Watts Calculation Method for 240V Circuits

Cybatrex

Member
I need assistance with determining my Watts correctly with Ohm's law using the amperage reading from my Fluke clamp meter. I welcome anyone who can correct the errors of my ways and offer a brief explanation so I can learn correctly.

Amperage Readings
L[SUP]1[/SUP]: 17.1 Amps
L[SUP]2[/SUP]: 17.0 Amps

Ohms Law
P(w) = V x I


Method 1: Calculated ohms law for each leg of the circuit, calculated them individually, then added the wattage together.
L[SUP]1[/SUP]: 17.1A x 120V = 2,052 Watts
L[SUP]2[/SUP]: 17.0A x 120V = 2,040 Watts

Total Watts[SUP]1[/SUP]: 4,092 Watts
[HR][/HR]
Method 2: Combined amperage for both legs and used 240V by combined average to calculate watts which is double Method 1.
L[SUP]1[/SUP]: 17.1 Amps
L[SUP]2[/SUP]: 17.0 Amps

L[SUP]t [/SUP]= 34.1 Amps x 240V = 8,184 Watts

Total Watts[SUP]2[/SUP]: 8,184 Watts


[HR][/HR]

Any help with this would be greatly appreciated.
:):):)
 

GoldDigger

Moderator
Staff member
I need assistance with determining my Watts correctly with Ohm's law using the amperage reading from my Fluke clamp meter. I welcome anyone who can correct the errors of my ways and offer a brief explanation so I can learn correctly.

Amperage Readings
L[SUP]1[/SUP]: 17.1 Amps
L[SUP]2[/SUP]: 17.0 Amps

Ohms Law
P(w) = V x I


Method 1: Calculated ohms law for each leg of the circuit, calculated them individually, then added the wattage together.
L[SUP]1[/SUP]: 17.1A x 120V = 2,052 Watts
L[SUP]2[/SUP]: 17.0A x 120V = 2,040 Watts

Total Watts[SUP]1[/SUP]: 4,092 Watts
[HR][/HR]
Method 2: Combined amperage for both legs and used 240V by combined average to calculate watts which is double Method 1.
L[SUP]1[/SUP]: 17.1 Amps
L[SUP]2[/SUP]: 17.0 Amps

L[SUP]t [/SUP]= 34.1 Amps x 240V = 8,184 Watts

Total Watts[SUP]2[/SUP]: 8,184 Watts


[HR][/HR]

Any help with this would be greatly appreciated.
:):):)
The problem with your second method is that the same current (approximately) is flowing through the same series circuit between L1 and L2. You only count that current once! That is where your extra factor of two came from.

Sent from my XT1585 using Tapatalk
 

Cybatrex

Member
The problem with your second method is that the same current (approximately) is flowing through the same series circuit between L1 and L2. You only count that current once! That is where your extra factor of two came from.

Sent from my XT1585 using Tapatalk
Thank you so much. To clarify,

I would take the amperage to either L1 or L2 and then multiply by 120V or 240V?

L1 X 240V = CORRECT 1? :D

(L1+L2) X 120V = CORRECT 2? :D
 

GoldDigger

Moderator
Staff member
Thank you so much. To clarify,

I would take the amperage to either L1 or L2 and then multiply by 120V or 240V?

L1 X 240V = CORRECT 1? :D

(L1+L2) X 120V = CORRECT 2? :D
1. Yes.
2. Yes.
For 240 you can use one L current value if the two are identical
If they are not identical, you can take the average and multiply it by 240. Algebraically this is 100% identical to the formula in 2. :)

Sent from my XT1585 using Tapatalk
 

gar

Senior Member
180526-0821 EDT

Cybatrex:

Change the word watts to volt-amperes. It may be watts, but only if the load is pure resistance like an incandescent bulb, or resistive heater,

.
 

ggunn

PE (Electrical), NABCEP certified
A common rookie mistake is to think that a 240V 100A 2 pole breaker delivers 200A @240V. 100A on L1 + 100A on L2.
 

Besoeker

Senior Member
I need assistance with determining my Watts correctly with Ohm's law using the amperage reading from my Fluke clamp meter. I welcome anyone who can correct the errors of my ways and offer a brief explanation so I can learn correctly.

Amperage Readings
L[SUP]1[/SUP]: 17.1 Amps
L[SUP]2[/SUP]: 17.0 Amps

Ohms Law
P(w) = V x I


Method 1: Calculated ohms law for each leg of the circuit, calculated them individually, then added the wattage together.
L[SUP]1[/SUP]: 17.1A x 120V = 2,052 Watts
L[SUP]2[/SUP]: 17.0A x 120V = 2,040 Watts

Total Watts[SUP]1[/SUP]: 4,092 Watts
[HR][/HR]
Method 2: Combined amperage for both legs and used 240V by combined average to calculate watts which is double Method 1.
L[SUP]1[/SUP]: 17.1 Amps
L[SUP]2[/SUP]: 17.0 Amps

L[SUP]t [/SUP]= 34.1 Amps x 240V = 8,184 Watts

Total Watts[SUP]2[/SUP]: 8,184 Watts


[HR][/HR]

Any help with this would be greatly appreciated.
:):):)
If it is 120-0-120V then I suppose L1*120 + L2*120 would be the more accurate. But the 0.1A would be in the neutral but probably not significant. If is is actually there.
 

Ingenieur

Senior Member
If it is 120-0-120V then I suppose L1*120 + L2*120 would be the more accurate. But the 0.1A would be in the neutral but probably not significant. If is is actually there.
that is more accurate
in this case the currents happened to be balanced
not always (or even usually) the case
 

kwired

Electron manager
I need assistance with determining my Watts correctly with Ohm's law using the amperage reading from my Fluke clamp meter. I welcome anyone who can correct the errors of my ways and offer a brief explanation so I can learn correctly.

Amperage Readings
L[SUP]1[/SUP]: 17.1 Amps
L[SUP]2[/SUP]: 17.0 Amps

Ohms Law
P(w) = V x I


Method 1: Calculated ohms law for each leg of the circuit, calculated them individually, then added the wattage together.
L[SUP]1[/SUP]: 17.1A x 120V = 2,052 Watts
L[SUP]2[/SUP]: 17.0A x 120V = 2,040 Watts

Total Watts[SUP]1[/SUP]: 4,092 Watts
[HR][/HR]
Method 2: Combined amperage for both legs and used 240V by combined average to calculate watts which is double Method 1.
L[SUP]1[/SUP]: 17.1 Amps
L[SUP]2[/SUP]: 17.0 Amps

L[SUP]t [/SUP]= 34.1 Amps x 240V = 8,184 Watts

Total Watts[SUP]2[/SUP]: 8,184 Watts


[HR][/HR]

Any help with this would be greatly appreciated.
:):):)
If you only have a two wire circuit you can't have a different current on one conductor then you have on the other, there is only one current path and current is same everywhere in the circuit. I'm sure accuracy of the measuring method is what resulted in the minor difference you did measure. That said and as others have been pointing out - you have ~17 amps at 240 volts and not ~17 amps at 120 volts - times two.

Add a third point in the system it gets more complicated, especially if you have 208/120 system. That situation you could possibly have 17 amps at 120 volts - times two, or even three. But that never can happen when there is only a two wire circuit.
 

Besoeker

Senior Member
that is more accurate
in this case the currents happened to be balanced
not always (or even usually) the case
Yes. And Gar's point about it being VA rather than W should be noted.

A point in passing. I usually use the letter or symbol for the unit - W rather that Watts.
This stems from losing a mark on an exam question for spelling out the unit in the answer.
I was told that spelling it out was the incorrect thing to do. IDK whether that's true or not. But it stuck with me.

Mods, forgive the digression.
 
I might be over-answering on this one, please forgive me if so.

Besoeker is correct - this is in units of VA, not watts, based on the inputs. Watts would actually be equal or less, but isn't relevant to this problem.

Method 1:
L[SUP]1[/SUP]: 17.1A x 120V = 2,052 Watts
L[SUP]2[/SUP]: 17.0A x 120V = 2,040 Watts

Total Watts[SUP]1[/SUP]: 4,092 Watts

Method 2:

L[SUP]1[/SUP]: 17.1 Amps
L[SUP]2[/SUP]: 17.0 Amps

L[SUP]t [/SUP]= 34.1 Amps x 240V = 8,184 Watts

Total Watts[SUP]2[/SUP]: 8,184 Watts
Method 1 is good, but Method 2 is toast.

With a 120/240 split-style transformer, each Line-to-Neutral leg is 120V, so it's 34.1 Amps (added across 120V legs), times 120V, not 240V, which is why the result doubled. Another way to look at it is - The average current on each side of the transformer - at (17.1+17.0)/2 - times the line-to-line voltage, 240V. 17.05A * 240V = 4,092VA

Revised Method 2 - 34.1A * 120V = 4,092VA
 

kwired

Electron manager
If it is 120-0-120V then I suppose L1*120 + L2*120 would be the more accurate. But the 0.1A would be in the neutral but probably not significant. If is is actually there.
I would say that is "more accurate" if there is a neutral conductor involved.

If it is just a two wire circuit - then it is simply V x A. The fact he measured .1 amps difference on one line vs the other is an accuracy issue with measuring method - the current has to be the same on both lines as there is no where for any differential current to go to, outside of potential fault current issues, but if those are present we have an unintended multiwire circuit and current can be flowing in more then just a single loop.
 

ggunn

PE (Electrical), NABCEP certified
I would say that is "more accurate" if there is a neutral conductor involved.

If it is just a two wire circuit - then it is simply V x A. The fact he measured .1 amps difference on one line vs the other is an accuracy issue with measuring method - the current has to be the same on both lines as there is no where for any differential current to go to, outside of potential fault current issues, but if those are present we have an unintended multiwire circuit and current can be flowing in more then just a single loop.
Kirchoff lives! :D
 

ggunn

PE (Electrical), NABCEP certified
A point in passing. I usually use the letter or symbol for the unit - W rather that Watts.
This stems from losing a mark on an exam question for spelling out the unit in the answer.
I was told that spelling it out was the incorrect thing to do. IDK whether that's true or not. But it stuck with me.
IMO you had a jerk for an instructor who warped you for life. :D
 

Cybatrex

Member
I might be over-answering on this one, please forgive me if so.

Besoeker is correct - this is in units of VA, not watts, based on the inputs. Watts would actually be equal or less, but isn't relevant to this problem.



Method 1 is good, but Method 2 is toast.

With a 120/240 split-style transformer, each Line-to-Neutral leg is 120V, so it's 34.1 Amps (added across 120V legs), times 120V, not 240V, which is why the result doubled. Another way to look at it is - The average current on each side of the transformer - at (17.1+17.0)/2 - times the line-to-line voltage, 240V. 17.05A * 240V = 4,092VA

Revised Method 2 - 34.1A * 120V = 4,092VA
Thats exactly what I needed to see. Perfect, thank you so much.
 
Top