Bonding

[bphawk48]

Hi, I'm an apprentice and I ran into something at work and had a Querstion. I asked a few guys at work, looked threw my code book and couldn't find a definitive answer. Any help or clarification would be greatly appreciated.

We were at work opening up panals and transformers. Some of them were 480V, and some 208V. The transformers were 30KVA and 75KVA.

For example, one set up they had was a 480/277V panel feeding a transformer. Then dropping the Voltage down to 208/120V feeding another panal.

There didn't seem to be any consistancy in which they used bonding bushings. A guy I worked with said for 120/208 in which you punch your own hole you didn't need the bushing. But for over 300V you needed a bonding bushing no matter what?

My Question is when do you use bonding bushings?

When can you get away with just the bonding locknut?

Does the Voltage, Concentric KO's, Line or Load matter?

Both sides of the flex(Panal and Transformer)?

Thanks

 

[augie47]

Hi, I'm an apprentice and I ran into something at work and had a Querstion. I asked a few guys at work, looked threw my code book and couldn't find a definitive answer. Any help or clarification would be greatly appreciated.

We were at work opening up panals and transformers. Some of them were 480V, and some 208V. The transformers were 30KVA and 75KVA.

For example, one set up they had was a 480/277V panel feeding a transformer. Then dropping the Voltage down to 208/120V feeding another panal.

There didn't seem to be any consistancy in which they used bonding bushings. A guy I worked with said for 120/208 in which you punch your own hole you didn't need the bushing. But for over 300V you needed a bonding bushing no matter what?

My Question is when do you use bonding bushings?

Perhaps no simple answer. Best references are 250.92 and 250.97
In general you use bonding bushings (or locknuts) to assure continuity on service raceways and on raceways with conductors over 250 volts to ground, unless the componets are listed so as not to need them, or you have hubs. (not an inclusive answer)

When can you get away with just the bonding locknut?

Short answer, a bonding locknut will normally suffice if you are bonding on a "punched" knockout. Bushing come into play more on concentric knockouts.

Does the Voltage, Concentric KO's, Line or Load matter?

all do. Other than service, you rarely need a grounding bushing on 250 volts or less unless you encounter a system where locknuts won't bond you raceway.

Both sides of the flex(Panal and Transformer)?

Since flex is not an approved ground path on higher currents, bonding bushings accomplish little with flex as you would only be bonding the connector. The flex is a weak link and normally a condition must be used to supplement it.

And WELCOME to the Forum.

 

[infinity]

Gus outlined most of the scenarios. Bottom line is, since you have FMC on both the primary and secondary bonding bushings are not required on either raceway.