CT meter accuracy if missized?

[Tracs]

A year and a half ago I had a 400 amp service put it. Ever since I feel the billed usage has been higher than ever before, especially in the low usage summer months.

I got talking to a retired power company employee and he was telling me that the CT transformer in the meter enclosure is sized based on the estimated electrical load. He said he thinks that if the CT was oversized it might not read accurately at low usage.

Mine is a 300:5 ratio based on my x60 bill multiplier.

My calculated demand load for house and shop are around 195 peak amps in the winter. Largest loads are a 18kw electric furnace in the house and same 18kw in the shop.

Wouldn't a 200:5 CT transformer be a better fit?

Would a 300:5 CT read inaccurate in the summer when I'm using a low amount of kWh?

 

[mm08822]

Typical revenue metering has an overall 0.5% accuracy.

You may have CT's at 0.5% accuracy but more likely 0.2% even 0.1% as the combined metering error can't exceed the 0.5%.

IIRC, the stated accuracy range is from ~5% ~125% of the stated ct value. This doesn't mean it isnt accurate outside of that range, but not guaranteed.

You would need the specs from the ct's and meter as a start.

Going to 200:5 ct's would put you at risk of exceeding the ct rating and even possibly burning them up. Admittedly, 200+ amps load is a lot, but someone determined 400a was needed.

Put a clamp-on on each leg during these "light load" periods and see how low it is.

Calling POCO about metering issues/concerns should get a fast response as they don't want complaints to the BPU.

Or buy your own metering system (emporia, sense, brultech) and compare the two.

5% of 300 is 15a and for 200 is 10a. Let's say the accuracy drifts to 2% in the 0 - 10/15a range. Are you really going to notice that for these intermittent light loads? (It could even be in your favor.)

ETA: My BPU only requires +/-2% accuracy (POCO or renewable metering) but POCO has been using 0.5% accuracy meters for at least the last 20 years.

 

[rlitman]

...Or buy your own metering system (emporia, sense, brultech) and compare the two...

Just be aware that split core CTs (clamp meters) will never meet the accuracy of solid core CTs.

 

[Chuckster in NJ]

5% of 300 is 15a and for 200 is 10a. Let's say the accuracy drifts to 2% in the 0 - 10/15a range. Are you really going to notice that for these intermittent light loads? (It could even be in your favor.)

I think this is why they have a "minimum charge" on commercial electric bills, because the 300:5 CT doesn’t "register" light loads.

 

[mm08822]

Just be aware that split core CTs (clamp meters) will never meet the accuracy of solid core CTs.

It was to see if there is a big disparity between billed and self-metered kwh. A few percent difference wasn't the point, but rather seeing a glaring difference between the two measurement systems. 5, 10, 15% would have me persue the concern.

The clamp meter was just to find if his lowest (perceived) usage patterns puts him on the 0-10/15 range. If he rarely is in this range then his worry is for naught.

My hunch is, OP is using more power than he realizes and POCO metering is correct.

 

[Innovate1]

Many meters accuracy is stated as a percentage of full scale. Not positive this applies to CTs but if so then having twice as big max rating gives half the accuracy at a given load. At low loads the accuracy goes down as a percentage of amount measured. As an example and using round numbers for a 100 A meter with 1% accuracy the accuracy would be within 1A. If you actually have 50A the accuracy is still 1A which is 2%. For 10 A the accuracy is still 1A which is 10% of the 10A. Accuracy is likely better than that at low loads but that's the rating.

Minimum charge is for infrastructure.

 

[wyliesdiesels]

I would get your own metering system and compare the 2 in real time and aggregate (when you get your bill). 195a peak demand is very high for a house....

 

[Tracs]

I would get your own metering system and compare the 2 in real time and aggregate (when you get your bill). 195a peak demand is very high for a house....

195a is only in the dead of winter when the house 18kw electric furnace and shop 18kw electric boiler simultaneously run along with oven and clothes dryer.

In retrospect I should never have went with a 400 amp service. For the cost I could have covered my electrical bill for 2 years and with load management, 200 amp service probably would have been sufficient.