Utility Demand Meter Question

O

olc

Senior Member
Typical utility demand meter (208V 3Ph using current transformers):

Is the measured demand instantaneous? (as opposed to averaged over half an hour or whatever)

If the load is not balanced, does it read the highest leg?

Usually KW or KVA? (I’m not sure how this would shake out off the top of my head, see please chime in if this is a factor. The email from the utility said KW.)



The reason I ask: Usually when you see the highest demand it is much less than the connected load. I just received a demand, from the utility, that I don’t think could be reached if everything in the building were on at once.
(This case is a small town highway garage with a lot of non-continuous loads like air compressor, truck lift, door openers. (the service size is 225A, the demand they gave is 89.6KW.)
 
In my experience, peak demand is measured in a 15 minute segment, consisting of 3 consecutive rolling 5 minute intervals. This may vary by utility.
3 phase power is measured, so imbalance due to single phase loads should not be a factor. Demand is expressed in KW, not KVA.
That demand number seems really high for that service. You need to ask the utility how they measure and calculate it.
 
By definition, demand load is the maximum recorded load averaged over some time period. The time period (in the US) is normally 15 min, 30 min or 60 min depending on the utility. It's not possible for the demand load to be more the ACTUAL connected load (during that billing cycle)

Some utilities have a "ratchet" so that the customer has to pay a charge based on the highest monthly demand for past 12 months.

That demand kW isn't impossible based on the service size, but does seem high. There is always a meter multiplier that has to be applied and with CT metering, the CT ratio is another source of error. You might ask the utility to double check their data. Does the kWh used seem reasonable?
 
Asking the utility what the demand interval is is going to be better than guessing or searching the internet.

I've read that some of the demand meters measure every five minutes but the demand charge may be the highest result of 3 consecutive measurements. Probably depends on how old the meter is, whether it's analog or digital, etc...

D. Castor said:
...

Some utilities have a "ratchet" so that the customer has to pay a charge based on the highest monthly demand for past 12 months.
Click to expand...
Or the "ratchet" resets monthly for a monthly charge.
D. Castor said:
That demand kW isn't impossible based on the service size, but does seem high. There is always a meter multiplier that has to be applied and with CT metering, the CT ratio is another source of error. You might ask the utility to double check their data. Does the kWh used seem reasonable?
Click to expand...
Agree. It's notable that the given demand of 89.6KW is higher than the nominal service rating (81kVA). Could happen for an hour without tripping the main, if the load was balanced, and especially if the voltage was running high. But just doesn't seem ... right.
 
olc said:
Typical utility demand meter (208V 3Ph using current transformers):

The reason I ask: Usually when you see the highest demand it is much less than the connected load. I just received a demand, from the utility, that I don’t think could be reached if everything in the building were on at once.
(This case is a small town highway garage with a lot of non-continuous loads like air compressor, truck lift, door openers. (the service size is 225A, the demand they gave is 89.6KW.)
Click to expand...
What state are you in and what utility? And what were measured demands from a few previous bills? Seems like they made an error to me.
 
As D.Castor alluded too,

1) Check to see if they recently replaced the meter. It is a common metering error to accidentially program the CT ratio incorrectly resulting in a correlating incorrect value.

2) New utility billing software can also create and error on the multiplier for the metering/billing data.

3) Use previous months billing as a metric. Is their average bill 89.6kW? For how long has it been this high? The utility should hold a record on that for a couple of years. If it jumped up by a ratio of 5:1, 20:1 etc. then you can guess it is a multiplier error.

If all that is not easily come by, put a power monitor on there for a week and see if the monitor gives you a similar value.
 
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