Help me understand 3 phase power

[ggunn]

Let's say, for example, I have 3 SMA SB7000 (7000 Watt) inverters configured to 277V connected one per phase (phase to neutral) to a 480VAC line. I have it connected to the service with #1 Cu wire (R = 0.154 ohms/1000') and the distance is 100'. What is my voltage drop?

a) 0.67V
b) 1.16V
c) 0.38V
d) none of the above

And should it be compared to 277V or to 480V to get the %VD?

 

[BillK-AZ]

If all 3 inverters are operating at a nominal 277V (25.3A) the voltage drop will be 'C'. There will be no neutral current if a balance exists.

If only one inverter is operating, the voltage drop will be twice as high as there will then be current in the neutral. This is why a full size neutral must be used.

The situation for two inverters operating is between these values but more complicated, there are formulas for this situation.

Current will be higher if the voltage is lower than normal, use the low limit of 244V for wire sizing. The inverters will not operate below 244V on a 277/480 system.

 

[ggunn]

If all 3 inverters are operating at a nominal 277V (25.3A) the voltage drop will be 'C'. There will be no neutral current if a balance exists.

If only one inverter is operating, the voltage drop will be twice as high as there will then be current in the neutral. This is why a full size neutral must be used.

The situation for two inverters operating is between these values but more complicated, there are formulas for this situation.

Current will be higher if the voltage is lower than normal, use the low limit of 244V for wire sizing. The inverters will not operate below 244V on a 277/480 system.

Thanks. "C" was my preferred answer. Thanks for the tip on considering the lower bound of the voltage; I hadn't thought of that.

 

[kwired]

If only one inverter is operating, the voltage drop will be twice as high as there will then be current in the neutral.

1.73 times is a more presice number.

 

[ggunn]

1.73 times is a more presice number.

Even more precisely, 1.73205081... There's that sqrt3 again!

 

[david luchini]

The voltage drop on a balanced 3 phase circuit (assuming 25.2A) is 0.67V.

The voltage drop from any phase to neutral would be 0.38V.

If only one inverter was operating, the voltage drop would be 0.77V, or twice the phase-neutral drop, as Bill points out.

But I'd say the correct answer is "A"

 

[BillK-AZ]

1.73205081 ????

1.73205081 ????

1.73 times is a more presice number.

The 1.73... does not apply in this situation with only one inverter operating. The 1.73 is used when calculating the currents in 1-ph vs. 3-ph circuits.

With only one inverter operating, on a single phase, the current in the neutral is the same as the current in the phase wire and if the gauge is the same the voltage drop will be double that of the situation will all three inverters operating equally. Power factor and harmonic issues may cause a drop in the neutral, but that is not the situation with UL Listed inverters.

 

[Besoeker]

With only one inverter operating, on a single phase, the current in the neutral is the same as the current in the phase wire and if the gauge is the same the voltage drop will be double that of the situation will all three inverters operating equally.

Won't each inverter have its own single phase two wire supply from the distribution board? It would be unusual to common the neutral at the load end for three separate single-phase items.
That being the case, you have 200' to consider regardless of how many are operating and you would have to treat them as three separate single-phase loads.
The minimum current taken by a 7000W load at 277V is 25.3A. Allowing for losses it would be higher but we have no information on efficiency.
The only answer that fits that is (d).

 

[BillK-AZ]

Won't each inverter have its own single phase two wire supply from the distribution board? It would be unusual to common the neutral at the load end for three separate single-phase items.
.......

In most cases with single phase inverters on a 3-phase service it is necessary to have a disconnect switch and likely a dedicated KWH meter. Running three single phase two wire circuits is not practical or needed.

Most of my experience with multiple single phase inverters on 480V/3-phase as in this posting is with SMA Sunny Towers with 6 inverters. Their install instructions specify a full size neutral because it is possible to only have a single phase loaded.

 

[ggunn]

Won't each inverter have its own single phase two wire supply from the distribution board? It would be unusual to common the neutral at the load end for three separate single-phase items.
That being the case, you have 200' to consider regardless of how many are operating and you would have to treat them as three separate single-phase loads.
The minimum current taken by a 7000W load at 277V is 25.3A. Allowing for losses it would be higher but we have no information on efficiency.
The only answer that fits that is (d).

What are you calling the load end? The inverters will connect to a nearby distribution panel, phase to neutral, one inverter for each phase, and the distribution panel feeds the interconnect 100' away. The neutral at the distro is common to the three inverters, and each inverter has its own two wire line to the distro. The conductor length to the distro from the inverters is negligible; it's the voltage drop in the line from the distro to the interconnect that I am getting at. In that line it matters not whether there are three single phase inverters or a single three phase inverter driving it.

I = P/V(sqrt3)

and

Vd = (sqrt3)LRI/1000'

By substitution:

Vd = (sqrt3)LR((P/V(sqrt3))/1000' = LRP/(1000')V = (100')(0.154 ohms)(21000W)/(1000')(480V) = 0.674V

 

[Besoeker]

What are you calling the load end? The inverters will connect to a nearby distribution panel, phase to neutral, one inverter for each phase, and the distribution panel feeds the interconnect 100' away. The neutral at the distro is common to the three inverters, and each inverter has its own two wire line to the distro. The conductor length to the distro from the inverters is negligible;

Not quite how it came across in your original post.

Let's say, for example, I have 3 SMA SB7000 (7000 Watt) inverters configured to 277V connected one per phase (phase to neutral) to a 480VAC line. I have it connected to the service with #1 Cu wire (R = 0.154 ohms/1000') and the distance is 100'. What is my voltage drop?

No mention of a local DB and negligible distance from that.
I apologise for not having taken account of the possibility of that negligible distance.
Clearly, my crystal ball failed me.

 

[ggunn]

Not quite how it came across in your original post.

No mention of a local DB and negligible distance from that.
I apologise for not having taken account of the possibility of that negligible distance.
Clearly, my crystal ball failed me.

Sorry if I didn't phrase the problem accurately. It is, however, a scaled down version of a real world situation for me for which I inadvertently left out details; I did not intentionally lead you astray. I appreciate your and everyone else's contribution to my self education.

When you get down to it, all education is self education.