what am I missing on the total amp calculation....!?

[Silvamp]

Folks, I've been doing a redesign of an emergency circuit in a building and when I was doing the new panelboard schedules I saw that the original design (less than 3 years old) had on its panelboard schedules figures that do not match what I've always used....

All voltages are 120/208

example1: panelboard X
TOTAL VA (ALL LOADS) = 137,368 VA
TOTAL AMPS (calculated by me) = 137368/208/1.732 = 381.3A
TOTAL AMPS (shown on original design drawings) = 423(!)

example2: panelboard y (same building, same design)
TOTAL VA = 173,261VA
TOTAL AMPS (MY CALCULATION) = 173261/208/1.732 =480.9A
TOTAL AMPS SHOWN ON ORIGINAL SCHEDULE (with above data) = 502

the difference is not even, lets say the same k factor... the design has several panels and none of them do match my calculation.. it is also interesting that the max phase amps matches the existing design (when I use 120v and single phase calculation with the most loaded phase)

please, can anyone show me what am I missing if anything...

thanks a lot.

 

[augie47]

Perhaps they took into consideration the load per phase. Your calculations assume the load is evenly distributed. If a portion of the load is single phase (120 or 208) and not evenly distributed between the 3 phases, they might be showing the phase with the highest ampere load as your feeder would need to be sized with that in consideration.
OR
they may be considering the portion of the load that is continuous at 125%.

 

[Steve Beranek]

Just a guess, but I would assume they are using a software to paste this into the drawing. The values entered vs values the came up with is near 10%. So maybe the software added 10% to the load ?