Building Power Distribution Issue

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rodneyl

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My company is stuck in an interesting situation, we have been in our rented building for the past 17 years. We are a builder of industrial machinery, the voltage and amperage loads on our building are constantly different depending on the equipment that we are building. Our current service panel is 480volts 225 amps, with 480 amps worht of breakers in it. There is a secondary distribution panel that is fed from the main panel with an 80A breaker, but the secondary distribution panel has multiple breakers (see attached Distribution Layout jpeg Distribution Layout.jpg totalling 430amps. So far to my knowledge we have not tripped the main breaker in the building, but now we need to add a new air compressor replacing the old one. The old air compressor is only breakered at 40 amps. The new compressor is going to be 60hp + additional loads for drying equipment. If i am correct the FLC per NEC is 77 amps just for the 60hp motor. I am very concerned about the service capacity of the building and am not sure of what the rules of power distribution are for a building like this! Can we just keep adding sub panels to our existing service? It appears as though the electricians that installed our HVAC equipment may have already cheated the panel. The Main panel breaker bus is setup so that larger breakers cannot be directly accross from smaller breakers, I assume so that the bus does not get overloaded. It appears as though they may have ground the tabs off of some of the larger breakers to make them fit opposing each other. Any help or advice would be greatly appreciated!!!

Thanks in advance...
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
The number of breakers that are installed is not relevant. The ratings of the installed breakers also are not relevant. What matters is the calculated load, or as an alternative, the measured load. The rules of power distribution for an existing building tell us that we can add load if there is spare capacity. One way to determine existing capacity is to perform a load calculation, per article 220. For existing buildings, that is sometimes hard to do, since the exact nature of all equipment in the building (or in your portion of the building) may not be known. Another way is to measure the peak demand over a 15 minute period, and repeat that measurement for 30 days. You take the highest measured value, add 25%, add a fudge factor (if appropriate) to adjust for seasonal differences, and see if there is still capacity to add your new load. The process is described in NEC article 220.87.
 

rodneyl

Member
Calcs

Calcs

Can this data be derived from our kw/H usage which at its highest is 15840 kw/h in July within the last 12 months. 12000 kwh is about our average.
Distribution Calcs based on kwH.jpg

If my math is correct thats only about 89amps of actual usage.

Thoughts???
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Sorry, but that information is not adequate. You need to know the peak, not the average, and not even the highest monthly average. It is like the following question:
  • A person drove for 200 miles along a single road.
  • The speed limit was 55 mph for that entire stretch of roadway.
  • The trip took them four hours.
  • Question: Did the driver break any speed limit laws during the trip?

You can figure out that the average speed was 50 mph. But you do not know whether drove at 90 mph for part of the trip, and stopped for lunch somewhere along the way.

What you need to know is the peak amount of power that was drawn each month. If the utility bill has that information, you can use it. But the average usage is not useful.
 

jrohe

Senior Member
Location
Omaha, NE
Occupation
Professional Engineer
Can this data be derived from our kw/H usage which at its highest is 15840 kw/h in July within the last 12 months. 12000 kwh is about our average.
View attachment 5890

If my math is correct thats only about 89amps of actual usage.

Thoughts???

kWh is not useful, as you can not determine what the peak usage is. However, utility companies are increasingly using meters that report peak demand loads. I would contact the utility company and ask if they have peak demand data. For projects I design, I ask for a minimum of 2 years worth of peak demand data to help ensure I am not pulling numbers from an unseasonable hot or cold season.
 
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