adjustable trip settings

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jackman435

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Location
Fort Dix, NJ
We will be installing a cb in an exist. MCC to provide gf & short circuit protection for a motor w. VFD. It is 100 hp, 460v, with a FLC of 124 amps. 124 amps X 250% = 310 amps. The next size standard breaker is 350 amps. Is it permissable to install a 300 amp adjustable trip setting breaker? Access will be by authorized personnel only.
 
Jraef

Jraef

Moderator, OTD
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Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Once you have a VFD, the CB for it need only be sized per the VFD listing. The VFD is protecting the motor from GF and SC, UL forced that change in 2005. So RTFM on the VFD and just put in whatever it says it is listed with.

But be forewarned, many VFDs require fuses, not CBs, if you want the SCCR to be high enough to match the available fault current and/or MCC rating. If you dig into VFDs listed inmCCs already, you will find that many of them will have oversized fuses in ADDDITION to the CB to attain that higher SCCR.
 
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