RVSS motor starter and branch circuit

I

Isaiah

Senior Member
Location
Baton Rouge
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Electrical Inspector
We have an existing 460V 3phase 60hp TEFC induction motor that needs to be upgraded to 75hp. (This application is for a torque motor that controls a large gate). The motor starter is a RVSS-reduced voltage soft starter- which will probably have to be increased in size. But does the motor branch circuit need to be upsized to the locked rotor current (I.e. 6 x motor FLA)
since it’s a high torque scenario? Or does the RVSS mitigate that requirement?


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Adding 15 hp load? The entire branch circuit, OCPD and any disconnects upsized absolutely. 75 hp = 96A (430.250)

RVSS does nothing to this required service.
 
herding_cats said:
Adding 15 hp load? The entire branch circuit, OCPD and any disconnects upsized absolutely. 75 hp = 96A (430.250)

RVSS does nothing to this required service.
Click to expand...

Agreed, but what about from the RVSS to motor itself?


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I was wondering about that. Most all soft starters are sized by LRA. I'll research. Good question thanks!
 
What is the conditions severity and starts per hour? Is the initial load heavy? Is it using a gearbox reduction?
 
I've done a lot of gate/valve operators and all of the electric ones have been gear motors. 75 HP sounds big. We did some butterfly valves for Philly Southwest wastewater treatment plant and they were big valves (combined wastewater/stormwater flow for that whole drainage area), and those were only like 20hp if that
 
That's what I'm trying to figure out. A gear reduction motor drive is not a "high torque" load. The biggest high torque loads are on massive air compressors, giant blowers, conveyor drives in mining, gravel etc. Extreme situations. I just want him to get the correct soft start for the upgraded 75 hp motor upgrade.
 
"High torque" and "soft start" are an oxymoron... You use a Soft Starter to REDUCE the torque. If the load REQUIRES high torque, then you are doing the opposite by adding a soft starter...

But moving on (assuming someone knew what they were doing in requiring it), then to address the OCPD situation, nothing about a Soft Starter changes the NEC requirements for sizing of the branch OCPD.

But does the motor branch circuit need to be upsized to the locked rotor current (I.e. 6 x motor FLA)
since it’s a high torque scenario? Or does the RVSS mitigate that requirement?
Click to expand...
That is not how the branch OCPD is selected to begin with... You read 430.52, do what it says, including TABLE 430.52 for sizing of the BOCPD.

The only thing worth noting that is different is that as of 2005, an RVSS will not have an Instantaneous Trip Breaker any longer, it will be an Inverse Time breaker or fuses, depending on their UL listing. As of 2005, UL no longer allows IT breakers (aka MCPs) in the combination listing for soft starters and VFDs. But that last bit is IMPORTANT: you must RTFM for the RVSS to fully understand the manufacturer's listing requirements for protection, because 110.3.B trumps all. In other words if the installation manual states that you must use fuses, then you cannot arbitrarily use a circuit breaker (and vice versa, but it never happens that way).
 
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