Open Forum > 2s Meter in a 120/208V service (12s application)

We found some 2s meters installed in a 4 jaw socket with a 120/208 service. It is a single phase, three wire service off of a three phase bank.There are several things wrong with the install but I can see how it happened. It should have been a 5 jaw socket and a 12s network meter. We are fixing the install but the question is, how far off was our metering and by how much did we under (maybe over but I don't think so) charge the customer? Thanks for any ideas.

August 9, 2012 | Unregistered CommenterCharlie Tracy

If you have only 208 volt loads connected to a 120/208 Network circuit with a form 2S meter, it will meter the loads correctly.

If 120 volt loads are connected to a 120/208 Network service, a form 2S meter will not meter these correctly. The 120 volt loads are measured at 75% which is a consistent error for these loads at unity power factor.

Thus, if you consider how the 120 volt loads are measured and want to measure combined 120 and 208 volt loads connected to a 120/208 Network service a form 2S meter, it cannot and will not meter these correctly based upon the 75% error factor.

The overall accuracy of the combined load, with this metering error, is totally dependent on the load balance between the 120 V loads and 208 V loads.

Higher 208 V loads will increase the overall accuracy and heavier 120 V loads will decrease the overall accuracy (with the worst case being at a 75% accuracy).

Note: With all three load set-ups, a slight additional error will be added on an electromechanical meter where the meter voltage coil is energized at 208 V rather than at the rated 240 V.

Thanks for the question and Be Safe All Ways,

By the way, in our Proofs & Truths book there are 3 examples of diagrams showing the 2S meter on a 12S Network and the math (with vectors) that proves these loads.

August 16, 2012 | Unregistered CommenterBrad

I'm a meter tech and found the opposite error. I found a 12s meter connected to a 120/240 service with the 5th jaw connected to neutral. I have the same question as to if the 12s meter was correctly metering. Btw the 12s meter found was an Alpha multi ranging 120v-480v. My feeling is it was metering correctly but wanted to verify .

January 3, 2013 | Unregistered CommenterDan Flosi

Thanks for your additional input concerning a 12S meter installed on a 120/240 volt, three wire single phase system. A Form 12S meter is the Blondel’s compliant way to meter this system and you are totally correct in your knowledge, that this meter will measure any of the connected loads that a customer might have.
According to Blondel’s Theorem a three wire system requires a two element meter to correctly measure all loads connected to this system. So, to be Blondel’s compliant a two element Form 12S meter would be the (Blondel’s) correct meter for this application.
We generally meter a single phase 120/240 volt, three wire single phase system with a Form 2S meter. In using this single element meter this way, we cheat Blondel’s (which is accepted by metering industry standards) as a correct metering alternative on a three wire system.
Be Safe All Ways,

January 6, 2013 | Unregistered CommenterBrad

Interesting posts, thanks. I have an unusual situation I'm not sure how to approach.

this is a commercial building with a single phase 120/240V, 800 service. there is a main breaker, feeding into an 800 amp, transfer switch. from the common side of the transfer switch, wiring runs into gutters which then have individual 100 amp single phase, 4 jaw meters connected to tenant load panels. a new building owner called and asked me to comment on the installation as he wants to make some meter changes due to the present transfer switch arrangement.

Obviously, all generator power is being metered and subsequently paid to the utility company. I explained this to him and he wants to correct it so he can expect some return on generator provided stand-by power. (please note there are no 'qualified' AHJ's here).

there is no physical space to install individual transfer switches, or an additional meter for gen power. the only way it can be done is to install a CT type 'master meter' before the main, then purchase and install individual meters and plug them in and make arrangements with tenants to be billed by landlord. this way landlord pays utility for all building consumption provided by utility company. then landlord provides billings to all tenants for power provided regardless of whether it is utility or generator provided.

however, in the course of investigating this, I was curious how they landed the neutrals in the (32 each) 4-jaw meter sockets, and at first was pleased to see the neutral passing straight through to tenant load centers and only the EGC was landed to meter base. THEN, i took another look at the generator. due to size needed to accommodate the 800 amp service, they used a 3-phase, 208/120 genset utilizing only two phases to transfer switch. so, when power is coming from utility the tenants have 120/240 power, and when on generator, they have 120/208 power.

regardless of all the numerous violations and non-standard wiring at this facility, my question(s) is/are, could i go ahead and recommend the master CT type meter for the utility 120/240 service, and conceivably install some standard I-70S type sub meters and expect them to meter properly if and when there is emergency standby 120/208 power from the generator?

or, do i just have a whole basket of problems here? (which i am beginning to believe i do).

hope this gets read by someone.



April 15, 2013 | Unregistered Commenterjoe c.

I am not certain that I have a total grasp on the installation you are inquiring about here or what needs to be done to correct it, but it sounds like your thinking as to how to approach the Master Meter is correct. I can, however in the mean time, speak to how to meter the single phase circuits no matter what the source is [120/240 or 120/208].

Are the single phase meter sockets that you are working with, new enough to retro-fit, so that you could purchase a kit for each of the sockets that will add the 5th terminal?

Once you have installed the kits, then you would proceed with installing 120 volt meters, using either the Form 12S or Form 25S. So, as you retrofit the sockets to have 5 terminals and install either the 12S or 25S meter, then the metering will be correct for both voltages and services.

The 120/240 single phase 3 wire circuits, supplied by the company, is supposed to be metered using a two element meter. This is according to Blondel’s Theorem, but we cheat Blondel’s often with a Form 2S meter. The 120/208 poly-phase 3 wire Network service is always metered with a 12S or 25S.

Does this make sense and does it seem like it will work?

If you are unable to make these changes to the meter sockets, then installing the Form 2S meters will be okay. The Form 2S will meter the 120 and 240 volt loads correctly when the source is from the company. Just remember, that the 120 volt loads, while the source is from the local generator, will only be metered at 75% and the 208 volt loads will meter close to accurate.

If you have further questions regarding this or any other installation, please contact me any time via e-mail or by posting in this forum or on our Q&A page.

Be Safe All Ways,

April 21, 2013 | Unregistered CommenterBrad