Peak Demand Calculation

[sauba]

I am trying to calculate the peak demand in amps to a space. The utility company has informed me that the max reading to the space in a 15 minute period was 29.7 kWh. If i have 120/208 V service to the space, am I correct in calculating this as:

I=kWh/(V*T)=29.7*1000/(.25*208*SQRT(3))= 329.76 amps?

This doesn't seem right. I am concerned about the time. Should this just be 1 hour? I am not familiar with exactly what they mean by a reading within a 15 minute period.

I appreciate any help you can provide.

 

[raisinbran]

Sauba,
kWh (kilowatt-hours)is a measure of energy usage, not peak power. Power (or peak demand)has the units of kW (kilowatts). Assume you meant the peak demand was 29.7 kilowatts, then kW = 1.732*(volts)*(amps)* (power factor)/1000. If you are talking a heating load, power factor = 1, otherwise you must estimate the power factor to work bakwards to find the current.
Hope this helps,
Raisinbran

 

[sauba]

raisinbran,

Thanks for the response. That makes more sense.

Just to make sure I have it straight, the peak demand represents the average maximum power delivered to the space over a defined period of time such as 15 minutes? Is this determined by averaging a number of instantaneous readings?

The utility company also indicated that the maximum daily reading was 400 kWh. Does this represent the maximum energy usage?

Thanks for your help.

 

[Senselessticker]

Knowing energy used (kilowatt-hours) in a given amount of time does not yeild a way to accurately calculate peak demand of a system in terms of current (amps).

For example: Let assume that "energy used" (kWh) measurement was taken for 1 hour for a system. Let assume that measurement was 60kWh. There are an infinite number of possibilities for peak amp demand of the system during that time: The system could be drawing 1kW every minute for the duration of 60 mins. Or the system could be drawing 2 kW for exactly half of the time. Or the system could draw all 60kW within one minute and draw nothing the remaining 59 mins.,etc...

Hope this helps.

 

[richanton]

It would be helpful to know your space. Is it an office area or industrial. If industrial, are there any very large loads that operate for a short time. Does it operate 24/7 or only one 8 hour shift.

Then you can use the 400kwh/day, 29.7kwh for 15 minutes to guess your max demand load, but you are still guessing.

For example, if an office space, the 29.7 may have occurred on a 98 degree day with all the air conditioners running.

 

[sauba]

I think I have it now. Lots of good information available on utility provider websites, so with a little research it all started to make alot more sense.

I am assuming I a misheard the utility company and he said 29.7 kw rather than kwh and this is the peak demand. From this, I calculated amps.

Thanks everyone for your help.

 

[abcd3286]

I believe peak demand is measured in terms of Kw NOT Kw Hr.

The calculation used by utilities may vary by utility.
Seattle City light uses the average of the peak in the last 3 5 minute periods -- I believe.

Now that you got a number for demand the rate can vary by the utility. They can multiply this by whatever factor they want to get the billing rate.

Check with your utility - best bet -- go see the meter and relay guys.


They are the ones who taught me.

 

[jraef]

abcd3286 is correct,

Demand is measured in kW, useage is in kWH. Think of it like this: the kWH (useage) meter is like the odometer in your car, it shows how much distance you have traveled, regardless of speed. The Demand meter is like a speedometer that has a red needle that shows the fastest speed you have gone this month, as long as you held that speed for at least 15 minutes, and stays there until the meter reader resets it.

The functional concept on that meter is a heater element that doesn't change temperature very fast. As power is used at an ever increasing rate, the heater heats up and moves the needle, but it takes 15 minutes to heat that element up after an increase in power. It is all done electronically now, but it is modeled after the same concept.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Rocketir]

Ask to talk with the Engineer at the utility, Not the customer rep. The Engineer will know the right answer for your application.

 

[geso52]

Sauba,
Utility companies disregard any pick loads that last less than 15 minutes and they record peak loads that last 15 minutes or more. That is why when they provided you with the pick load (which more likely was in KW) they mentioned the 15 minutes interval.
Geso52