load calculation

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paul hodge

Member
Location
atl
Panel information:
150 amp MCB
208/120
3 Phase
Phase A Load = 14.5 KVA
Phase B Load = 11.5 KVA
Phase C Load = 11.9 KVA
Connected KVA = 37.9
Dem. Kva = 26.2
Dem. Amps = 72.8

I have 17 additional circuits at 12 amps a piece per equipment name tag to add. My calculations appear to show to much load being added for a 150 main breaker.
150 amp 208/120 panel Fed from 45 kva transfomer. Transformer fed by 70/3 phase 480 volt
 
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paul hodge

Member
Location
atl
Dennis Alwon said:
You need to do a load calculation. Will those 17 circuits be on at the same time at 100%?
Click to expand...
yes, all loads more than likely will be on at same time. Laptop charging stations. The figures given for the panel are current loads before the additional 17 New circuits which of course will be divided between all 3 phases. I calulated 1.4 kva (1400 watts) for each circuit placing 6 A; 6B;5C phase for the 17.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Does that "Dem." stand for to be demolished or removed? If so you'll have a connected load of 11.7kVA afterwards and you will be adding 24.5kVA (assuming 120V) for a total of 36.2kVA total upon completion, which will allow for some imbalance.
 
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paul hodge

Member
Location
atl
Smart $ said:
Does that "Dem." stand for to be demolished or removed? If so you'll have a connected load of 11.7kVA afterwards and you will be adding 24.5kVA (assuming 120V) for a total of 36.2kVA total upon completion, which will allow for some imbalance.
Click to expand...

The dem was taken straight from the print using panel schedule. To be perfectly honest I don't know what it stands for. I'm a little confused the connected load is 37.9 kva which I assumed was used to calulate the ampere, but 26.2 kva × 1000 ÷ 360= 72.8 Dem amps. What have I missed?
 
charlie b

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Semi-Retired Electrical Engineer
I would surmise that "Dem" means "demand." It would be the result of starting with the connected load and applying any available demand factors.
 
charlie b

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Semi-Retired Electrical Engineer
I have no way of discerning how the demand KVA value of 26.2 was determined. But let's take that at face value. You are adding 17 circuits, each rated 12 amps, and each being (I presume) rated 120 volts, single phase. That is a new load of 24,480 VA. Adding that to the existing 26,200 VA, you would have a total load of 50,680 VA. That is too much for a 45 KVA transformer. Here is what I think you need to do:
  1. Set up a 30 day load study, to determine a "legally useable" value for the existing load. Refer to NEC 220.87.
  2. Verify the load values for the new circuits. I have sincere doubts that a laptop charging station would draw 12 amps. I would expect a value closer to 2 amps. You need to check.
  3. If the results of steps 1 and 2 confirm that the proposed installation would overload the existing 45 KVA transformer, then you need to talk with the owner (or your client, whoever that is) about upgrading the transformer (including its primary and secondary conductors) and upgrading the panel.
 
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paul hodge

Member
Location
atl
charlie b said:
I have no way of discerning how the demand KVA value of 26.2 was determined. But let's take that at face value. You are adding 17 circuits, each rated 12 amps, and each being (I presume) rated 120 volts, single phase. That is a new load of 24,480 VA. Adding that to the existing 26,200 VA, you would have a total load of 50,680 VA. That is too much for a 45 KVA transformer. Here is what I think you need to do:
  1. Set up a 30 day load study, to determine a "legally useable" value for the existing load. Refer to NEC 220.87.
  2. Verify the load values for the new circuits. I have sincere doubts that a laptop charging station would draw 12 amps. I would expect a value closer to 2 amps. You need to check.
  3. If the results of steps 1 and 2 confirm that the proposed installation would overload the existing 45 KVA transformer, then you need to talk with the owner (or your client, whoever that is) about upgrading the transformer (including its primary and secondary conductors) and upgrading the panel.
Click to expand...

I personally checked the charging station tag and it states 12amps. These are large cabinets used in a school district with literally I think 30 to 40 Laptops charging in each cabinet at the end of the school day. I thought about putting an amp meter on a cabinet to get exact amp draw but common sense told me go with nameplate information. No short cuts when it comes to kids and safety. Anyway, I certainly appreciate your input and will use your suggestions as a discussion point. Thanks
 
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Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
Paul...

If you are designing an add-on system to an existing one, using given or assumed kV x A = kVA, loads, then your results will be in error! By error, I mean non-linear loads containing harmonic currents will distort "distort" expected values!

Assumed kVA will not match kVA as found from measuring kW and kVAr or PF! Since you already have existing "like-loads" I suggest you measure the existing loads, better yet, record them to formulate "true" demand requirements.

Regards, Phil Corso
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Phil Corso said:
Paul...

If you are designing an add-on system to an existing one, using given or assumed kV x A = kVA, loads, then your results will be in error! By error, I mean non-linear loads containing harmonic currents will distort "distort" expected values!

Assumed kVA will not match kVA as found from measuring kW and kVAr or PF! Since you already have existing "like-loads" I suggest you measure the existing loads, better yet, record them to formulate "true" demand requirements.

Regards, Phil Corso
Click to expand...
The NEC permits using a recording or demand record for existing loads under the optional method of 220.87... but it does not permit one to add new loads under the same premise. One is required to use nameplate values, typically, and apply demand only as specifically permitted.
 
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Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
Smart$...

"The NEC permits using a recording or demand record for existing loads under the optional method of 220.87... but it does not permit one to add new loads under the same premise. One is required to use nameplate values, typically, and apply demand only as specifically permitted."

So you expect a jr Elec-Tec has enough knowledge to handle NEC Art's without a Heads-up!

Phil


 
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