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Joined: Sep 2001
Posts: 806
Will a standard single phase mechanical watthour meter work properly when used on 2 phases of a 208Y/120 system?

I am installing a bunch of instrumentation, a cabinet air conditioner, and some lighting in a shed located about 300' away from a building. The feeder run to the shed from the building is 2 phases, neutral, and ground of a 3 phase 208V system in the main building. The site owner wants a watthour meter on the side of the shed, so the instrument owner can be billed for the power consumed. Will a standard mechanical watthour meter read properly if the phase difference between the legs is 120 degrees rather than 180, or do i need the more expensive electronic CT type metering here?

Joined: May 2002
Posts: 30
The short answer to your first question is "no". The single phase meter will meter the 208v loads accurately, but not the 120 volt loads accurately. The 120 volt loads will meter approx 75%, but it will vary up or down based upon the power factor.

For a 200 amp or less service, the meter you should use for this is a 5 jaw "network" meter.

The cost difference between the meters is approx $90.00 (single phase -$30 vs network meter - $120).

I hope this helps

[This message has been edited by ameterguy (edited 01-06-2005).]

Joined: Apr 2002
Posts: 2,527
I agree with ameterguy. Ideally, a network meter is used for the described service, but there is a possibly less costly option that is also intended for the application.

Look at for a (remanufactured/tested) 120V form-12 class-100 or -200 meter. A 5-jaw socket is needed.

Another source is Austin International at

Joined: Sep 2001
Posts: 806
Thanks for the info! Ordering a 5 lug meter from Hialeah...

All the loads here will be 120V. One phase for lighting and A/C, the other one for the electronics.

Joined: Nov 2004
Posts: 138
Is it mandatory to isolate the electronics on its own phase?

Joined: Sep 2001
Posts: 806
I figured that it would help to protect the instrumentation from possible spikes caused by the A/C compressor cycling on and off. There will be a UPS unit backing some of the more critical gear up, as well.

All the electronics are inside a NEMA 12 rack enclosure, with an air conditioner hung on the side of it to maintain the internal temperature. The equipment will be running 24/7/365 for at least the next few years.

Joined: Dec 2005
Posts: 866
Likes: 4
Blondells theorem

n-1= meter elements : where n are the phases and neutral, subtract one that tells you how many elements the meter needs.

in your case 2ph + Neutral - 1 = 2 element meter for the correct voltage of the service

The product of rotation, excitation and flux produces electricty.
Joined: Sep 2005
Posts: 202
WFO Offline
"The site owner wants a watthour meter on the side of the shed, so the instrument owner can be billed for the power consumed. "

There might be some legallities here that your owner may want to check into. Not sure if this is permitted everywhere.

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