30 day load study/220.87

[shawnbebout]

all,
Thank you for reading this and taking the time to educate me. We recently purchase a fluke 1736 data logger. Mainly to ensure our loads our balanaced in our facility, but I know they will be asking me how much I can add to the panel. I have completed a 4 day load study to use the information as a training tool, and I will attach a screen shot as a reference. Now I did have the interval set @ 5 seconds, not 15 minutes as spec out in 220.87. I think the loads are pretty well balanced. 120/208 volt 400 amp panel. Industrial panel. 3 phase 4 wire.

So in theory, if it was a 15 minute interval instead of a 5 second.

400 amp minus 110 max equals 290 amps. 290 divided by 1.25 equals 232 amps. So the max amount of loads I could add to this panel would be 232 amps.

Is my formula right?

 

[shawnbebout]

or is the calculation 110 amps (max on B phase) times 125% equals 137.5

So
400-137.5+262.5 amps

So max we could add is 262.5 amps in theory,

but in reality probably 200 amps. (262.5 divided by 125%)= 210 amps

 

[Smart $]

or is the calculation 110 amps (max on B phase) times 125% equals 137.5

So
400-137.5+262.5 amps

So max we could add is 262.5 amps in theory,

but in reality probably 200 amps. (262.5 divided by 125%)= 210 amps

Only if all continuous load.

 

[NewtonLaw]

Min Avg Max

Min Avg Max

The 15 minutes referred to in the NEC recognizes that this is the normal heating cycle for most electrical equipment. I.E. a 15 Minute Thermal demand is usually the true limit for most equipment. I think as an example, if you have 50 amps of load on for 7.5 minutes and 0 amps for 7.5 minutes, the 15 minute thermal demand (heating capability of the current flow) would be 25 amps. The same heating effect of carrying 25 amps for a full 15 minutes. You can still calculate this equivalent from the data you have with your 1736.

Also, the data logging of a Fluke 1736 is collected by sampling near 15K/second. If your "write" interval was set for 5 seconds, then the Fluke 1736 has 75,000 readings and it reports three things to you. The Maximum that occurred during the interval, the average of all readings over the interval and the minimum over the interval. So I see you used the 110 amp maximum reading instead of the 71.5 amp average. Do you think the 110 amp level that may have occurred for one or two cycles is the correct load to use? Just asking.

 

[iwire]

The 15 minutes referred to in the NEC recognizes that this is the normal heating cycle for most electrical equipment. I.E. a 15 Minute Thermal demand is usually the true limit for most equipment.

I am finding this unlikely, do you have a source for this?

I think 15 minutes was chosen because that is what the POCO often uses for demand metering.


The 'heating cycle' is going to vary greatly dependent on the thermal mass of the equipment.

An oil filled pad mount transformer of going to have a much different heating cycle than a receptacle.

 

[shawnbebout]

Only if all continuous load.

When our industry was at its peak, we were running pretty much 24/7. So I am edging on the side of safety and considering all loads continuous. Our industry is coming back, so eventually our load will pick back up.

 

[NewtonLaw]

I am finding this unlikely, do you have a source for this?

I think 15 minutes was chosen because that is what the POCO often uses it for demand metering.


The 'heating cycle' is going to vary greatly dependent on the thermal mass of the equipment.

An oil filled pad mount transformer of going to have a much different heating cycle than a receptacle.

The NEC allows numerous ways to calculate customer loading. For the purposes of allowing additional load to be connected to an existing service or feeder, you can use actual maximum kVA demand figures to determine the existing load instead of using the standard calculation. This type of data may be secured from your local POCO usually on a two year or greater history. The NEC requires at least one year of data. The demand that is given to you from the POCO will be a 15 minute thermal demand. This comes from the POCO's need to match the thermal ratings of their electrical equipment to the expected and actual loads of their customers. The NEC identifies this as the load to be used to determine a customer's actual load. This times 125% + new load is then used to determine if existing facilities are capable of additional loading. My POCO will provide the 15 minute demands every 15 minutes giving the kW & kVAR readings to any commercial customer requesting that data. They keep it for two years to five years depending on the POCO's need for the data.

From 220.87 "Determining Existing Loads" The calculation of a feeder or service load for existing installations shall be permitted to use actual maximum demand to determine the existing load ....

Under the Exception note it states: If the maximum demand data for a 1-year period is not available, the calculated load shall be permitted to be based on the maximum demand (the highest average kilowatts reached and maintained for a 15-minute interval) continuously recorded over a minimum 30-day period .....

Hope this helps

 

[steve66]

or is the calculation 110 amps (max on B phase) times 125% equals 137.5

So
400-137.5+262.5 amps

So max we could add is 262.5 amps in theory,

but in reality probably 200 amps. (262.5 divided by 125%)= 210 amps

Yes, you have to use 125% of the existing load so you start with 400-137.5 = 262 amps.

Then, if you wanted to be conservative, you can also add 25% to the new load, so you would be correct, 210 amps would be the limit for added load.

The 5 second peak demand will always be larger or equal to the 15 min demand. It will usually be quite a bit larger than the 15 min. demand because of motor starting and other short duration loads. Therefore, you will have a very conservative estimate of how much load can be added.