', AWG 4 (uncoated) Copper

VD = (2 x 220' x .308 ohms x 43 amps)/1000 = 5.82 volts

VD% = (5.82/240) x 100 = 2.425 %

I'm thinking the answer is the two outside phase conductors, but I can't visualize it. Any insight is appreciated.

When you use the L-L voltage and 2?length, you are calculating L-L voltage drop. When or if the circuit is balanced, the voltage drop L-N will be one half the L-L value, but at one half the voltage. As such, your L-N percentage will be the same as the L-L percentage. However, in the extreme unbalanced condition, your L-N voltage drop will double, making the L-N percentage twice that of the calculated L-L percentage.

 

[broadgage]

If the loads are mainly 240 volt without connection to the neutral, then calculate on the basis of voltage drop in the 2 outer or phase conductors.

If the loads are mainly small 120 volt loads, well balanced between the two phase or hot consuctors, then again calculate the voltage drop between phase or hot conductors.

If the loads are 120 volts, but few in number and large in size, then it possible that one hot hot could be unloaded, and the other fully loaded. In that case the voltage drop between the neutral and one phase or hot conductor should be calculated.

As an example, consider a domestic service service that feeds several large 240 volt loads (central A/C, range, water heater) and numerous 120 volt lighting and small appliances branch circuits.
The 240 volt loads cant impose significant load on the neutral, and the 120 volt circuits are most unlikely to be badly balanced, and are probably a fairly small total load anyway.

At the opossite extreme, consider a 120/240 volt 3 wire feeder that supplies two large 120 volt water pumps, one on each hot wire, and a couple of lamps.
Use of one pump only is entirely forseeable, and voltage drop calculations should reflect this.
(and as an aside, 240 volt pumps would probably be a better choice)

 

[ejerrym]

Thanks guys....I appreciate the insight. Since this is the output circuit of a PV inverter and the grid is the load, we can probably presume that the loads are fairly balanced. In such a case the VD between the hot legs and neutral are insignificant and the VD calculation should be done using the 2 outer legs.

 

[Smart $]

Thanks guys....I appreciate the insight. Since this is the output circuit of a PV inverter and the grid is the load, we can probably presume that the loads are fairly balanced. In such a case the VD between the hot legs and neutral are insignificant and the VD calculation should be done using the 2 outer legs.

While I understand what you are saying, it's technically incorrect. Even for a balanced load, the L-N Vd is one half the L-L Vd...

I don't work with PV systems, but I do understand for the most part how they work, the equipment involved, and how they are wired. I have a training certificate saying I'm PV installation qualified, yet I've never installed one . Nevertheless, I am of the impression most output only L-L. I've not heard of any that have L1-N-L2 output. So if there are such systems, please help me adjust my impression...???

 

[broadgage]

No common 240 volt PV grid tied inverter needs a neutral connection.
Two hots and an EGC only.
The circuit might well incorporate a neutral in case needs change.

Voltage drop (voltage RISE to be pedantic) should be calculated at 240 volts.