Flexable Metal Conduit and bonding at transformers

[MattMill]

I am trying to locate where in the code that the FMC needs to be bonded when it enters a dry transformer. I have a journeyman who does not put on a bond bushing, and states that it is not required. I got out the trusty code book and cant find anything definitive on this. Thanks

 

[RUWired]

Matt Welcome to the forum. Thats a good question and it sort of hard to find. The article is 250.97 exception # 3 for the primary side if 480 volt. Basically, if using approved fittings for FMC and you punch out or use a factory KO, You don't need any bonding jumpers.

As for the secondary side, FMC is not listed as an EGC when circuits over 20 amps are installed in the raceway, therefore a bonding bushing would do no good unless its connection is loosely installed. A lot of guys think the secondary side of the transformer is like a service and want to follow those rules for bonding on the line side, but it's not nessesary.

 

[infinity]

I agree bonding bushings are not required on a transformer. Even if you installed them on FMC what purpose would they serve?

 

[stevee]

Matt Welcome to the forum. Thats a good question and it sort of hard to find. The article is 250.97 exception # 3 for the primary side if 480 volt. Basically, if using approved fittings for FMC and you punch out or use a factory KO, You don't need any bonding jumpers.

As for the secondary side, FMC is not listed as an EGC when circuits over 20 amps are installed in the raceway, therefore a bonding bushing would do no good unless its connection is loosely installed. A lot of guys think the secondary side of the transformer is like a service and want to follow those rules for bonding on the line side, but it's not nessesary.

So, I guess I'm "one of those guys." I was always told the secondary of a transformer was now a new service and so bonding had to be ensured. In most cases the inspectors here in California want bonding bushings and jumpers installed on the secondary side of a transformer between the Xfmr and the Panelboard it is supplying.

Now, lets look at the purpose of bonding on the line side of a service. Bonding is more stringent on the line side because whenever a ground fault occurs in the system the service equipment will conduct that current. If the SBJ and GEC connections are made in the transformer then there are two paths back to the SBJ for ground fault current. The first path being thru the EBJ and the second path being the metal raceway between the Xfmr and the Panelboard. Now if this metal raceway is a FMC or EMT or even Liquid tight FMC and the connections are not good due to connector type arcing can occur which may start a fire or other problems. Don't you think ensuring the bonding of the metal raceway is important?

 

[augie47]

I would have to side with the "what purpose" folks. If the FMC is not allowed to be an equipment ground for circuits over 20 amps, what purpose could the bond bushing serve ? You would be assuring the connection to a 20 amp conductor.
Although the analogy of SDS to services has some limited merit, I know of no Code section which requires the transformer secondary bonding to be treated the same as a service.

 

[don_resqcapt19]

So, I guess I'm "one of those guys." I was always told the secondary of a transformer was now a new service and so bonding had to be ensured. In most cases the inspectors here in California want bonding bushings and jumpers installed on the secondary side of a transformer between the Xfmr and the Panelboard it is supplying.

Now, lets look at the purpose of bonding on the line side of a service. Bonding is more stringent on the line side because whenever a ground fault occurs in the system the service equipment will conduct that current. If the SBJ and GEC connections are made in the transformer then there are two paths back to the SBJ for ground fault current. The first path being thru the EBJ and the second path being the metal raceway between the Xfmr and the Panelboard. Now if this metal raceway is a FMC or EMT or even Liquid tight FMC and the connections are not good due to connector type arcing can occur which may start a fire or other problems. Don't you think ensuring the bonding of the metal raceway is important?

Guven the low current carrying abilty of FMC or LFMC, I don't see any advantage to use a bonding bushing. For the non-flexible metallic raceways, you may get a better path with the bushing, but it is not required.

 

[infinity]

I would have to side with the "what purpose" folks. If the FMC is not allowed to be an equipment ground for circuits over 20 amps, what purpose could the bond bushing serve ? You would be assuring the connection to a 20 amp conductor.
Although the analogy of SDS to services has some limited merit, I know of no Code section which requires the transformer secondary bonding to be treated the same as a service.

That's my take on it. The FMC cannot be used as an EGC in the first place so what would a bonding bushing actually do? IMO it's kind of like installing a bonding bushing on a PVC terminal adapter.

 

[MattMill]

Thanks to all for the quick response!! We do havea jurisdiction that requires (at this point...requests :grin both the line side and load side of the transformers to be bonded. I spent quite a few hours on this reviewing this forum and nose in the code book. I enjoy using this forum (it makes me look smarter :roll..............Thanks again

 

[infinity]

Thanks to all for the quick response!! We do havea jurisdiction that requires (at this point...requests :grin both the line side and load side of the transformers to be bonded. I spent quite a few hours on this reviewing this forum and nose in the code book. I enjoy using this forum (it makes me look smarter :roll..............Thanks again

What do you mean by bonded? Are the raceways required to be bonded? Is this a local code rule?

 

[Bea]

I know of no Code section which requires the transformer secondary bonding to be treated the same as a service.

I dont have my code book in front of me but acording to 250.30 and 250.106 the equipment bonding jumper between transformer and panel board secondary is sized in accordance to 250 . 66 much the same as services. I belive I have quoted correct articles not sure about 250.106 but I believe that is the article number addressing equipment bonding jumpers. If wrong please don't mind correcting me.

 

[raider1]

I dont have my code book in front of me but acording to 250.30 and 250.106 the equipment bonding jumper between transformer and panel board secondary is sized in accordance to 250 . 66 much the same as services. I belive I have quoted correct articles not sure about 250.106 but I believe that is the article number addressing equipment bonding jumpers. If wrong please don't mind correcting me.

Actually it is 250.102(C) not 250.106. 250.106 is for lightning protection systems.

You are correct about the sizing of equipment bonding jumpers on the secondary side of a transformer. 250.30(A)(2) requires that if an equipment bonding jumper of the wire type is installed then it must be sized in accordance with 250.102(C).

That section does not address bonding of FMC on the secondary of a transformer.

Chris