Motor branch circuit OCP/ conductors too small?

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ssandoval

ssandoval

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League City,TX
Hello everyone,
New to the forum, but I could use any and all input. I have a 5hp AHU controlled by a VSD controller, 3ph/480v, FLA 7.6A. The contractor has installed a 3-pole/50amp and the conductors feeding the controller are #10 THHN awg. The contractor and even MEP rep says this is fine, due to not wanting start-up and in-rush current to trip the breaker. I realize that OCP has to be sized at 125% and even 175% on occasion depending on other factors, but in no case due any of my calculations allow for a 50 amp breaker to protect #10 wire. Am I getting this wrong? They also have an OAHU VSD fed with a 70 amp and #8awg. I've got them on warranty no matter what because the as-builts show these to be a 30 amp and a 50 amp respectively, but I'd still like to know exactly what the NEC allows as far as overprotecting conductors in this manner. Any help would be appreciated an this.
 
ssandoval said:
I realize that OCP has to be sized at 125% and even 175% on occasion depending on other factors, but in no case due any of my calculations allow for a 50 amp breaker to protect #10 wire.
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The breaker does not protect the conductors, the motor OLs do, or in this case the settings in the VSD.

That said the NEC requires the conductors supplying a VSD to be sized based on the VSD ratings and not the motors.

If you happened to run a 1 HP motor from a 50 HP VSD the supply conductors to the VSD would be sized on 50 HP.
 
I'm not sure I understand; your saying the VSD is rated for a certain HP? If it is a 5HP VSD, then how do you size the feeder conductors? Do you use the ampere ratings in the tables in sec 430 to determine the ampacity of the conductors that you will need? Also, my real question that I sill don't understand is, do #10awg conductors protected by a 50amp CB meet code, and is it the fuses in the VSD that actually protect (not only the motor) but the conductors between the breaker and the VSD disconnect switches. I really appreciate any input because we have two new high schools that are full of VSD's but the contractor didn't connect them to the CB sizes shown on the prints. So they are having to change them under warranty, it's just that the MEP guys I talk to all give me different answers when we talk about these issues, and depending on which EE I talk to, their explanantions always vary. Thanks again.
 
ssandoval said:
I've got them on warranty no matter what because the as-builts show these to be a 30 amp and a 50 amp respectively . . .
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Maybe yes, maybe no. The VSD discussion aside, most modern HVAC equipment comes labelled with MCA (minimum circuit ampacity) and MCP (maximum circuit protection) numbers.

The former controls the smallest conductor size permitted, and the latter controls the largest beraker/fuse size permitted. If those ratings are complied with, you may have no warranty issues.
 
The supply conductors to the VFD must be sized at at least 125% of the rated input amps. The branch circuit short circuit and ground fault protective device is sized based on the full load current of the motor as found in Tables 430.247 through 430.250 and Table 430.52 for the OCPD.
 
LarryFine said:
Maybe yes, maybe no. The VSD discussion aside, most modern HVAC equipment comes labelled with MCA (minimum circuit ampacity) and MCP (maximum circuit protection) numbers.

The former controls the smallest conductor size permitted, and the latter controls the largest beraker/fuse size permitted. If those ratings are complied with, you may have no warranty issues.
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Larry,
The original question was about an air handling unit...I have never seen those types of lables on an air handling unit. I have only seen them on equipment that includes a compressor.
 
Don, I have indeed seen MCA/MOC numbers on air handlers, including those containing strip heaters.

I'll try to snap a pic next time I see one.
 
LarryFine said:
Don, I have indeed seen MCA/MOC numbers on air handlers, including those containing strip heaters.

I'll try to snap a pic next time I see one.
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Likewise, most of the A-Hs I see have this data, along with heat strip data or instructions to affix same.

The attached picture is poor quality, but descriptive of the nameplate.View attachment 2124
 
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