voltage drop on 120/240 V circuit

[art]

Hello All,
I have a spreadsheet that calculates voltage drop segment-by-segment. One of the inputs is the Wattage and from there the current is calculated. All my branch circuits are 120 Volts. Since it's 120/240 V, do I multiply current times 120 or by 240 to get the wattage to start with?

Should say that there are two loads. one at 30 Amps and another at 15. Both 120 Volts

 

[LarryFine]

To "start" with, total power is that at the line-to-line voltage.

 

[ptonsparky]

Add your all your wattages together, divide by 240.

10 1200 watt items = 12000 watts. 12000/240=50 amps.

 

[art]

Thanks. Makes sense

Sent from my LM-Q610(FGN) using Tapatalk

 

[LarryFine]

Keep in mind that only current is directly relevant to voltage drop.

Should say that there are two loads. one at 30 Amps and another at 15. Both 120 Volts

With this added detail, you have (120*15) 1.8kw and (120*30) 3.6kw.

The source must be capable of the greater of the two for (240*30) 7.2kw.

Thanks. Makes sense

We do that sometimes.

 

[ggunn]

Hello All,
I have a spreadsheet that calculates voltage drop segment-by-segment. One of the inputs is the Wattage and from there the current is calculated. All my branch circuits are 120 Volts. Since it's 120/240 V, do I multiply current times 120 or by 240 to get the wattage to start with?

Should say that there are two loads. one at 30 Amps and another at 15. Both 120 Volts

If the loads are balanced, the power (wattage) is current X240V. If not, the power is the current in L1 X120V plus the current in L2 X120V. Actually, it's always the current in L1 X120V plus the current in L2 X120V, but in a balanced system the current in L1 and L2 are equal.

 

[art]

one more:
what if one load is 120 and the other 240?

 

[LarryFine]

one more:
what if one load is 120 and the other 240?

Same applies. Figure each load separately, and then add them. One line will share both loads.

 

[oldsparky52]

Are you doing VD for branch circuits or feeders?

 

[art]

this is for a feeder.

 

[Besoeker3]

Add your all your wattages together, divide by 240.

10 1200 watt items = 12000 watts. 12000/240=50 amps.

Assuming unity power factor.

 

[ptonsparky]

Assuming unity power factor.

You win!! I was wondering who and how long.

 

[LarryFine]

We shoulda known! :lol:

 

[kwired]

How much precision do you want? If you are mostly interested in selecting conductor size to assure a maximum amount of VD, then your worst case scenario should be when the 30 amp load is operating and the 15 amp load on the other line is not. If both loads are operating you have both 120 volt and 240 volt paths to take into consideration and it isn't quite so simple as a two wire circuit to figure out. This assumes conductors are all same size, if you have different sizes, (reduced neutral in some cases, but probably not here) that complicates things a little as well when it comes to precision on voltage drop for different points in the system.