Calculating Load on a Existing 277-480v 3 Phase Panel

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Europoljuice

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I was reading Charlie's directions and formulas for Calculating a Load on a existing panel. See Below

If you are starting with a new design project, and if you are given a set of loads (in units of amps), some single phase 120, some single phase 208, and some three phase 208, then what you do is,
  • Add up all the single phase 120 current values, and mutiply the sum by 120,
  • Add up all the single phase 208 current values, and mutiply the sum by 208,
  • Add up all the three phase 208 current values, and multiply the sum by 208, and then multiply that result by 1.732,
  • Add up the three values calculated above,
  • Divide that sum by 208, and divide again by 1.732.
  • You now have a design value for the current that will be drawn on each of the three phases (assuming you can balance the loads closely enough). That is the value you use as a design basis for selecting the panel rating, the feeder to the panel, and the feeder breaker from the upstream panel. Be sure to add some margin for future load growth. Most of my clients have asked for 20% above the calculated load.
My Question: Does this Apply to 3 phase 277-480 v Panels as well???
 
You would have thought with all the new APP's that are out there, someone would have designed simple excel friendly App to do all these existing Load Cal. I feel like I am back in Math Class. Thanks "Petersonra"
 
Europoljuice said:
If you are starting with a new design project, and if you are given a set of loads (in units of amps), some single phase 120, some single phase 208, and some three phase 208, then what you do is,
  • Add up all the single phase 120 current values, and mutiply the sum by 120,
  • Add up all the single phase 208 current values, and mutiply the sum by 208,
  • Add up all the three phase 208 current values, and multiply the sum by 208, and then multiply that result by 1.732,
  • Add up the three values calculated above,
  • Divide that sum by 208, and divide again by 1.732.
  • You now have a design value for the current that will be drawn on each of the three phases (assuming you can balance the loads closely enough). That is the value you use as a design basis for selecting the panel rating, the feeder to the panel, and the feeder breaker from the upstream panel. Be sure to add some margin for future load growth. Most of my clients have asked for 20% above the calculated load.
My Question: Does this Apply to 3 phase 277-480 v Panels as well???
Click to expand...

The short answer is yes, with 277 replacing 120, and 480 replacing 208 respectively. Replace these with variables Vpn and Vpp respectively, to generalize this calculation, which you could then apply to 230/400V systems, or any other 3-phase grid.

The first two bullet points only strictly apply, if they are balanced among all three phases, and phase shift power factor is unity. You generally should aim to do this where you can, as it allows better utilization of the total amp and kVA ratings of the equipment.

There is a way to add up the phase imbalance, if it is of interest to do so:
Add up the A to N loads, call it I_AN_0
Add up the B to N loads, call it I_BN_0
Add up the C to N loads, call it I_CN_0
Add up the 3-phase loads, call it I_3ph

From the above you can calculate the individual phase-to-neutral current, neglecting the 2-pole single phase loads.
I_A0 = I_AN_0 + I_3ph
I_B0 = I_BN_0 + I_3ph
I_C0 = I_CN_0 + I_3ph

Now work with the phase-to-phase loads:
Add up the A to B loads, call it I_ab
Add up the B to C loads, call it I_bc
Add up the C to A loads, call it I_ca

There is a square root formula that accounts for the phase shift as it assigns the A to B loads onto phases A and B. There is symmetry to this formula for producing the versions for the other phases. Once you calculate the square roots, add on the previous phase totals, to get the final phase-totals.
I_A = I_A0 + sqrt(I_ab^2 + I_ca^2 + I_ab*I_ca)
I_B = I_B0 + sqrt(I_ab^2 + I_bc^2 + I_ab*I_bc)
I_C = I_C0 + sqrt(I_bc^2 + I_ca^2 + I_bc*I_ca)
 
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