Factory Wired Equipment Sizing

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Br4nd0n

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Location
Indianapolis, IN, USA
I'm installing a piece of testing equipment that has a 10 HP/240v-3P motor. There is no equipment nameplate and the manual refers you to the motor's. This equipment has been factory wired to the line side of a small VFD using a flexible cord, but no plug.

The equipment is on castors, so is it more common to install a plug on the cord with a receptacle? Or would hard-wiring be the preferred method?
What qualifies equipment or motors for cord and plug?
Would hard-wiring violate 400.8(1) or does this qualify for "frequent interchange" in 400.7(A)(6)?

For conductors: I've figured Table Amps (28A) X 125% = 35A. The conduit will have 7 current carrying conductors, so after derating 70%, it seems I need 3x#8 THHN.
For OCPD: It's fed out of a standard panel with a Cutler Hammer type BA breaker (inverse time right?) so 28A x 250% = 70A breaker
EGC: #8 here to match the ungrounded conductors right? Sized off 70A, but not to exceed ungrounded conductors 250.122(A).
Do these calculations look valid?

If a receptacle is the practicle solution, my question is with sizing it.The plug and receptacle are part of the wiring, so are they considered protected by the overloads? If so - 50A? If not, I don't see how you could size it from the OCPD at 70A.

I know I'm missing something here and just need a little guidance. Thanks in advance!
 
RichB

RichB

Senior Member
Location
Tacoma, Wa
Occupation
Electrician/Electrical Inspector
Ok I'll take a shot at this:
Using a cord and plug in this situation is common. I don't think it would violate 400.8(1) and it fits 400.7(6) especially as it was factory wired with a cord.
For the conductors,OCPD, and EGC these all look right.
For the recepticle, sizing at 50A{FLA(125%)=35A}would be ok as the start up inrush is for a very,very short period.

My questions:
1. How many recepts would you need to mount?
A. Is this going to be used in a close area, i.e., for instance is it only going to move 20 or 25 feet or is it going to move to several widely spaced locations?

If it is going to be rolled around a close area the I think hard wired with a longer cord may be best(like a roll around welder).

If it going all over the place the strategically placed recepts would probably be best.

If I missed anything or have read things wrong, hopefully some of the other guys will correct and educate me:?
 
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Br4nd0n

Member
Location
Indianapolis, IN, USA
Thank you so much for the response.
I honestly don't think they will be moving it at all. It's a test bench that sits behind their sales counter. So only one receptacle or junction box to hard wire to. Hard wiring seems to be the cheapest and easiest way, but I was concerned that hardwiring specifically might violate 400.8.
So for clarification, the reason a 50A receptacle on a 70A breaker would be ok is because the overloads are considered to protect it as well? The code book doesnt seem clear on this.
 
RichB

RichB

Senior Member
Location
Tacoma, Wa
Occupation
Electrician/Electrical Inspector
In that case I would say hard wire it.

Yes

No problem
 
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Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If you are wiring to the line side of the VFD, 430.122 requires that the conductors be sized at 125% of the VFD Max Amp rating, not the motor FLC. You don't say what the max amp rating is of the VFD, so we can't comment on the right conductor size.
 
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