# MWBC calculation



## inetdog (Apr 13, 2016)

TOOL_5150 said:


> My brain went totally blank on how to find the total Watts consumed on a MWBC when I know the voltage (120/240) and the amperage of each leg (L1 - 13.2A, L2 - 10.2A, N - 11.6A). My goal is to find out the kWh used for this circuit, and the load is constant. Can someone help me out?


Those numbers sound a lot more like two legs of 208Y/120 three phase than 120/240 single phase.
If it really is 120/240, then the power factors are off the wall wierd and so calculating watts rather than VA will require more information than you have given.

If all you need is VA and you have only line to neutral loads, just take 13.2 times 120 plus 10.2 times 120.

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## telsa (May 22, 2015)

TOOL_5150 said:


> My brain went totally blank on how to find the total Watts consumed on a MWBC when I know the voltage (120/240) and the amperage of each leg (L1 - 13.2A, L2 - 10.2A, N - 11.6A). My goal is to find out the kWh used for this circuit, and the load is constant. Can someone help me out?


You crazy.

The current on the Neutral has NOTHING to do with Watts calculations. 

It's a 'dump line' energy wise.

Just work out the V L-N for each leg and use the power wheel formula:

Watts = E x I

DONE.

( 13.2 + 10.2 ) x 120VAC =

You tell me.


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## Dennis Alwon (May 9, 2009)

TOOL_5150 said:


> My brain went totally blank on how to find the total Watts consumed on a MWBC when I know the voltage (120/240) and the amperage of each leg (L1 - 13.2A, L2 - 10.2A, N - 11.6A). My goal is to find out the kWh used for this circuit, and the load is constant. Can someone help me out?


Where did these numbers come from. On a multiwire branch circuit the neutral will carry the unbalanced load.. L1= 13.2 amps, L2= 10.2 amps then the neutral will only carry 3 amps not 11.6 amps. 

For kwh just convert amps to watts and multiply by the number of hrs it will be on.


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## hardworkingstiff (Jan 22, 2007)

Dennis Alwon said:


> Where did these numbers come from. On a multiwire branch circuit the neutral will carry the unbalanced load.. L1= 13.2 amps, L2= 10.2 amps then the neutral will only carry 3 amps not 11.6 amps.
> 
> For kwh just convert amps to watts and multiply by the number of hrs it will be on.


If it is 2 legs of a 208Y/120 distribution the math for the neutral calculates to about 12 amps. PF and rounding errors should account for the math vs real world difference.


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## telsa (May 22, 2015)

The neutral is irrelevant.

Watts is Watts.

The Amps down the Hots dictate the Watts.

These are NOT Hot to Hot loads... so the E is 120 VAC.


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## Dennis Alwon (May 9, 2009)

hardworkingstiff said:


> If it is 2 legs of a 208Y/120 distribution the math for the neutral calculates to about 12 amps. PF and rounding errors should account for the math vs real world difference.


Good point - I was thinking single phase but obviously, it is a 3 phase system

I generally don't find a multiwire branch circuit with 2 hots on a 3 phase system


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## HackWork (Oct 2, 2009)

Can someone please tell Dennis that to convert to Kwh one needs to divide the result of his equation by 1000. Thanks.


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## hardworkingstiff (Jan 22, 2007)

HackWork said:


> Can someone please tell Dennis that to convert to Kwh one needs to divide the result of his equation by 1000. Thanks.


Power Factor might come into play, depending.


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## frenchelectrican (Mar 15, 2007)

Dennis Alwon said:


> Good point - I was thinking single phase but obviously, it is a 3 phase system
> 
> I generally don't find a multiwire branch circuit with 2 hots on a 3 phase system


It is more common than you think Dennis .,, especally with the 208Y120 or 480Y277 circuits. ( 415Y240 circuits in my location ) 

I have see it pretty often on wye system but delta system it get little more tricky depending on if wired for three or four wire delta.

but as far for OP sistuation the quickest way is add L-N loads from each phase then / 1000 X hours that will result a Kwh.


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## HackWork (Oct 2, 2009)

frenchelectrican said:


> but as far for OP sistuation the quickest way is add L-N loads from each phase then X 1000 X hours that will result a Kwh.


Divide by 1000 to turn watts into kilowatts.


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## frenchelectrican (Mar 15, 2007)

HackWork said:


> Divide by 1000 to turn watts into kilowatts.


Opps sorry.,, ran out of coffee .,,


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## HackWork (Oct 2, 2009)

frenchelectrican said:


> Opps sorry.,, ran out of coffee .,,


I forgive you, but not Dennis :thumbup:


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## hardworkingstiff (Jan 22, 2007)

Dennis Alwon said:


> Good point - I was thinking single phase but obviously, it is a 3 phase system
> 
> I generally don't find a multiwire branch circuit with 2 hots on a 3 phase system


Most single-family homes (at least where I've worked) are like the OP stated his voltages were, 120/240. The OP must have made and assumption about the voltage instead of actually checking.

I hate that we call 2 phases of a 3 phase system single-phase.


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## backstay (Feb 3, 2011)

hardworkingstiff said:


> Most single-family homes (at least where I've worked) are like the OP stated his voltages were, 120/240. The OP must have made and assumption about the voltage instead of actually checking.
> 
> I hate that we call 2 phases of a 3 phase system single-phase.


What's this we stuff?


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## HackWork (Oct 2, 2009)

backstay said:


> What's this we stuff?


A mouse in your pocket!


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## hardworkingstiff (Jan 22, 2007)

backstay said:


> What's this we stuff?


Just about everyone keeps telling me when you feed a home with 2/3rds of a wye system it's a single-phase service. Am I wrong about what I'm being told? Are they right about what they are telling me? 

I don't know if you remember but this has come up here and MH's multiple times and the point made by people more educated than I so I gave up.


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## Bird dog (Oct 27, 2015)

hardworkingstiff said:


> I hate that we call 2 phases of a 3 phase system single-phase.


Granted it's not technically correct, but, it's a quick way of communicating a type of circuit. We're tradespeople not lawyers.


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## hardworkingstiff (Jan 22, 2007)

Bird dog said:


> Granted it's not technically correct, but, it's a quick way of communicating a type of circuit. We're tradespeople not lawyers.


So does that mean you are part of "we" that backstay was questioning about?


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## Bird dog (Oct 27, 2015)

hardworkingstiff said:


> So does that mean you are part of "we" that backstay was questioning about?


Hmmm.. not sure. I would not assume just because I call something one thing that everyone does. When pulling wire or talking about equipment, I will use the phrase "single phase, 208v". But I don't know if everyone does. What phrases would you use?


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## HackWork (Oct 2, 2009)

In a 120/208v system we would normally say:

Three phase 208 for all 3 hots.
Single phase 208 for 2 hots.
And 120 for a hot and neutral.

In a 277/480v system we would say:

Three phase 480 for all 3 hots.
Single phase 480 for 2 hots.
And 277 for a hot and neutral.


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## frenchelectrican (Mar 15, 2007)

Sometime I will say 208 volt network so that is the other clue being a single phase 208 volts.

Same thing on 480 or 415 volt side as well.

some case it can be called spur.,,


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## hardworkingstiff (Jan 22, 2007)

Bird dog said:


> Hmmm.. not sure. I would not assume just because I call something one thing that everyone does. When pulling wire or talking about equipment, I will use the phrase "single phase, 208v". But I don't know if everyone does. What phrases would you use?


IDK, .67 3-phase? Actually maybe offset single-phase, but .... if "we" all agree 120/208 should mean 2 of 3 phases of a 3-phase wye, 120/240 should be a "traditional" singe-phase, and 208Y/120 is the whole 3-phase wye package.

I only questioned this because backstay ask who "we" was when I mentioned that I didn't like that "we" called 2 of 3 legs of a wye 3-phase source single-phase. Seems like a lot of people are calling it single-phase, I guess it is OK to call them we?

IDK, what say you backstay, enough people satisfy the "we" usage?


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## HackWork (Oct 2, 2009)

Liberals can't stop themselves from ruining technicals threads either.


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## inetdog (Apr 13, 2016)

hardworkingstiff said:


> IDK, .67 3-phase? Actually maybe offset single-phase, but .... if "we" all agree 120/208 should mean 2 of 3 phases of a 3-phase wye, 120/240 should be a "traditional" singe-phase, and 208Y/120 is the whole 3-phase wye package.
> 
> I only questioned this because backstay ask who "we" was when I mentioned that I didn't like that "we" called 2 of 3 legs of a wye 3-phase source single-phase. Seems like a lot of people are calling it single-phase, I guess it is OK to call them we?
> 
> IDK, what say you backstay, enough people satisfy the "we" usage?


One place it is confusing to call it 208 single phase is that some people will still expect current cancellation in the grounded wire (neutral).

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## MechanicalDVR (Dec 29, 2007)

HackWork said:


> Divide by 1000 to turn watts into kilowatts.


Divide kilowatts by 1000 to get watts.


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## HackWork (Oct 2, 2009)

MechanicalDVR said:


> Divide kilowatts by 1000 to get watts.


:no: You have to multiply kilowatts by 1000 to get watts.


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## telsa (May 22, 2015)

mechanicaldvr said:


> *multiply* kilowatts by 1000 to get watts.


f i f y.


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## RePhase277 (Feb 5, 2008)

frenchelectrican said:


> ( 415Y240 circuits in my location )


You see.... 'cause he's in the Philippines.


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## RePhase277 (Feb 5, 2008)

I've been think about 2 hots and a neutral from wye connected system for a while. I also don't like how it's called "single phase" because you can indeed get 3-phase power out of it.

At last I've thought of a term which might do: Bi-phase.

(Not the kind Dennis went through in his 20s.)


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## HackWork (Oct 2, 2009)

RePhase277 said:


> I've been think about 2 hots and a neutral from wye connected system for a while. I also don't like how it's called "single phase" because *you can indeed get 3-phase power out of it*.
> 
> At last I've thought of a term which might do: Bi-phase.
> 
> (Not the kind Dennis went through in his 20s.)


How so?


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## RePhase277 (Feb 5, 2008)

HackWork said:


> How so?


Well, it isn't standard, but let's say you had two 120 volt heaters and a 208 volt heater wired like a delta and connected to your bi-phase circuit, the currents and power would flow according to 3-phase equations.

Taken even further, you could connect three transformers to your circuit and get 3-phase from the secondaries.


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## HackWork (Oct 2, 2009)

RePhase277 said:


> Well, it isn't standard, but let's say you had two 120 volt heaters and a 208 volt heater wired like a delta and connected to your bi-phase circuit, the currents and power would flow according to 3-phase equations.
> 
> Taken even further, you could connect three transformers to your circuit and get 3-phase from the secondaries.


I like the way you've already adopted the term "bi-phase" without even putting it to a vote


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## Going_Commando (Oct 1, 2011)

hardworkingstiff said:


> Just about everyone keeps telling me when you feed a home with 2/3rds of a wye system it's a single-phase service. Am I wrong about what I'm being told? Are they right about what they are telling me?
> 
> I don't know if you remember but this has come up here and MH's multiple times and the point made by people more educated than I so I gave up.


Single family dwellings are typically only fed with 1 phase, not 2. On the primary side of the tub they hit one phase conductor, and hit the neutral on the other side. On the secondary you have two ends of your coil are you 2 "hots" and the center of the coil is grounded to give you 120/240.


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## RePhase277 (Feb 5, 2008)

HackWork said:


> I like the way you've already adopted the term "bi-phase" without even putting it to a vote


Every revolution has a first step.


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## frenchelectrican (Mar 15, 2007)

RePhase277 said:


> You see.... 'cause he's in the Philippines.


Yup .,, and just keep in your mind we are on 60 HZ as well .,, no 50 HZ stuff over here yet.,,


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## RePhase277 (Feb 5, 2008)

frenchelectrican said:


> Yup .,, and just keep in your mind we are on 60 HZ as well .,, no 50 HZ stuff over here yet.,,


I'll wrap that tidbit up in a nice neat little bow and swallow it down in case I need to order equipment for a job in the Philippines.


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## MechanicalDVR (Dec 29, 2007)

HackWork said:


> :no: You have to multiply kilowatts by 1000 to get watts.


Tried to squeeze that in quick as I was heading out, wanted to multiply rather than divide.


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## TOOL_5150 (Aug 27, 2007)

I shouldn't post when I'm that tired... It is 2 legs of a 120/208v 3 phase service. Thank you for the clarification fellas!


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## backstay (Feb 3, 2011)

hardworkingstiff said:


> IDK, .67 3-phase? Actually maybe offset single-phase, but .... if "we" all agree 120/208 should mean 2 of 3 phases of a 3-phase wye, 120/240 should be a "traditional" singe-phase, and 208Y/120 is the whole 3-phase wye package.
> 
> I only questioned this because backstay ask who "we" was when I mentioned that I didn't like that "we" called 2 of 3 legs of a wye 3-phase source single-phase. Seems like a lot of people are calling it single-phase, I guess it is OK to call them we?
> 
> IDK, what say you backstay, enough people satisfy the "we" usage?


I work solo, no "we" here! Other than voltage, if you had that panel open, you couldn't tell if it was single phase or 3 phase with a regular meter. Maybe that's why "you" call it single phase. 

Why do "you" call one leg of a single phase 120/240 system, a phase?


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## hardworkingstiff (Jan 22, 2007)

Smh


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