# how to calculate voltage drop for 240 volt

Solar Expert Posts: 1,052 ✭✭✭✭
I need clarification on how to figure voltage drop ect. for 240 volt. Think i might have done it wrong using this voltage calculator from our host. http://www.windsun.com/Hardware/Voltage_Calc.htm My gt inverters maxium output is 240 vac at 14.2 amps. I was entering 240 volt and 14.2 amp. to get wire size and voltage drop results. Got to thinking that 240 is 2 wires l1 and l2. So do you enter 120 vac and 7.1 amp to get the answer?
Solarvic

• Solar Expert Posts: 1,052 ✭✭✭✭
Re: how to calculate voltage drop for 240 volt

Well! I answered my own question. I run the calculations both ways. 120 vac 7.1 amp and 220 vac. 14.2 amp. Got same results.
:Dsolarvic:D
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: how to calculate voltage drop for 240 volt
solarvic wrote: »
Well! I answered my own question. I run the calculations both ways. 120 vac 7.1 amp and 220 vac. 14.2 amp. Got same results.
:Dsolarvic:D

It all depends on what you are trying to measure. The voltage drop on one wire of the 240 volt circuit at 14.2 amps will be the same as the voltage drop for one wire of a 120 volt circuit at 14.2 amps. The percentage will be different since the voltages are different.
However, if you put a load on only one side of a center-tapped 240 volt source, you will need to figure for the voltage drop in the phase wire and in the neutral wire, so you multiply that single-wire drop by 2.
On the other hand, if you put the same load on both sides of the 240 volt source, you will only figure the one wire drop for each of them, since there will be no current in the neutral wire.
Looking at it from a different point of view, the voltage drop for the two identical 120 volt loads in series will be the same as the voltage drop for a single 220 volt load with the same current.

The calculator results are a potentially misleading in this situation, since they are based on % voltage drop rather than absolute voltage drop. It may be easier for you to understand the results if you also look at the detail line (engineering specs) that tells you the actual voltage drop.
For example,
Using 120 volts: "3.6 volts maximum allowable voltage drop at 3%"
Changing only the voltage to 240: "7.2 volts maximum allowable voltage drop at 3%"
SMA SB 3000, old BP panels.