Adjustable Drive Systems Feeder Calculations

[mikedionne]

In NEC Article 430 Section X - Adjustable Speed Drives. They provide methods to size the conductors and the disconnects. My question here is what is the proper way to size the main feeder branch circuit protection to a system with a combination of loads that included VFD's & Full Voltage Motor Starters? What I have been doing is taking the largest overcurrent device and then adding all the other loads to it. I use the nameplate rating of the VFD (if it is the largest device I do not use the input fuse or circuit breaker) or the the largest Branch Circuit / Short Circuit device if it is a Full Voltage Starter and then add all the other loads to it. How has everyone else been doint it?

 

[skeshesh]

Can you clarify the situation a bit more? what do you mean by 'main feeder branch circuit protection'? I assume thats the Main OCPD of some sort of distributin unit. I would think that you have a MCC (existing MCC maybe) with some motor loads to remain and your adding new loads equipped with VFDs. In any case, a bit more detail would be great.

 

[mikedionne]

Adjustable Drive Systems Feeder Calculations

What I am talking about is sizing the Main Feeder OCPD for a combination of VFD & FVNR Motor Loads. Everyone seems to do it differently and has an opinion. I am just trying to get a feel for what everyone else is dong. A good example would be 5 VFD's and 5 FVNR Starters. If one (1) VFD is the largest motor how do you calculate the Feeder OCPD?

 

[skeshesh]

As a first step I would check the feeder size ampacity calulated like you said (125% of largest load - be it a motor FLA or a VFD input - plus 100% of the rest of the load). That would give you the ampacity of your feeder and you can select a minmium OCPD size which would be the next size up. Afterwards comes the hard part which is using SKM, ETAP or what-have-you or (if you can actually manage it) get the motor performence curves from the mfg to see the starting characteristics and damage curves. Then you can check the time-current curves starting with the minimum OCPD size and work your way up untill you have a cooridnated system.

 

[mikedionne]

VFD Feeder Calculations

VFD Feeder Calculations

Thank you for your comments. What brought me to post this is that we have one customer that always undersizes their feeders. They always use the FLA of the motors and not the input of the VFDs. An example is they will have a 3HP Drive with a 1/8 Hp motor and only ise the fla of the 1/8 Hp motor not the rated input of the drive. I will come up with a 90 Amp Feeder and they will have a 50 Amp Feeder. I have sited the NEC to them, UL 508A, NFPA 79 etc. but as they say the customer is always right.

 

[skeshesh]

I agree with you based on the NEC, but I do believe the smaller conductor would work find with a smaller load. I'm not an expert on VFDs by any means and there several members on this forum who really are, so maybe they'll chime in. In any case, I hope if and when the client decides they need a larger motor for some reason they check the conductors and don't just read 3HP off the nameplate of the VFD and not realize the conductors feeding the VFD are actually undersized.

 

[mikedionne]

VFD Feeder Calculations

VFD Feeder Calculations

I agree with you on that. That is what I try to tell them but I guess they will have to find out the hard way.