Farm load calculation, T220.102, help with understanding, please.

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sw_ross

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Background:
-We do work on a farm that actually has 4- 400 amp meters (utility here uses Ronk brand, site isolating device, meters) spread throughout a fairly large farmstead.
-There are a number of typical buildings related to cattle, cold storage buildings related to storing equipment, grain bins with fans, utility buildings, and a dwelling unit, each building, bin site, etc., has a panel set up as a service fed from one of the Ronk's.

Objective:
- they want a generator sized appropriately to feed 2 of the Ronk meter loads (the Ronk's are set up for an auxillary power input, i.e.-manual transfer switch supplied by the Utility), he's already been talking to a supplier about a generator in the 110-125 kw range.
-the farmer might want everything to be automatic (we're thinking 2 ATS's sized for the 2 Ronk meter's loads)
-Instead, we're trying to talk him into setting it up to use the Manual Transfer Switch's that are already in place, we would only have to trench from the generator to each meter box, he could select the loads he needs to operate under the conditions and it would allow him to use a smaller generator.
-He likes the idea of it being automatic, he's not sure employee's will be able to manually operate the system.

Issue:
-we need to do a load calc for the loads he wants to be able to operate off the generator if using an ATS system (2 of the 400 amp Ronk meters).
-I spent half the day at the farm yesterday, the other half in the office working on the load calculation
-I'm having a hard time understanding how to use Table220.102

Our largest load is about 12 grain bin fan motors, in the 5 to 7.5 hp range. Normally, multiple motor loads would be sized at 125% of largest, then 100% remaining motor loads. This table gives you directions to choose the largest of three calculations, one of which is "100% of all loads expected to operate simultaneously". This would be our fan motors as the largest that the Table requires us to use, it doesn't say anything about 125% of largest, then 100% of remaining.

Am I understanding this correctly?

Once I get this part understood, I need to figure out how to blend this load situation into the next table, T220.103, as to the largest load @ 100%, 2nd lardest @ 75%, etc.

Any input from those experienced with using these 2 tables would be appreciated.

thanks,
Sky
 
You want to be very careful with using a standard NEC calc. to size a generator. Standard calcs are more suited to using a "stiff" utility supply. Some seem to think that if they have a 100 KW load @8 PF they can just go get a 100 KW machine and will be good. This can get you in real trouble as a genset is not stiff like a utility. There are a lot of factors involved such motor sizes, order of starting, acceptable freq. and voltage drop, etc.

The good news is all the major genset manufacturers have good software to assist in sizing that can sort out all the variables, such as starting order, etc.

I would work with Kohler, Cat, Cummins and I dare say Generac (Industrial) as they all have mature software tailored to their machines that help you greatly to end up with a good result.

This can also help you even if you decide on a manual system as it can help you write the instructions for start up order of equipment.
 
sw_ross said:
Background:
-We do work on a farm that actually has 4- 400 amp meters (utility here uses Ronk brand, site isolating device, meters) spread throughout a fairly large farmstead.
-There are a number of typical buildings related to cattle, cold storage buildings related to storing equipment, grain bins with fans, utility buildings, and a dwelling unit, each building, bin site, etc., has a panel set up as a service fed from one of the Ronk's.

Objective:
- they want a generator sized appropriately to feed 2 of the Ronk meter loads (the Ronk's are set up for an auxillary power input, i.e.-manual transfer switch supplied by the Utility), he's already been talking to a supplier about a generator in the 110-125 kw range.
-the farmer might want everything to be automatic (we're thinking 2 ATS's sized for the 2 Ronk meter's loads)
-Instead, we're trying to talk him into setting it up to use the Manual Transfer Switch's that are already in place, we would only have to trench from the generator to each meter box, he could select the loads he needs to operate under the conditions and it would allow him to use a smaller generator.
-He likes the idea of it being automatic, he's not sure employee's will be able to manually operate the system.

Issue:
-we need to do a load calc for the loads he wants to be able to operate off the generator if using an ATS system (2 of the 400 amp Ronk meters).
-I spent half the day at the farm yesterday, the other half in the office working on the load calculation
-I'm having a hard time understanding how to use Table220.102

Our largest load is about 12 grain bin fan motors, in the 5 to 7.5 hp range. Normally, multiple motor loads would be sized at 125% of largest, then 100% remaining motor loads. This table gives you directions to choose the largest of three calculations, one of which is "100% of all loads expected to operate simultaneously". This would be our fan motors as the largest that the Table requires us to use, it doesn't say anything about 125% of largest, then 100% of remaining.

Am I understanding this correctly?

Once I get this part understood, I need to figure out how to blend this load situation into the next table, T220.103, as to the largest load @ 100%, 2nd lardest @ 75%, etc.

Any input from those experienced with using these 2 tables would be appreciated.

thanks,
Sky
Click to expand...
I would be willing to suggest to not back up the grain bin fan motors. They likely are not that critical of a load, to warrant the generation capacity, even for an anticipated outage of a week or so. They often are a pretty significant amount of load as well. Simple method is to not supply the bins from the transfer switch, unless it is necessary to supply unloading equipment for livestock feeding, but a separate feeder for standby power is allowed or additional control wiring for load shedding would work as well.
 
Thanks for the replies/comments.

I think at this point our strategy is to get an overall load calculation for what the farmer "thinks" he wants to run off the generator, then show the farmer how costly that will be to set up, thereby using that as ammunition to convince the farmer to pare down his grandiose plans into a more manageable/affordable layout.

The other purpose of getting the overall load calc, is that if we end up doing some sort of ATS/load shedding scenario the inspector is going to want to see the numbers, and that we based our sizing decisions on something other than a "gut feeling".

The ec&m mag article is helpful. I started studying the example, it's clarifying some questions. Thanks.
 
Still have questions

Still have questions

I read the linked article, and contemplated it related to the farm load calc that I'm working on, and still have some confusion.

Regarding an overall farm load calculation, that involves a number of misc loads/panels spread throughout the farm, including a dwelling, I'm not sure what for/how to use Table 220.102 and how to convert anything I get there into Table 220.103.

Currently, I am using Table 220.103, I've done an individual load calc for each of the panels spread throughout the farm (taking into consideration the sq. ft. of structure and a va, plus 180va for each recep, plus calcs for all motor loads, etc). This allowed me to rank the loads to identify the largest, second largest, third largest, etc. and use Table 220.103 to come up with a total farm load calc, to with I added the dwelling unit load calc.

I'm not confident that I've done this correctly, as I haven't really used table 220.102, (not sure how to incorporate it into what I've done so far)

Any thoughts or comments would be appreciated,
Thanks
 
sw_ross said:
I read the linked article, and contemplated it related to the farm load calc that I'm working on, and still have some confusion.

Regarding an overall farm load calculation, that involves a number of misc loads/panels spread throughout the farm, including a dwelling, I'm not sure what for/how to use Table 220.102 and how to convert anything I get there into Table 220.103.

Currently, I am using Table 220.103, I've done an individual load calc for each of the panels spread throughout the farm (taking into consideration the sq. ft. of structure and a va, plus 180va for each recep, plus calcs for all motor loads, etc). This allowed me to rank the loads to identify the largest, second largest, third largest, etc. and use Table 220.103 to come up with a total farm load calc, to with I added the dwelling unit load calc.

I'm not confident that I've done this correctly, as I haven't really used table 220.102, (not sure how to incorporate it into what I've done so far)

Any thoughts or comments would be appreciated,
Thanks
Click to expand...
Are you luck enough for the POCO to have a demand recording meter and can use the data from it? That would be the best. Something tells me that NEC is not up to par on modern farm loads, especially if grain storage is involved. The past 5-10 years I have seen major increases in sizes of aeration fans used for a typical grain storage bin. Some of this is because the new bins are getting larger and need more air, but they are also adding fans or increasing size of existing fans as well, bottom line is they want more air through the bin than they used to. Many times the grain storage area has enough load that it has it's own utility supply, and maybe even is 480/277 volts where the rest of the farm may only be 120/240 single phase. I can think of one farm that has separate service to the grain storage area that has 8 - 20 hp fans, a 75 hp motor on the dryer, plus additional conveyors if the dryer is running that can all easily be running at same time.

My grandparents would have never envisioned it even possible to use that kind of power on a farm, I was too young to know some of this stuff when they still had the farm, but would bet it was supplied with only 60 or 100 amp single phase service at the time they sold the farm and retired.
 
When I was first assigned to look into this farm load calc I went out to the farm to look things over from a generator perspective, we've done a fair amount of work for this farmer previously so we're fairly familiar with the layout.

My first attempt at doing a load calc was a little overwhelming so I call up the utility company and was able to get the peak demand recorded for the two meters that he wants to operate off the generator. The meter that feeds the fan motors had a peak in October 2012 (harvest season - full bins) of 92 kw, I know that was because of the fan motors running. The other meter the farmer wants on the generator feeds the dwelling unit, the cow/calf part of the farm, and the storage quanset huts. Its peak demand was in the spring time period (calving season) of around 28 kw.

This information is as important to me as adding up all the motor FLC's, and square footage VA's of buildings.

I agree the farms around here are getting industrial in nature. I only know one farm in our area that has 480/277, 3-phase, but I do know a number of farms that have big bins with 3-phase fan motors and phase converters. I think it's only a matter of time before the utility is going to be pressed to provide 3-phase (at least in certain utility corridors).
 
sw_ross said:
When I was first assigned to look into this farm load calc I went out to the farm to look things over from a generator perspective, we've done a fair amount of work for this farmer previously so we're fairly familiar with the layout.

My first attempt at doing a load calc was a little overwhelming so I call up the utility company and was able to get the peak demand recorded for the two meters that he wants to operate off the generator. The meter that feeds the fan motors had a peak in October 2012 (harvest season - full bins) of 92 kw, I know that was because of the fan motors running. The other meter the farmer wants on the generator feeds the dwelling unit, the cow/calf part of the farm, and the storage quanset huts. Its peak demand was in the spring time period (calving season) of around 28 kw.

This information is as important to me as adding up all the motor FLC's, and square footage VA's of buildings.

I agree the farms around here are getting industrial in nature. I only know one farm in our area that has 480/277, 3-phase, but I do know a number of farms that have big bins with 3-phase fan motors and phase converters. I think it's only a matter of time before the utility is going to be pressed to provide 3-phase (at least in certain utility corridors).
Click to expand...
I note that you are considering using 1 genset to supply more than 1 service point. This has implications regarding weather this will need to be a SDS due to having grounded neutrals at each service. I also note that you mentioned the POCO uses Ronk double throw switches with meter sockets. If so, I'm assuming that you have a service disconnect after the Ronk meter/switch as the Ronk is not fusible.
If I have described this correctly, and you intend to use the Ronk switches as the transfer switch(s), that would require the genset to be a SDS as the transfer switch(s) are on the line side of the disconnecting means.
If you choose to go the ATS route (leaving the Ronk MTS unused and as is), you will need to use an ATS with a switched neutral as it would be on the load side of the service disconnecting means and you would be supplying more than one service.
 
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