size ATS based on load recordings

[jaime.A]

Some ATS are to be replaced, so load recoding was performed. The load recorded was set to a sample current every second but I have gotten used to the the sample rate set every 5 minutes. The load recorded has the option to display 5 and 1 second sample rate. The issue I have is that in the 1 second sample rate, the max is close to 100A but the average is around 15A. when you zoom in on the 100A peaks, they seem to last 2 seconds.

Would you size based on the average or the maximum?

 

[wayne440]

If by "ATS" you mean an automatic transfer switch, I would size it equal to (or higher) than the rating of the overcurrent devices feeding it, and the requirements of your local approving authority.

 

[waross]

Does the recorder take a sample every second or every 5 minutes or does it record the maximum value for each preceding window of 1 second or 5 minutes
What wayne440 said.
Size it according to the breaker rating or the fuse holder rating of the feed to the ATS.
If the ATS fails and does insurable damage you may have a difficult discussion with your insurance adjuster as to why the ATS was not properly protected.
Your building insurance may be voided.
Internal heating is predominantly I[sup]2[/sup]R of the switch and contacts.
100 Amps [sup]2[/sup]/15 Amps[sup]2[/sup] = 44.4 times as much heat generated by your 100 Amp peaks.
What may be insignificant heating at 15 Amps may become significant at 100 Amps.
Further more, if the ATS opens during a 100 Amp peak, it may cause a serious arc in a grossly undersized switch.




--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

What kind of loads does the ATS serve? The short spike looks a bit like motor starting. Motor starting currents typically exceed the steady-state rating of circuits. I had not looked at ATS ratings to see how they are specified to handle opening during motor starting. It seems like have a motor randomly starting during the transition would be less severe than the ATS immediately re-opening after closing when significant inrush is still occurring.

You can find the equivalent thermal loading on the circuit by calculating the root-mean-square of the heating losses. For example, having a 1 second spike to 100 A once per 45 seconds generates about double the heat losses a steady state load of 15 amps.

An additional setting to check is that the meter was recording harmonics in addition to the fundamental current. High levels of harmonics increases the internal heating losses.

As the other pointed out, the minimum size is dictated by code requirements. NEC 220.87 has provisions for using 1 year of actual load recordings as a basis for feeder sizing.

 

[jaime.A]

The recorder was set to sample every second. If needed, the ATS can be upgraded along with conductors and the OCPD.

The question is whether to size based on max or average. waross would you still go with max even if its 1 or 2 max second duration?

 

[waross]

What is the protection ahead of the ATS?

NEC 220.87 has provisions for using 1 year of actual load recordings as a basis for feeder sizing.

There is a similar provision in the Canadian code.
It is most often applied when an industrial plant wants to add load to a system that is maxed out according to demand factor calculations but the actual load is less than the calculated demand.
The rule allows load to be added without increasing the size of the main service or sub service.

waross would you still go with max even if its 1 or 2 max second duration?

No. I would go by the rating of the protection ahead of the ATS.
I may also check the calculated demand, using the appropriate factors and tables in the applicable code.
You have not told us what the unusual load is, nor what the nameplate rating of the unusual load may be.

Parable;
John was tasked with unloading boxes from a trailer using a hoist and a sling.
The boxes weighed between 15 and 100 lbs.
John selected a sling based on the average, or 57.5 lbs.
To add a margin of safety John selected a sling with a breaking strength of 60 lbs.
From time to time, the sling would break.
Each time John would recheck his calculations and select another 60 lbs breaking strength sling.
For the weight of boxes calculating the mean was straight forward.
Heating is based on the square of the current and you must use Root-mean-square calculations.
The number comes out quite a bit higher than the simple mean.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!