Motor switch size

[schan05]

We are installed three motors with 650A FLA @ 460V, each motor is to be controlled by a VFD within motor vicinity, and there are two old 3p/1600A fusible switches available on the distribution switchboard, neither switch is equipped with GF protection.

I am thinking of two options, one is to connection 2 of 3 motors to one of the 3p/1600A switchs, and then have same-size tap conductors wired to each motor. As per NEC, the conductor for a group motor can be sized at 125% of the largest motor plus 100% of the remainder (1.25*650+650=1462.5A @ 460V), and connect the 3rd motor to the other 3P/1600A switch. If we go with this option, we will have to retrofit both switches with GF protection sensor and trip mechanism. The the option is to replace the existing switches with (3) new insulated power breakers, one for each motor.

My questions is: As per 2005 NEC 430.110(A), the disconnect for motors can be sized at 115% of the NEC FLA table value (table 430.247-430.250) , however the overcurrent protection for the motor circuit have to be 125% of the NEC FLA table value. Another words, the fuses or circuit breaker would be bigger than the switch. Is my interpretation of the NEC correct?

If 115% is ok, we will not have to install 1000A power breakers, 800A will be adequate, do we still have to provide GF protection? since they're not 1000A or move @ 460V. Much Thanks.

 

[rbulsara]

You have a few things mixed up.

1. Where did you get the requirement of a GFP on a motor feeder? None is required by NEC. The requirement you are quoting is for "main disconnects for a builing or structures only".

2. The 115% permitted switch size is for non-fused disconncet, intended to manually disconncet the motor only for service. This assumes that you have a separate upstrem branch overcurrent protection. If you are using the Switchfuse used of overcurrent (short circuit) protection of a motor branch circuit as the local motor disconnect as well then it has be sized for accepting properly sized fuses.

3. A proper solution could be installa 1600A MLO board properly sized breakers for each motor and feed them indidually.

 

[weh3]

The 1000A GF requirement is for service entrances (230-95). You still need O/C protection and a disconnecting means for each motor.

The disconnect must be at least 115% of the FLA, unless it is rated for the horsepower of the motor. In your case, the disconnect must be either 800A or 525 hp.

I would replace the 1600A switches and use three 800A breakers. You could probably even use 700A breakers.

William

 

[schan05]

What about NEC 215.10 Ground Fault Protection for equipment? I thought that would apply here. Please clarify for me. Thanks.

 

[rbulsara]

See Exception No.3 to 215.10 and if you have handbook, read the commentary following it. If you have a GFP on mains (or ahead of the feeder) a separate GFP is not required.

 

[schan05]

rbulsara, my apology for not mentioning this earlier, this is an older building with old electrical distribution system, the upstream or electric service switch doesn't have any GFP. Also, in your opinion, should the fusible switchs I mentioned earlier be at least 125% rated in order to accommodate the fuses? Thanks.

 

[rbulsara]

Do the VFD have manual bypass? The fusing shall meet requirement of a motor feeders as you have it for multiple motors and also for individual motor protection ( like 175% of FLA if using dual element fuses..) which ever is greater. You can not put a fuse larger than the switch rating.

Also rememeber that you need not retroactively install a GFP on existing equipment. But if easy enuff do it. I still think you should find way to replace those fusible disconncets with a proerly sized new switchboard.

 

[schan05]

VFDs don't have manual bypass. I believe 175% is the maximum allowable rating for the dual element fuses for normal starting motors.

Is it specifically spelled on the NEC that existing equipment need not to be retrofitted for GFP? I couldn't find it on my handbook. Thanks for your time.

 

[rbulsara]

Whether or not Code are retroactively applicable are usually mentioned in ammendments or the laguages of the 'laws' passed by state legislatures who in fact adopt these 'standards' as 'Codes' or laws. NEC as it stands is merely a standard not a 'Code' untill adopted by law.

And most codes are not retroactive.

 

[schan05]

Thanks.

 

[schan05]

William, how was 700A breaker considered? can you please enlight me on that? thanks.

 

[weh3]

With an FLA of 650A, your O/C can be as high as 2000A. The 250% FLA is a maximum value for O/C, but it can be less than that. If you are using soft starts or VFD's, then your protection can be lower. I would look for a way to get three molded-case CB's in those two 1600A cubes.

William