Was wondering if someone can answer a Breaker sizing question for me. I have a panel with 4 x 75 HP VFDs, cooling fans,1kva xfmr, and a few indicator lights. I am needing a Main disconnect breaker for this panel. How do I calculate the size. My VFD' s Use 125 A Breakers (specs by MFG). Do I simply take the input currents of these Drives, add them up and do a 1.25 service factor? Or am I way off? Thanks in advance!!
Multi VFD Panel Main Sizing
- Thread starter chazmicks
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Even though you have VFDs and technically, many of the issues may not apply, there is nothing allowing that to be any different than 430.110(C), except that because of the VFDs, there is no way to start multiple motors at their Locked Rotor Current, so you can ignore that (unless there are Bypass Starters for the VFDs).
So you would take the FLCs from all of the MOTORS (not the VFD inputs) based on the HP and per the appropriate table (usually table 430.250), plus the full load currents of any other devices, x 1.15 as a minimum. If the cooling fans and indicators are all powered FROM that 1kVA transformer, you would not add them again by the way, it's all part of the transformer load current (as determined at the primary side).
I should add however that if you just took 125% of all of the VFD input currents + the transformer primary, that would likely exceed that number. The only issue is the sizes of conductors going from that main to the individual drives and transformer and propoerly applying the Feeder Tap Rules to them.
So you would take the FLCs from all of the MOTORS (not the VFD inputs) based on the HP and per the appropriate table (usually table 430.250), plus the full load currents of any other devices, x 1.15 as a minimum. If the cooling fans and indicators are all powered FROM that 1kVA transformer, you would not add them again by the way, it's all part of the transformer load current (as determined at the primary side).
I should add however that if you just took 125% of all of the VFD input currents + the transformer primary, that would likely exceed that number. The only issue is the sizes of conductors going from that main to the individual drives and transformer and propoerly applying the Feeder Tap Rules to them.
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