kWh vs. Demand load

[pete m.]

I am curious as to how many design professionals and inspectors would accept kWh in place of "maximum demand" with respect to 220.35? I would think that kWh calculated back into amps would only reflect the average usage for a given month and not the "maximum demand" but I have had discussions with some local inspectors who will accept kWh with respect to 220.35. Just looking for thoughts and/or opinions.

Thanks, Pete

 

[charlie b]

Re: kWh vs. Demand load

The State of Washington is fairly clear on this question. So too, I believe, is the NEC article you cited. The issue is ?power,? not ?energy.?

 

[ron]

Re: kWh vs. Demand load

kWH and Demand each have different information.
For the purpose of calculating a service or feeder, I think the kWH is not very helpful and not code compliant with 220.35.
The AHJ's that accept kWH; what periods of kWH is used? Are they using kWH over several three hour periods (measured at 3 hour increments) to determine continuous load?

 

[bonding jumper]

Re: kWh vs. Demand load

Just a note, most meters the PO CO uses contain infornmation that includes the Peak KW reached durring the last however many months.

 

[pete m.]

Re: kWh vs. Demand load

The way it was described to me by our local utility co. is that demand is measured in 30 minute increments at various times of the day. I do believe that the NEC is very conservative in it's required calculations. I have yet to see a service that has actually come close to the connected load or the calculated demand load in accordance with article 220. The NEC in 220.35 I believe is giving a very leniant way to determine existing load for the purposes of adding new. Would I be correct in saying that the kWh would simply give an average for the month and not the maximum demand? I appreciate the responses.

Pete

 

[charlie]

Re: kWh vs. Demand load

WOW! You guys are all over the place.

The kWh has nothing to do with anything except power over a period of time. During that period of time, the amount of power may fluctuate all over the place. The kWh total is only the amount of energy used during that time period.

If you wish to measure the total instantaneous demand and the meter has a wheel, you may use this method.

1. Look at the meter for a Kh (secondary watt-hour constant) number, this is the meter constant and indicates the energy used by the load for each revolution of the disk. If you are looking at a CT cabinet, someone will have marked a multiplier and is used to increase the Kh value for the CT ratio.

2. Count the revolutions of the disk for one minute, if you want a little more accuracy, use a little more time.

3. Multiply the number of revolutions in one minute by the Kh, and then by 60. Divide the resultant by 1000.

Example:

You are at a service that has a meter with a Kh of 3.6 and time the turns at 68 per minute.

(68)(3.6)(60) / 1000 = 14.688 kW

If this is a single phase service, the load in amperes is

14.688 / 0.24 = 61.2 amperes

This is just an instant in time and is of no use for any kind of load study.

In general, residential and small commercial meters do not have the ability to register the demand. Additionally, the pulses are every 15 seconds instead of 30 minutes for the demand meters.

 

[pete m.]

Re: kWh vs. Demand load

Thanks Charlie that should settle some debate for me. I very well could be mistaken in the time aspect for the demand readings, but I understood from our utility that the measurements were taken during a 30 minute period and I am not sure of the frequency during that 30 minutes? (Sorry, I just dont understand the workings of the cash register for the utility ) but your explanation of the kWh in relation to the demand is exactly what I was after.

Pete

 

[steve66]

Re: kWh vs. Demand load

My 2 cents:

The exception makes it clear that the "maximum demand" is the measure of average power demand over a 15-minute period. Many commercial meters are capable of measuring this "average demand over a 15-minuite period", and the power company usually records the maximum demand for billing purposes. Thus, one can frequently use 220.35 if the meter has been in place for a year or more.

Steve

 

[charlie]

Re: kWh vs. Demand load

Steve, it depends on the utility. Generally, we are looking for over 50 kW demand before we go to a demand rate and have that information available. I am sure that other electric utilities have different levels to obtain that rate.