Voltage drop question

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sw_ross

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Having a discussion with a coworker about voltage drop related to a feeder. He’s got a 120/240 feeder going to a panel. The ONLY loads on that panel are 120v circuits.

He was using the Southwire app for voltage drop and when it asked for voltage he didn’t know if he should say 120 or 240.
I told him my thinking was that if the loads are mostly balanced in the panel there’s very little current on the neutral and most current is on the L1 & L2 lines so he should choose 240v in the app.
He was having a hard time understanding the theoretical perspective that I was saying. He kept saying, “Yeah, but the loads are all 120v”.

Am I misunderstanding the theory behind the voltage drop situation related to this scenario?

Thanks
 
sw_ross said:
Having a discussion with a coworker about voltage drop related to a feeder. He’s got a 120/240 feeder going to a panel. The ONLY loads on that panel are 120v circuits.

He was using the Southwire app for voltage drop and when it asked for voltage he didn’t know if he should say 120 or 240.
I told him my thinking was that if the loads are mostly balanced in the panel there’s very little current on the neutral and most current is on the L1 & L2 lines so he should choose 240v in the app.
He was having a hard time understanding the theoretical perspective that I was saying. He kept saying, “Yeah, but the loads are all 120v”.

Am I misunderstanding the theory behind the voltage drop situation related to this scenario?

Thanks
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You're right. Voltage drop only relates when a conductor has current. It would be a 240v circuit with neutral

Once that feeder reaches a point of distribution, then the individual loads have their own minimal voltage drop.
 
Try using a simple drawing with two different line loads.
What is the netural load?
Then do the drawing again with a balanced line to line load.

For fun take the calculated VA for that panel and map it out.
See what the maximum unbalanced load could be.

Consider what type of loads are on each leg. Let say lighting,recpt, equipment.
If a house use dryer and range netural loads at 70%.
There is examples for netural load sizing in the index.

Then you can take the calculated possible netural load and do VD @ 120v.
See what you have.
Putting all continuous lighting loads on same line can change the unbalanced load calc. Then balance these loads and calculate the difference.

You did say all 120 and I did mention range and dryer as a learning exercise. I also assumed this was single phase 120/240.
 
You have it right.

If the loads are perfectly balanced, the net result is a 240V load composed of 120V parts in series.

The thing about 'all 120V loads' is that there is some chance that the loads ad used are not balanced even if the panel is balanced. The customer could simply pick a bad combination of loads, all on one leg of the panel.

The installer would probably need to guess at the possibility of unbalanced loading, but if this is a small feeder (30A residential to a 6 space panel) then the chance of unbalanced load is likely pretty high, whereas a 200A feeder likely has lots more load diversity and will likely be more balanced.

If the installer guesses there is a good chance of unbalanced loading, then it would probably pay to do a 240V full load calc and a 120V maximum unbalanced load calc.

Jon
 
Let's say this is a house and all 120 volt loads in one panel. All this load one one line.
Take square foot of house *3
2000*3=6000 VA/120=50 amp
Use this for max unbalanced.
The use 25 amp balances at 240. Then solve for distance.
Use 60c and #1 Al.
50 amp @120
25 amp @ 240
Then you can build on this by adding the other require load for a residential job.
Fun exercise and teach a thing or two about panel loading and balancing.
Example add two fridge,two furnace, circuits etc.
 
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