220.87 - Maximum Demand from Power Company?

[kwired]

What is the chance that all three will be on at the same time within a random 15 minute interval? Assume a 50% duty cycle. My 2.5 pulls about 20A.

If there's a power failure they will be all be on for a while but that happens rarely.

When you hit the max demand designed area, say a day where temp reaches 100 deg F or more, I would expect to see all three running nearly 100% of the time during the worst part of the day. I would also expect to see similar on the other end of things. Electric heating would run near 100% of the time if temps reach designed or even fall below designed low temps. If units run short cycles when at these extremes then those units are going to be oversized in most cases and usually will not be as efficient for the application as a properly sized unit would be either.

 

[Hv&Lv]

Here's a simplified version of this problem.


You have a sampling ammeter that records in 15 min. intervals.


Once/month a guy throws a 4 hr party during which time his house draws much more current than normal. Four hours is 16 ea. 15 minute chunks.


You don't know when the party will be and you have one month to look for this peak current draw.


If you ran the ammeter continuously, that is 2880 15 minute chunks, you have a 100% chance of detecting this peak current draw.
If you ran the ammeter once for 15 minutes you have 16 chances out of 2880 (0.00555, ~0.6%) of detecting this peak current draw so you have 1-0.00555= 0.994 (99.4% chance) of not detecting it.

How many times (x) would you have to sample within the month to have a 50% (0.5) chance of detecting the peak? How 'bout for a 95% (0.05) chance?


This is 0.994^x = 0.5 and 0.994^x = 0.05.
x = (log 0.5)/(log 0.994) = 115 times
and
x = (log 0.05)/(log 0.994) = 498 times

Nice math. Too bad it is irrelevant for one simple reason...
The book says :

the calculated load shall be permitted
to be based on the maximum demand (measure of average
power demand over a 15-minute period) continuously recorded over a minimum 30-day period using a recording
ammeter or power meter connected to the highest loaded
phase of the feeder or service, based on the initial loading
at the start of the recording.


I could simply turn on everything in the house at once and get readings, but that isn't the way the book requires us to do it.

 

[G._S._Ohm]

I screwed up.
Read thoroughly, then post. . .:blink: