Voltage Drop

From a theoretical point of view, you need to consider both the 240V balanced case and the 120V unbalanced case separately. The load for the 120V unbalanced case may be less, but the base voltage is half as much, so to comply with a given percent voltage drop criterion, you'd only be allowed half the voltage drop in volts.

When the 120V loads are numerous and balanced between both legs, the theoretical worst case of all loads L1-N being on and all loads L2-N being off may be statistically negligible. And if some of the L2-N loads are on, that load will cause L2-N voltage drop, which raises the L1-N voltage, meaning the L1-N voltage drop will be less than theoretical worst case. I don't know what a reasonable allowance for this in practice would be.

Cheers, Wayne
 
wwhitney said:
From a theoretical point of view, you need to consider both the 240V balanced case and the 120V unbalanced case separately. The load for the 120V unbalanced case may be less, but the base voltage is half as much, so to comply with a given percent voltage drop criterion, you'd only be allowed half the voltage drop in volts.

When the 120V loads are numerous and balanced between both legs, the theoretical worst case of all loads L1-N being on and all loads L2-N being off may be statistically negligible. And if some of the L2-N loads are on, that load will cause L2-N voltage drop, which raises the L1-N voltage, meaning the L1-N voltage drop will be less than theoretical worst case. I don't know what a reasonable allowance for this in practice would be.

Cheers, Wayne
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So I'd enter 240v in my situation above?
 
Alwayslearningelec said:
So I'd enter 240v in my situation above?
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You didn't provide the load calc in the OP.. But suppose it came out to 80A on each line conductor and 50A on the neutral. Then for thoroughness you'd do one VD calc with 240V and 80A, and a second VD calc with 120V and 50A. [Although using the full NEC calculated load may be overly conservative.]

For the second calc, if your neutral and line conductors are different sizes, that would complicate things. You could handle that by doing two calculations, one with each conductor size, using half the distance in each calculation, and then add the results.

For the reasons described in the second paragraph of my earlier post, this 120V calculation may be over conservative.

Cheers, Wayne
 
wwhitney said:
You didn't provide the load calc in the OP.. But suppose it came out to 80A on each line conductor and 50A on the neutral.
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Maybe I'm just having a high density day, but if the currents in the line conductors are equal does that not mean that it is balanced and the neutral current should be zero?
 
ggunn said:
Maybe I'm just having a high density day, but if the currents in the line conductors are equal does that not mean that it is balanced and the neutral current should be zero?
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Yes, if there is no phase shift between the two line currents. In general the neutral current will be the difference between the two line currents (for an appropriate choice of sign convention).

But we were discussing a load calculation. The load calculation is the maximum possible current on each line (after demand factors), not necessarily simultaneously. So with a load calc of L1 80A, L2 80A, N 50A, you'd most likely have a maximum simultaneous current in one case of 80A, 80A, 0A; and in another case of 0A, 50A, 50A (or the two line currents swapped).

Those two cases would be the two cases I'm saying it's appropriate to do a voltage drop calculation for. The first is 80A @ 240V; the second is 50A @ 120V. For those numbers the second case will control (unless you make the neutral conductor larger than the ungrounded conductors, then the first case might control the ungrounded conductors).

Cheers, Wayne
 
wwhitney said:
Yes, if there is no phase shift between the two line currents. In general the neutral current will be the difference between the two line currents (for an appropriate choice of sign convention).

But we were discussing a load calculation. The load calculation is the maximum possible current on each line (after demand factors), not necessarily simultaneously. So with a load calc of L1 80A, L2 80A, N 50A, you'd most likely have a maximum simultaneous current in one case of 80A, 80A, 0A; and in another case of 0A, 50A, 50A (or the two line currents swapped).

Those two cases would be the two cases I'm saying it's appropriate to do a voltage drop calculation for. The first is 80A @ 240V; the second is 50A @ 120V. For those numbers the second case will control (unless you make the neutral conductor larger than the ungrounded conductors, then the first case might control the ungrounded conductors).

Cheers, Wayne
Click to expand...
Oh, so you did not mean simultaneously.
 
What’s the requirements for the item you’re trying to power up?
If it’s 120 V then use 120 V. If 240 then use 240 V.
Logically thinking….
 
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