Determining instantaneous peak demand

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Coppersmith

Coppersmith

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Location
Tampa, FL, USA
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Electrical Contractor
I'm trying to determine if a homeowner needs a service upgrade. It's a 150 amp service on a 4500 sq ft house. They don't have any issues currently, but want to add a pool heater and a small instantaneous water heater. I can go over there and measure the amperage being used, but that won't really tell me their peak usage.

The POCO gave me their peak monthly usage of 2765 KW (or is this KW/hrs?), but they don't have any data on peak daily or hourly usage which is what I really need to see if a main breaker is going to get overloaded. I don't own a recording amp meter, but even if I did, I would have to record a year's worth of data to be sure of the number.

Any suggestions? I'm thinking I will tell them to install the additional stuff on the existing service and see if the main trips. If it does, they need an upgrade.
 
Do a load calc for the residence. With the dwelling unit load calc, most of the load is based on prescriptive methods such as VA/SF, laundry circuits, etc. that should give you some idea of how close you would be to fully loading the service without needing to track down every specific load in the house.

The value from the utility company is kWh, and doesn't do any good in determining the demand. If the load really was 2765 kW on a 240V, single phase service, you would be looking at a 12,000 A service.
 
Coppersmith said:
...but they don't have any data on peak daily or hourly usage...
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If they gave you the peak value, you do not care if it is hourly or daily or monthly, after all peak is peak.
Divide your kW value by your L-L voltage to see your peak current draw.
 
Coppersmith said:
I'm trying to determine if a homeowner needs a service upgrade. It's a 150 amp service on a 4500 sq ft house.
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I came back from looking at the house and of course the HO lied to me. There are two 150 amp panels on a 300 amp service. Still, I'm curious how to determine actual peak usage, not just what the NEC calculations say it should be.

One further note: Since there are two panels, neither panel carries all the load so I assume you can't use the 83% method when determining wire size from the meter. One of the panels just has two pole breakers in it, a lot of them. There are four A/C systems here.
 
jim dungar said:
If they gave you the peak value, you do not care if it is hourly or daily or monthly, after all peak is peak.
Divide your kW value by your L-L voltage to see your peak current draw.
Click to expand...

I would think I do care about hourly because the average usage over the month does not show if everything in the house is turned on all at once for short periods which might exceed the main breaker size.
 
electrofelon said:
300 amp service, I wouldn't worry about anything personally.
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When I arrived and looked at the service and saw that it was 300 amps I wasn't as worried about the 60 amp W/H and 60 amp pool heater they wanted to add. Then the HO got to talking about adding a spa, backyard lighting, an outdoor kitchen, and upgraded pool pump. I figure I need to run a 100 amp subpanel out to the pool area to handle all of that. Now I'm worried again, especially since the service is divided into two 150 panels and I'm not sure how much draw is on either of them. Both are full up with breakers.
 
Coppersmith said:
...since the service is divided into two 150 panels and I'm not sure how much draw is on either of them. Both are full up with breakers.
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What the utility gave you almost doesn't matter anymore, once you determined there are multiple panels fed from a single meter.
 
Coppersmith said:
When I arrived and looked at the service and saw that it was 300 amps I wasn't as worried about the 60 amp W/H and 60 amp pool heater they wanted to add. Then the HO got to talking about adding a spa, backyard lighting, an outdoor kitchen, and upgraded pool pump. I figure I need to run a 100 amp subpanel out to the pool area to handle all of that. Now I'm worried again, especially since the service is divided into two 150 panels and I'm not sure how much draw is on either of them. Both are full up with breakers.
Click to expand...

I have all of that except the instant water heater on my 200A service. 3 - 100 A subpanels, one each in the house , garage, and pool house.
 
Coppersmith said:
What if the 30 days I measure are during Fall and they use much more power during Summer?
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WA State has a form that deals with this. You put the max measured demand on the first line. You then adjust that value by multiplying that value by two factors to account for the potentially higher readings you would get in a different time of year, or if the facility were more fully occupied. There is a third factor that you can included if there is some special circumstance. You get to pick the values, and you should say something about how you picked them.
 
Demand is demand. You measure 30 days with a recording power meter. You look for the highest 15 minute average amperes in that 30 days. You don’t care about the other 29 days 23.75 hours. If you want VA multiply by 240.

The utility gave you kwhr for the month. That is average kw x 30 x 24. Kw = V x A x power factor but it’s a monthly average NOT 15 minute peak, and it’s kw, not VA. The panels only count A, not kw.

Finally you may have to measure both panels unless you plan on adding a third sub panel to the two existing because at each panel you have 150 A, not 300.
 
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