Motor Grounding

[T-Wragg]

I recently worked on a 208v three phase air compressor motor in which the electrician before me had grounded the motor with a one foot piece of metal flex conduit from the grounded motor control box. The flex has ends listed for grounding, but enters the motor junction box through reducing washers. Is it required by code to use a grounding bushing or grounding locknut where the flex enters through the reducing washers? If it is necessary could a bonding wedge also be used? Thank you.

 

[Dennis Alwon]

I hope it isn't necessary since I have never used a grounding bushing when I used reducing washers. As far as I know they are suitable for grounding.

This is from the white book

Outlet Bushings and Fittings (QCRV)−Continued Metal reducing washers are considered suitable for grounding for use in circuits over and under 250 V and where installed in accordance with ANSI/NFPA 70, ‘‘National Electrical Code.’’ Reducing washers are intended for use with metal enclosures having a minimum thickness of 0.053 in. for non-service conductors only. Reducing washers may be installed in enclosures provided with concentric or eccentric knockouts, only after all of the concentric and eccentric rings have been removed. However, those enclosures containing concentric and eccentric knockouts that have been Listed for bonding purposes may be used with reducing washers without all knockouts being removed.
CARTON MARKINGS
Fittings for use with flexible cords and marked ‘‘Liquid-Tight’’ on the carton indicates suitability for the use where directly exposed to oil spray or to
rain.

 

[tom baker]

250.118 allows the use of FMC for the conductive path - EGC - if the OCPD is 20 amperes or less and 6 ft or less in length.
There are listed reducing washers available.
but if flexibility is required after installation, and EGC of the wire type must be installed.

 

[Dennis Alwon]

250.118 allows the use of FMC for the conductive path - EGC - if the OCPD is 20 amperes or less and 6 ft or less in length.
There are listed reducing washers available.
but if flexibility is required after installation, and EGC of the wire type must be installed.

Tom are you saying there are reducing washers that are not listed for grounding? Wouldn't the white book have made reference to listed washers if that was the case?

 

[wirenut601]

reducing washers

reducing washers

i dont have my code book handy but i believe ground bushings are only required on fmc connectors for over 250v circuts.

 

[Dennis Alwon]

i dont have my code book handy but i believe ground bushings are only required on fmc connectors for over 250v circuts.

I don't see that but it may be there. FMC can only be used as a ground on circuits that are protected by 20 amps or less.

 

[tom baker]

Tom are you saying there are reducing washers that are not listed for grounding? Wouldn't the white book have made reference to listed washers if that was the case?

Some reducing washers are stamped with UL and some are not. I bet the answer is to look at the box they come in, which I don't have.

 

[don_resqcapt19]

Tom are you saying there are reducing washers that are not listed for grounding? Wouldn't the white book have made reference to listed washers if that was the case?

I thought that everything talked about in the UL White Book was a listed or classified item.

 

[TT009]

i dont have my code book handy but i believe ground bushings are only required on fmc connectors for over 250v circuts.

You are correct 250.97

 

[Dennis Alwon]

You are correct 250.97

That is for circuits 250 volts to ground. Thus a standard 3 phase 208V or 240v system is not affected by this article.

 

[TT009]

That is for circuits 250 volts to ground. Thus a standard 3 phase 208V or 240v system is not affected by this article.

That is correct. The way I read it, 250.96 just state that the frame needs to be effectively grounded. The use of FMC would suffice according to 250.118

250.97 "Over 250V" requires you to use a grounding bushing or other listed devices (250.92(B))

 

[T-Wragg]

The motor draws 20.6 amps, and has a OCPD of 30 amps. From the code citations you guys are listing it appears that an individual ground wire should have been run in the last foot of fmc. The motor says it should be grounded with a #8. It is an industrial motor that can be used either 208-230v/460v. The circuit that supplies it is a 208v, three phase circuit with # 10 wire. A #10 ground wire runs from the panel to the motor control box. The grounded motor control box is bolted to the side of the compressor. The only place a #10 ground was not included was from the control box to the motor. I don't know if this makes any difference.

 

[jumper]

The motor draws 20.6 amps, and has a OCPD of 30 amps. From the code citations you guys are listing it appears that an individual ground wire should have been run in the last foot of fmc. The motor says it should be grounded with a #8. It is an industrial motor that can be used either 208-230v/460v. The circuit that supplies it is a 208v, three phase circuit with # 10 wire. A #10 ground wire runs from the panel to the motor control box. The grounded motor control box is bolted to the side of the compressor. The only place a #10 ground was not included was from the control box to the motor. I don't know if this makes any difference.

If the MFG. instructions call for a #8 than the #10 is a violation, 110.3(B). I know that is probably overkill, but that is the way it is.

The FMC cannot be used since it is more than 20 amps, but that is a moot point at this time.

 

[augie47]

my 2? worth...
The FMC from the starter to the motor (and pressure switch) look like they may be factory wired as part of the compressor with field wiring coming to the starter only. If that were the case then there would be no NEC violation, except for the fact that you brought up the #8 to the motor. If this is part of the manufacturers install instructions, then it should be there. (I have connected a lot of compressors and never ran across such a requirement).
If the FMC is filed installed, then, as mentioned it would not meet the NEC requirements as a ground as it is a 30 amp circuit, and, under "normal" conditions a #10 would need to be installed along with the circuit conductors.
If you have an EGC to the starter and everything is fastened to the steel frame, and the motor FMC is factory installed I would not be concerned unless the aforementioned #8 is a "requirement".