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07-07-2011, 11:24 AM   #1
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## 240 V question

Hi Everyone,
I am trying to understand more about how 240V circuits draw power.

Say I have a 30A (dual-pole?) 240V breaker running to a subpanel.

Does this mean I either have 30A I can use at 240V or I have 60A I could use at 120V (if it were split evenly)?

I am trying to figure out if it is better to use 240V devices or 120V devices if I have the option (For stuff like A/C and lights).

For example, if I have a 6.1 Amp 240V AC unit, does it draw around 3.05 Amps from each 120V line evenly? On a 30A 240V breaker would that give me 26.95A left to work with on each side (forgetting the 80% guideline for simplicity)?

OR
Does the 240V A/C unit draw 6.1 Amps from both 120 lines?

Does running 240V devices effectively let me get more power with thinner gauge wire due to only needing half the amps to run?

Thanks for helping me learn!

07-07-2011, 11:27 AM   #2
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A 240 volt 30 amp circuit supplies 30 amps. If you have a 120/240 volt circuit with a neutral the circuit could supply 30 amps on each 120 volt leg. No way you can get 60 amps on one leg.

07-07-2011, 12:14 PM   #3
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Quote:
 Originally Posted by brric A 240 volt 30 amp circuit supplies 30 amps. If you have a 120/240 volt circuit with a neutral the circuit could supply 30 amps on each 120 volt leg. No way you can get 60 amps on one leg.
Thanks for responding. I think you misunderstood my question. I am not wanting to get 60 amps on one leg. I am trying to understand how 240V draws power from each leg.

If I have a 6.1 Amp 240V AC unit, does it draw around 3.05 Amps from each 120V leg evenly? On a 30A 240V breaker would that give me 26.95A left to work with on each side (forgetting the 80% guideline for simplicity)?
OR
Does the 240V A/C unit draw 6.1 Amps from both 120 lines?

07-07-2011, 12:16 PM   #4
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Quote:
 Originally Posted by mulltron thanks for responding. I think you misunderstood my question. I am not wanting to get 60 amps on one leg. I am trying to understand how 240v draws power from each leg. If i have a 6.1 amp 240v ac unit, does it draw around 3.05 amps from each 120v leg evenly? On a 30a 240v breaker would that give me 26.95a left to work with on each side (forgetting the 80% guideline for simplicity)? Or does the 240v a/c unit draw 6.1 amps from both 120 lines?
yes...

07-07-2011, 01:29 PM   #5
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Quote:
 Originally Posted by brric yes...
Thanks for responding brric, but you answered yes to two questions that have the word OR in between them. Both of the questions can't be answered with yes.

If I have a 6.1 amp 240v ac unit, does it draw around 3.05 amps from each 120v leg evenly? On a 30a 240v breaker would that give me 26.95a left to work with on each side (forgetting the 80% guideline for simplicity)?
Or
does the 240v a/c unit draw 6.1 amps from each of the 120 lines?

 07-07-2011, 01:35 PM #6 Member     Join Date: Mar 2005 Location: Welland, Ontario Posts: 12,600 Rewards Points: 11,992 Blog Entries: 11 A 240 volt circuit drawing 6.1 amps will draw 6.1 amps from both hots and nothing from the neutral. 30 amps at 240 volts = 7200 watts. 30 at 120 volts times 2 = 7200 watts. There is no energy savings. The savings and advantage come from the fact that a device that can run on either voltage will draw 1/2 the amps a 240 volts( watts is the same). Therefore you can use smaller wires and smaller breakers which are usually cheaper to buy and easier to work with. For example a motor that can run 240v 10 amp or 120v 20 amps would only need a 15 amp circuit with #14 cable at 240 volts as opposed to a 30 amp circuit with #10 cable at 120 volts. Last edited by joed; 07-07-2011 at 01:38 PM.
 The Following User Says Thank You to joed For This Useful Post: mulltron (07-07-2011)
07-07-2011, 01:37 PM   #7
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Quote:
 Originally Posted by mulltron Thanks for responding brric, but you answered yes to two questions that have the word OR in between them. Both of the questions can't be answered with yes. If I have a 6.1 amp 240v ac unit, does it draw around 3.05 amps from each 120v leg evenly? On a 30a 240v breaker would that give me 26.95a left to work with on each side (forgetting the 80% guideline for simplicity)? Or does the 240v a/c unit draw 6.1 amps from each of the 120 lines?
Equal amperage on each leg at 240 volts.

07-07-2011, 01:58 PM   #8
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Quote:
 Originally Posted by joed A 240 volt circuit drawing 6.1 amps will draw 6.1 amps from both hots and nothing from the neutral. 30 amps at 240 volts = 7200 watts. 30 at 120 volts times 2 = 7200 watts. There is no energy savings. The savings and advantage come from the fact that a device that can run on either voltage will draw 1/2 the amps a 240 volts( watts is the same). Therefore you can use smaller wires and smaller breakers which are usually cheaper to buy and easier to work with. For example a motor that can run 240v 10 amp or 120v 20 amps would only need a 15 amp circuit with #14 cable at 240 volts as opposed to a 30 amp circuit with #10 cable at 120 volts.

Thanks Joed!

That helps! I think I got it now, but let me bounce it off the experts one last time to make sure. The only real advantage to 240V is the ability to use smaller wiring and breakers from the subpanel to the devices (and it is nice that it draws evenly from both sides), but we will need to use the same gauge of wires from the main panel to the subpanel regardless of whether we go with 120V or 240V, because in the end it draws the same number of total amps, it is just a matter of whether the amps get drawn all from 1 of the 120V lines or whether they are split evenly between the 2 lines.

Is that right?

 07-07-2011, 02:44 PM #9 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,493 Rewards Points: 4,566 I am not quite sure you get it yet. You said "but we will need to use the same gauge of wires from the main panel to the subpanel regardless of whether we go with 120V or 240V". This is incorrect. If you plan to wire your subpanel to handle 7200 watts maximum, the subpanel would draw 30A at 240 volts, and could be wired using #10 copper. The same subpanel could be wired using one leg (120 volts), but would require 60A at 120 volts, which would require #4 copper wire (and the panel would need to be rated for 60A). For 240 volt only panels, you need three wires, two hots and a ground. For 120 volt only panel, you need three wires, one hot, one neutral, one ground. For 120/240 panels, you need four wires, two hots, one neutral and one ground. Most of the time people wire subpanels for 120/240 volt service.
 07-07-2011, 03:24 PM #10 Member     Join Date: Mar 2005 Location: Welland, Ontario Posts: 12,600 Rewards Points: 11,992 Blog Entries: 11 Let me try and explain it this way. Electricity has to return back to the source. A 240 volt device draw 30 amp on one and returns it on the other. the wave forms are 180º out of phase. When one is coming the other is going. 120 volt device draw 30 amps from one hot and return it through the neutral. Here is the wierd part for most people. If two 120 volt circuits are drawing 30 amps from each hot then the neutral current is zero. It works like a 240 device where one is pushing and one is pulling. And then there are the devices like your dryer that have both 120 and 240 users inside them. Typically the heaters are 240 and the motor and timer are 120. Last edited by joed; 07-07-2011 at 03:26 PM.
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07-07-2011, 06:32 PM   #11
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Quote:
 Originally Posted by Daniel Holzman I am not quite sure you get it yet. You said "but we will need to use the same gauge of wires from the main panel to the subpanel regardless of whether we go with 120V or 240V". This is incorrect. If you plan to wire your subpanel to handle 7200 watts maximum, the subpanel would draw 30A at 240 volts, and could be wired using #10 copper. The same subpanel could be wired using one leg (120 volts), but would require 60A at 120 volts, which would require #4 copper wire (and the panel would need to be rated for 60A). For 240 volt only panels, you need three wires, two hots and a ground. For 120 volt only panel, you need three wires, one hot, one neutral, one ground. For 120/240 panels, you need four wires, two hots, one neutral and one ground. Most of the time people wire subpanels for 120/240 volt service.
I think I do get it. I just think I might not be explaining my thoughts well enough for others to understand what I am trying to say. I apologize for not knowing the lingo very well.

We have 240V going to the subpanel, which is 2, 120 legs coming from a 240V breaker. We also have a neutral and a ground from the main board to the service panel for 120/240 service.

Right now, I plan to run all 120V devices, using both legs as equally split as possible. I added up all the AMP ratings on all my 120V devices if they are all on at the same time and get 56.16 A which will be split almost evenly between the 2, 120 circuits. This is too close for comfort for continuously running devices. The solution I have so far is to upgrade to #8 wire for the run between panels and change the breaker on the main unit to 40A 240V. I was trying to see if there is an advantage to buying any 240V devices and stick with the #10 wire between panels.

I am not trying to run 56.16A off of one leg of 120V, which is the impression I somehow keep giving people. I am wondering if there is an advantage to running 240V devices instead of running 120V devices with a similar power rating split over the 2 120V legs. Each device would only be on one leg of 120V but I would put an equal number of similar devices on each 120V leg.

Case 1: If I run 56 Amps split up as 28 Amps on each 120V leg, I am under the 30A breaker for each leg, but past the 80% recommendation for continuous load.
Case 2: If I run the same power on 240V devices it is 28 Amps that flow through both power wires and is still past the 80% recommendation for the 30A breaker.

So in both cases, it appears to me that regardless of whether I go 120V or 240V devices, I will need to upgrade the wires from the main panel to the subpanel to #8 and the ground to #10. That is what I meant by "but we will need to use the same gauge of wires from the main panel to the subpanel regardless of whether we go with 120V or 240V"

Does that make sense? Is my science correct or am I off in that statement?

 07-07-2011, 08:57 PM #12 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,493 Rewards Points: 4,566 I prefer 240 volt devices versus 120 volt when the devices draw a lot of power, like a table saw, a well pump, or a large motor. Since power is I X V, where I is the current and V is the voltage drop, you run half the amperage through a 240 volt device versus a 120 volt device of the same power. Losses through the wire are computed by I * I * R, where R is the resistance, so you get 1/4 the line loss through a 240 volt device as opposed to an equivalent power 120 volt device, assuming resistance is the same (not always the case). This equates to higher overall efficiency, although you need better insulation for the higher voltage device, so it can cost more. This is the reason that large amounts of power are transmitted at very high voltage, sometimes as high as 500,000 volts - it reduces line loss, which is significant over long distances. Line loss may not be important in your case.
 The Following User Says Thank You to Daniel Holzman For This Useful Post: mulltron (07-08-2011)
 07-07-2011, 08:58 PM #13 Member     Join Date: Mar 2005 Location: Welland, Ontario Posts: 12,600 Rewards Points: 11,992 Blog Entries: 11 You got the idea perfect in the last 5 sentences.
 The Following User Says Thank You to joed For This Useful Post: mulltron (07-08-2011)
07-07-2011, 09:00 PM   #14
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Quote:
 Originally Posted by Daniel Holzman I prefer 240 volt devices versus 120 volt when the devices draw a lot of power, like a table saw, a well pump, or a large motor. Since power is I X V, where I is the current and V is the voltage drop, you run half the amperage through a 240 volt device versus a 120 volt device of the same power. Losses through the wire are computed by I * I * R, where R is the resistance, so you get 1/4 the line loss through a 240 volt device as opposed to an equivalent power 120 volt device, assuming resistance is the same (not always the case). This equates to higher overall efficiency, although you need better insulation for the higher voltage device, so it can cost more. This is the reason that large amounts of power are transmitted at very high voltage, sometimes as high as 500,000 volts - it reduces line loss, which is significant over long distances. Line loss may not be important in your case.

There is nothing to gain in this case since the same power/amps is needed at both 120 and 240.

 The Following User Says Thank You to joed For This Useful Post: mulltron (07-08-2011)
07-08-2011, 12:13 AM   #15
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Joe and Daniel,
Thank you so much for all of your help! I am good to go!!!

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