# Sub-metering: need neutral or not?



## Rainwater01 (Oct 6, 2015)

Have you looked for 2 and 3 wire meters? My thought is a 2 wire meter would know there’s an unbalanced load and do the math to figure out what the unbalanced load is. Somebody else may know how the meter calculates it but every single phase meter I’ve seen has been 2 wire and I doubt they would have made it this long if either the customer or power company were getting ripped off because that’s what would be happening right?


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## SWDweller (Dec 9, 2020)

I worked around a Honeywell control system, long ago. They monitored one phase and then did math for the other two. We found the system to be wildly in accurate. IMO if your monitoring electricity you need to monitor all of the phases.

You fail to mention if this is a utility grade situation. If your going to bill from this monitor be as accurate as you can. Follow all of the local regs and laws.


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## ScooterMcGavin (Jan 24, 2011)

Blondel’s Theorem dictates the rules for accurate electrical metering. 

Simply stated: Count the number of wires in any circuit (including the neutral). One less stator than the total number of wires in the circuit is required to correctly meter the energy flowing in the circuit. 

For example: 

A 2-wire circuit requires a single stator meter. 

A 3-wire circuit requires a two-stator meter. 

A 4-wire circuit requires a three-stator meter. 

In practice, Blondel’s Theorem is not strictly adhered to in all metering applications. Meter manufacturers have found ways to design special meters that allow adequate accuracy without the required number of stators. One such meter is the common (form 2S) house meter. It is a single stator meter that is specifically designed to meter a 3-wire circuit. In addition, some metering circuits may be connected in special configurations, which may also provide adequate levels of accuracy without the required number of stators. 

A quite appropriate question to ask is, "Was Blondel wrong"? The answer is, "No"! Any time a metering circuit does not strictly adhere to Blondel’s Theorem, it is subject to inaccuracies when the voltages present in the circuits are not balanced. Once again, to emphasize the previous sentence, when cheating on Blondel’s Theorem, it is the balance of the voltages that are important, not the balance of currents flowing in the circuits! Most voltages in modern alternating current circuits are adequately balanced to permit these deviations from the rules laid forth by Blondel.


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## joe-nwt (Mar 28, 2019)

burn bhad black said:


> I'm metering a subpanel (240V, 1 Phase, 3 Wire), does the meter need to also also monitor the neutral because of the unbalanced 120V loads ?


Does the meter monitor the neutral on the utility meter? Why would a meter on a subpanel be any different?


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## micromind (Aug 11, 2007)

joe-nwt said:


> Does the meter monitor the neutral on the utility meter? Why would a meter on a subpanel be any different?


Exactly.....

The PUCO uses 4 jaw meter sockets for 120/240 single phase, the neutral passes through and is not connected to the meter. 

If the single phase is 2 legs of a Y, then it does need the neutral reference.


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## Almost Retired (Sep 14, 2021)

micromind said:


> Exactly.....
> 
> The PUCO uses 4 jaw meter sockets for 120/240 single phase, the neutral passes through and is not connected to the meter.
> 
> If the single phase is 2 legs of a Y, then it does need the neutral reference.





burn bhad black said:


> I'm metering a subpanel (240V, 1 Phase, 3 Wire), does the meter need to also also monitor the neutral because of the unbalanced 120V loads ?
> 
> 
> Many meters only monitor the L1 & L2 without a neutral. This would be ok for 240V loads as both legs draw the same current, but the 120V loads will mostly be unbalanced (eg: L1 could draw 10 amps & L2 could draw 5A ) so without a neutral would a 2 wire meter give a true kWhr reading.
> ...


the current from both supply legs is monitored as it comes in to the system, equal or not does not matter. Monitoring the neutral would require that it be subtracted to prevent it from canceling part of the incoming reading. Think about trying to amp clamp a lamp cord without splitting it, it cancels and shows zero.


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## burn bhad black (Apr 4, 2018)

Great, thank you very much for all your input. Solved my question. 

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## micromind (Aug 11, 2007)

Almost Retired said:


> the current from both supply legs is monitored as it comes in to the system, equal or not does not matter. Monitoring the neutral would require that it be subtracted to prevent it from canceling part of the incoming reading. Think about trying to amp clamp a lamp cord without splitting it, it cancels and shows zero.


If you used 2 Amprobes, you'd get the true current. 

That's how a PUCO 4 jaw meter works.


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## getting old (Mar 26, 2021)

5 or 10 years ago they started having us install 5 jaw meter cans, with the 5th jaw a neutral. Started in the capital, and worked its way from there. Never knew what the 5th jaw was for, as they still installed the digital ones in the 4 jaw bases, and I'm pretty sure they just install 4 jaw meters in the 5 jaw bases. Anyone here know what's up with that? No longer live in that state, so haven't thought about it till now.

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## CraziFuzzy (Jul 10, 2019)

getting old said:


> 5 or 10 years ago they started having us install 5 jaw meter cans, with the 5th jaw a neutral. Started in the capital, and worked its way from there. Never knew what the 5th jaw was for, as they still installed the digital ones in the 4 jaw bases, and I'm pretty sure they just install 4 jaw meters in the 5 jaw bases. Anyone here know what's up with that? No longer live in that state, so haven't thought about it till now.
> 
> Sent from my SM-G970U using Tapatalk


Pretty sure the areas that required the 5 jaws did so for the possibly of using more advanced metering down the road, then never did so. 5 jaw meters are, I believe, are more accurate with highly imbalanced loads, because the imbalance will cause a different voltage drop for each leg, and a 4 jaw meter won't know.


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## micromind (Aug 11, 2007)

getting old said:


> 5 or 10 years ago they started having us install 5 jaw meter cans, with the 5th jaw a neutral. Started in the capital, and worked its way from there. Never knew what the 5th jaw was for, as they still installed the digital ones in the 4 jaw bases, and I'm pretty sure they just install 4 jaw meters in the 5 jaw bases. Anyone here know what's up with that? No longer live in that state, so haven't thought about it till now.
> 
> Sent from my SM-G970U using Tapatalk


Around here, the 5th jaw is used only if the source is 2 legs of a 120/208 Y. Even the new 'smart meters' still use only 4 jaws if it's 120/240.


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## joe-nwt (Mar 28, 2019)

I've been told a single phase meter with 4 jaws are designed for split phase and cannot properly measure the voltage on 208 because of the phase angle. But I'm no metering expert.


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## ComTowerSparky (May 28, 2021)

micromind said:


> Around here, the 5th jaw is used only if the source is 2 legs of a 120/208 Y. Even the new 'smart meters' still use only 4 jaws if it's 120/240.


Same here......5th jaw for 120/208......kinda rare but see it occasionally.


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## oldsparky52 (Feb 25, 2020)

I guess the reason for the 5th jaw is because of the phase angles of the supply.

Single phase 120/240, a 20 amp load across the two ungrounded conductors is the same KW (ignoring PF) whether it is connected as two 1-pole circuits (on opposite phases) or one 2-pole circuit. 20x120x2=4,800kw and 20x240x1=4,800. 

Single phase 120/208 would need to know the neutral current because if the two ungrounded phases each had a 20-amp load; connected as two 1-pole circuits (on opposite phases) would be 20x120x2=4,800kw and connected as one 2-pole circuit would be 20x208x1=4,160kw. 

But if they were measuring neutral current wouldn't there be 6 jaws and not 5? 

It's amazing how much I don't know about a trade I made a living in.


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## joe-nwt (Mar 28, 2019)

They don't measure neutral current. The "tickler" is just a small wire from the neutral to the 5th jaw. If anything, it's so the 120V metering elements actually see 120V for a more accurate reading. Two 120V elements in series on 1PH 240V will see 120V each whether there is a neutral connection or not. Without the neutral on 208 1PH each element in series would only have 104V across it even though 120V is being supplied.


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## McClary’s Electrical (Feb 21, 2009)

I have never seen a meter measure the neutral. I have seen fifth jaw meters, but they operate off the neutral, not measure it. I don’t believe a 6 jaw meter exist, which is what would be needed to measure the neutral in your application.


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## CraziFuzzy (Jul 10, 2019)

The neutral jaw is used for voltage measurement, so each measured line can be accurate to the voltage and phase. I believe the before mentioned 120/208 measuring people are mentioning is not 3-phase to that service, but two legs of a 3-phase 120/208 Y service acting "like" a 120/240 split. Because the lines aren't 180 deg off each other, the voltage is needed to add the two lines to calculate a proper total kW.


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## VELOCI3 (Aug 15, 2019)

Just go on the EKM Metering site. They have different meters and specific wiring requirements. 









Submetering Solutions For Any Scale & Budget


EKM designs and sells smart submeters and metering systems for electric kWh energy, water, and gas.




www.ekmmetering.com






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## oldsparky52 (Feb 25, 2020)

CraziFuzzy said:


> The neutral jaw is used for voltage measurement, so each measured line can be accurate to the voltage and phase. I believe the before mentioned 120/208 measuring people are mentioning is not 3-phase to that service, but* two legs of a 3-phase 120/208 Y service acting "like" a 120/240 split.* Because the lines aren't 180 deg off each other, the voltage is needed to add the two lines to calculate a proper total kW.


Isn't the problem that they really don't act like each other?


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## CraziFuzzy (Jul 10, 2019)

oldsparky52 said:


> Isn't the problem that they really don't act like each other?


Yes. Thus the need for the 5th jaw so it can measure how they are different.


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## oldsparky52 (Feb 25, 2020)

joe-nwt said:


> They don't measure neutral current. The "tickler" is just a small wire from the neutral to the 5th jaw. If anything, it's so the 120V metering elements actually see 120V for a more accurate reading. Two 120V elements in series on 1PH 240V will see 120V each whether there is a neutral connection or not. Without the neutral on 208 1PH each element in series would only have 104V across it even though 120V is being supplied.


Do you know how a 5 jaw meter knows whether 20 amps on each ungrounded conductor is to be invoiced at 4,160 kw (one 2-wire circuit) or 4,800 kw (two 2-wire circuits)?


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## CraziFuzzy (Jul 10, 2019)

oldsparky52 said:


> Do you know how a 5 jaw meter knows whether 20 amps on each ungrounded conductor is to be invoiced at 4,160 kw (one 2-wire circuit) or 4,800 kw (two 2-wire circuits)?


Volts times amps would be my guess.


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## joe-nwt (Mar 28, 2019)

oldsparky52 said:


> Do you know how a 5 jaw meter knows whether 20 amps on each ungrounded conductor is to be invoiced at 4,160 kw (one 2-wire circuit) or 4,800 kw (two 2-wire circuits)?


If the voltage reference for each leg is 120V, then 120V x 20A  x 2 = 4800watts. Without the neutral reference it would be 104V x 20A  x 2 = 4160watts. 

What I'm tying to say is I think even though a 4 jaw meter has no reference to neutral, it must have two 120V elements in series and can use each element as the 120V for each leg. I think the 5th jaw is simply a wire to the center of those two elements.

Lets say you only had 20A on one leg on a 120/208 single phase.

The load is referenced to neutral so 120V x 20A =2400watts. Without that neutral reference at the meter what would it use, 208V or 104V to calculate watts?


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## oldsparky52 (Feb 25, 2020)

joe-nwt said:


> If the voltage reference for each leg is 120V, then 120V x 20A  x 2 = 4800watts. Without the neutral reference it would be 104V x 20A  x 2 = 4160watts.
> 
> What I'm tying to say is I think even though a 4 jaw meter has no reference to neutral, it must have two 120V elements in series and can use each element as the 120V for each leg. I think the 5th jaw is simply a wire to the center of those two elements.
> 
> ...


Okay, I need you to clear up some terms for me. What are these "elements" in the meter you are referring to?


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## oldsparky52 (Feb 25, 2020)

joe-nwt said:


> If the voltage reference for each leg is 120V, then 120V x 20A  x 2 = 4800watts. Without the neutral reference it would be 104V x 20A  x 2 = 4160watts.
> 
> What I'm tying to say is I think even though a 4 jaw meter has no reference to neutral, it must have two 120V elements in series and can use each element as the 120V for each leg. I think the 5th jaw is simply a wire to the center of those two elements.
> 
> ...


How does the meter distinguish between a 4,160 watt load wired on 208 and two 2400 watt loads on 120V and different phases? Both send 20 amps through the meter on the ungrounded conductors. How does the meter know the difference when it does not monitor the neutral?


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## joe-nwt (Mar 28, 2019)

A meter has a voltage element and a current element. combined they give you a wattage reading. 

I'm pretty sure they are called elements but it's been 35years and you have me doubting myself now....  

I'll get back to your other question after supper.


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## emtnut (Mar 1, 2015)

Picture helps.
This is the standard 1.5 element meter. If it didn't have a neutral reference, then the voltage element reads 208V, and then would base the metering for the 120V legs at 104V. Clear it up at all ?


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## oldsparky52 (Feb 25, 2020)

Okay, let me ask this a different way. What looks different to the element between the two loads I'm talking about. The current sensor part of the element will sense the 20 amps on each leg, the voltage sine wave would be the same, the voltage would be the same, ... what is different that the element can detect?


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## emtnut (Mar 1, 2015)

oldsparky52 said:


> Okay, let me ask this a different way. What looks different to the element between the two loads I'm talking about. The current sensor part of the element will sense the 20 amps on each leg, the voltage sine wave would be the same, the voltage would be the same, ... what is different that the element can detect?


On a 240V service, the voltage element reads 240V. Without a neutral, it just splits that to 120V each leg. That will be accurate.
On a 208V service, the voltage element reads 208V. Without a neutral, it splits that and assumes 104V each leg. That will NOT be accurate.


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## oldsparky52 (Feb 25, 2020)

emtnut said:


> Picture helps.
> This is the standard 1.5 element meter. If it didn't have a neutral reference, then the voltage element reads 208V, and then would base the metering for the 120V legs at 104V. Clear it up at all ?
> 
> View attachment 158778


What's the difference in those two drawings?


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## oldsparky52 (Feb 25, 2020)

emtnut said:


> On a 240V service, the voltage element reads 240V. Without a neutral, it just splits that to 120V each leg. That will be accurate.
> On a 208V service, the voltage element reads 208V. Without a neutral, it splits that and assumes 104V each leg. That will NOT be accurate.


I understand that. That is not my question.

How does it know when when a 20 amp load (this is a 120/208 single phase service) on each leg is connected to 4800 watts on 2 equal 120V loads or when it's connected to a 208V circuit with is 4160 watts.


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## emtnut (Mar 1, 2015)

oldsparky52 said:


> I understand that. That is not my question.
> 
> How does it know when when a 20 amp load (this is a 120/208 single phase service) on each leg is connected to 4800 watts on 2 equal 120V loads or when it's connected to a 208V circuit with is 4160 watts.


It doesn't. All the meter knows is current on each leg, and the phase to phase voltage


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## emtnut (Mar 1, 2015)

oldsparky52 said:


> What's the difference in those two drawings?


Just the comments about the Blondel compliance. It's the same meter, it's just part of a powerpoint presentation.


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## joe-nwt (Mar 28, 2019)

Maybe this chicken scratch will make more sense. Blue is 120/240 1PH, Red is 120/208 1PH


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## joe-nwt (Mar 28, 2019)

If you take away either 6ohm load you still have a 20amp load at 120 volts on the other leg but the voltage element for the metering for that leg would only be 104V.


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## oldsparky52 (Feb 25, 2020)

If you have 20 amps through both legs on this 120/208 single phase service. How many watts are you going to be charged for? 


joe-nwt said:


> Maybe this chicken scratch will make more sense. Blue is 120/240 1PH, Red is 120/208 1PH
> 
> View attachment 158782


What is the WT unit and why is there a wire leaving it going to the loads?


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## joe-nwt (Mar 28, 2019)

Wt is watts total. The neutral has an arrow on either side of WT to indicate no connection. 

Hey, I told you it was chicken scratch.....


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## joe-nwt (Mar 28, 2019)

Obviously there will be some added gadgetry, mechanical or electronic, to convert watts to watthours.


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## oldsparky52 (Feb 25, 2020)

joe-nwt said:


> If you take away either 6ohm load you still have a 20amp load at 120 volts on the other leg but the voltage element for the metering for that leg would only be 104V.


So the power company would undercharge for a 120V load in that scenario?


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## oldsparky52 (Feb 25, 2020)

emtnut said:


> It doesn't. All the meter knows is current on each leg, and the phase to phase voltage


Wouldn't it know the phase to neutral voltage from the 5th jaw?


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## oldsparky52 (Feb 25, 2020)

joe-nwt said:


> Wt is watts total. The neutral has an arrow on either side of WT to indicate no connection.
> 
> Hey, I told you it was chicken scratch.....


No, the problem is I never went to school.


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## oldsparky52 (Feb 25, 2020)

joe-nwt said:


> Maybe this chicken scratch will make more sense. Blue is 120/240 1PH, Red is 120/208 1PH
> 
> View attachment 158782


One more observation question. If this was a 208V feed, the 0A shown on the neutral would change to 20A.


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## emtnut (Mar 1, 2015)

oldsparky52 said:


> Wouldn't it know the phase to neutral voltage from the 5th jaw?


Sorry Oldsparky ... I thought you were wondering why a standard meter (4 jaw) wouldn't work (be accurate) on a 120/208V feed.

Yes, on a 5 jaw meter, it would have both phase to neutral voltages, along with the 2 CTs, and all is measured properly.


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## oldsparky52 (Feb 25, 2020)

I think I figured out the answer to my question. Peak amperage in a 208V circuit will happen when the line to neutral voltage is 104V and peak amperage on on two 120V loads happens when line to neutral voltage is at 120V.

I don't know why it took me so long to comprehend this. 

This helped me


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## joe-nwt (Mar 28, 2019)

oldsparky52 said:


> This helped me


No it didn't. It was the chicken scratch.


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## oldsparky52 (Feb 25, 2020)

joe-nwt said:


> No it didn't. It was the chicken scratch.


That started it.


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## joe-nwt (Mar 28, 2019)

LOL! No! MY chicken scratch!

All in good fun. Sound like you're starting to get a grip on the 5jaw meter thing?


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## CraziFuzzy (Jul 10, 2019)

oldsparky52 said:


> Wouldn't it know the phase to neutral voltage from the 5th jaw?


Yes... The discussion is "Why the 5th Jaw". The answer is this.


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