Real Time KW readings on an Electric Meter

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Shujinko

Shujinko

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I have a project that has an existing electrical meter measuring electrical values on a utility transformer. I am trying to read the real time KW on the meter digital display. It shows a small value for KW and the multiplier label on the meter is blank. My question is how do you obtain the correct multiplier to obtain the correct KW reading of the meter. Is the following correct?

KW x (meter multiplier x current transformer ratio x potential transformer ration) = total KW

The meter multiplier is given by the meter manufacturer and is unique to each meter?
 
I know many residential analog meters use a watthour constant of 7.2, meaning every rotation of the disc is 7.2 watthours.

I'm not aware of any digital meters having any such conversion. I know that the Itron meters have an IR LED on the top that blinks once per watthour, and each "blip" of the segments on the LCD is also equal to one watthour.

Given that the multiplier is blank, I would assume a multiplier of 1, however I have no experience with meters that give a reading of instantaneous watts.
 
Shujinko said:
I have a project that has an existing electrical meter measuring electrical values on a utility transformer. I am trying to read the real time KW on the meter digital display. It shows a small value for KW and the multiplier label on the meter is blank. My question is how do you obtain the correct multiplier to obtain the correct KW reading of the meter. Is the following correct?

KW x (meter multiplier x current transformer ratio x potential transformer ration) = total KW

The meter multiplier is given by the meter manufacturer and is unique to each meter?
Click to expand...


I recently was looking at a meter for the same info. It was marked with the multiplier. It cycled thru values and gave watt, total kwh, and demand. It wasn't clear which was demand and which was watts. I had to watch a few cycles to see which one changed.
 
Self contained (non transformer rated) meters (no CT's) will not have a multiplier. They will be rated CL200 (200A), 240V, 3W or some such. Instrument rated (CT) meters will be CL20 (20A) and voltage rating could be 120V or 120-480V. Older meters were not rated line voltage and required voltage transformers. Newer solid state meters are multi-voltage rated. If a 120V 4W meter needs to be used on 277V 4W, it would need 2.5:1 VT's (some folks still use PT, but it's the same thing). Some utilities use VT's on 277/480 and some do not. You're correct....multiplier = CT ratio X VT ratio. A meter using 200:5 CT's would have a multiplier of 40. if 2.5 VT's are used for 277/480, the multiplier would be 100. Digital meters will have a Ke value that indicated watthours per pulse. Demand is peak demand averaged over a set interval (usually 15 min). The disk emulator can be a blinking LCD, a scrolling series of LCD's or some other form of visual simulation of a disk. The IR LED is for use on a test bench that uses an IR pickup to measure Watthours per pulse. Just get the nameplate info from the meter face and go online to the manufacturer's website and it will explain what's displayed. Not all manufacturers do it the same way. The numerical displays are programmable, and will display whatever info the POCO programs.

Now, aren't you sorry you asked?:happyyes:
 
meternerd said:
Self contained (non transformer rated) meters (no CT's) will not have a multiplier. They will be rated CL200 (200A), 240V, 3W or some such. Instrument rated (CT) meters will be CL20 (20A) and voltage rating could be 120V or 120-480V. Older meters were not rated line voltage and required voltage transformers. Newer solid state meters are multi-voltage rated. If a 120V 4W meter needs to be used on 277V 4W, it would need 2.5:1 VT's (some folks still use PT, but it's the same thing). Some utilities use VT's on 277/480 and some do not. You're correct....multiplier = CT ratio X VT ratio. A meter using 200:5 CT's would have a multiplier of 40. if 2.5 VT's are used for 277/480, the multiplier would be 100. Digital meters will have a Ke value that indicated watthours per pulse. Demand is peak demand averaged over a set interval (usually 15 min). The disk emulator can be a blinking LCD, a scrolling series of LCD's or some other form of visual simulation of a disk. The IR LED is for use on a test bench that uses an IR pickup to measure Watthours per pulse. Just get the nameplate info from the meter face and go online to the manufacturer's website and it will explain what's displayed. Not all manufacturers do it the same way. The numerical displays are programmable, and will display whatever info the POCO programs.

Now, aren't you sorry you asked?:happyyes:
Click to expand...

So basically if I had a multiplier of 40 on a meter, I would simply multiply the meter readings by 40 (reading x 40) and that would be the final KW? Or is there an intermediate step that I am missing?
 
Shujinko said:
So basically if I had a multiplier of 40 on a meter, I would simply multiply the meter readings by 40 (reading x 40) and that would be the final KW? Or is there an intermediate step that I am missing?
Click to expand...

Wow...I haven't been on here for a while.....but your statement is kinda correct. You multiply the reading by the multiplier, but you are reading KWh, not KW. Demand is in KW, but it's peak KW within a period of time, usually 15 min. Meaningless for what you want.
 
meternerd said:
You multiply the reading by the multiplier, but you are reading KWh, not KW. Demand is in KW, but it's peak KW within a period of time, usually 15 min. Meaningless for what you want.
Click to expand...
You don't know (NEI). Meter could be set to display peak kW, instantaneous kW, both or even sub-interval kW.
 
Shujinko said:
I have a project that has an existing electrical meter measuring electrical values on a utility transformer. I am trying to read the real time KW on the meter digital display.
Click to expand...

If your client is the existing POCO customer, I would suggest simply asking client for 12 months of utility bills. If billing is demand based, the peak demand for each month will be listed on the bill and you will have a code basis for claiming maximum demand load on the system. This is what we do when we go in to add more equipment to an existing marginally-sized service.
 
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