Calculation of loads on a 4000a switchgear?
- Thread starter Michael.D
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drktmplr12
Senior Member
- Location
- South Florida
- Occupation
- Electrical Engineer
How did someone come up with the 1200 amp for the MCC? You do the same thing with the switchgear. Add up all continuous loads and add 25% of the largest continuous load.
gadfly56
Senior Member
- Location
- New Jersey
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- Professional Engineer, Fire & Life Safety
You can't back-calculate the continuous loads, you have to know what they are going in.
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
Is this a new install, or existing?
We use loggers quite a bit, if it's existing and we have time to do it.
We use loggers quite a bit, if it's existing and we have time to do it.
thanks, this was a wood pellet plant built around 2007, our company just aquired it Jan 2019, looking to upgrade. i did the same calculations as MCC's, numbers looked alittle high for me so i thought i would ask this forum..One question, you use 25% continuous load of the highest MCC load? this is how I figured it..
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drktmplr12
Senior Member
- Location
- South Florida
- Occupation
- Electrical Engineer
you can do it a couple ways.
first is you can put a power meter on it for 30 days that logs demand every 15 minutes. take the max value from that data set and multiply it by 125%. that is your minimum feeder size. add to it any new loads if you need to install new equipment.
second.. say your MCC has 5 x 25 hp pumps that operate continuously. you also have a 100 hp fan that operates continuously. assuming a 460V system, your 25 hp pumps each draw 34 amps (according to the NEC). the fan draws 124 amps. add up all continuous loads to determine connected load: 5 x 34 + 124 = 294 amps. last we have to add 25% of the largest continuous load: 124 * 0.25 = 31 amps for determinig the minimum feeder size.. which would be 294 + 31 = 325 amps.
repeat this process for each MCC and add up all of their connected loads. then determine the largest motor connected to any one of the MCC's and add 25% of that to determine the minimum feeder size for the switchgear. for example, if you had 5 MCC's, all identical to the example above, the connected load for the switchgear would be 294 x 5 = 1470 amps. last we add 25% of the largest motor (100 hp fan): 1470 + 124 x 0.25 = 1504 amps minimum feeder size.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
What are you trying to accomplish exactly? If you want to determine existing loads to find extra capacity, then you need a power logger. I don't really see the point in doing an NEC calculation on an existing system, all it will do is over estimate your usage by a factor of 2 to 2.5.
drktmplr12
Senior Member
- Location
- South Florida
- Occupation
- Electrical Engineer
we regularly run into this exact issue because we use NEC calculations to make additions. there is not always time or money to place a logger. NEC calc's always over estimate the load, and rightly so. we make what adjustments we can based on anticipated operation, but there is no way to anticipate the diversity factor without logging the demand over a long period. the only short coming to this method (logging) is you must take into consideration the annual operation of the plant. a waste water plant in florida will have higher peak flows during the travel season. whereas a wood pellet facility demand will go up and down based on how many units the plant must produce.
if the switchgear is the service entrance point for the plant, consider contacting the utility authority and requesting demand data for the last 12 months. they should be able to tell you what the peak demand was during that time period.
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