Bonding of 24VDC instrument junction box

[Dale001289]

Recently, the inspector cited junction boxes that were not bonded. He said the shield on the circuits could be used for bonding the box. However, there are two 3/4" LFMC attached to the box with locknuts on the inside. LFMC is a recognized EGC per 250.118, so why cant this be considered sufficient bonding for the box?

 

[petersonra]

Recently, the inspector cited junction boxes that were not bonded. He said the shield on the circuits could be used for bonding the box. However, there are two 3/4" LFMC attached to the box with locknuts on the inside. LFMC is a recognized EGC per 250.118, so why cant this be considered sufficient bonding for the box?

I think the inspector is wrong. You do not want to use the shield as a bonding conductor. For one thing it is way too small. The smallest bonding conductor allowed is #14.

 

[Isaiah]

I think the inspector is wrong. You do not want to use the shield as a bonding conductor. For one thing it is way too small. The smallest bonding conductor allowed is #14.

Agreed - I’ve never seen one done like that either


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[Dale001289]

I think the inspector is wrong. You do not want to use the shield as a bonding conductor. For one thing it is way too small. The smallest bonding conductor allowed is #14.

But what about using the flex as a bond?


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[CoolWill]

But what about using the flex as a bond?


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As long as it isn't over 6 feet in length you're good.

 

[petersonra]

Let me ask another question. These are probably class 2 circuits. Is there any code requirement to actually run an EGC with class 2 circuits?

 

[Dale001289]

Let me ask another question. These are probably class 2 circuits. Is there any code requirement to actually run an EGC with class 2 circuits?

The circuits come off a class 1 power supply in the PLC which my understanding is they automatically default to class 1 but they are all 24VDC


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[jaggedben]

If your 24VDC is separately derived and ungrounded then the only thing accomplished by bonding the box is to ensure that if there's a fault in the box then everything else bonded to the box becomes energized at 24V to the other DC conductor. Seems low stakes and pretty ticky-tack on the part of the inspector.

I suppose 250.4(A)(3) still requires the box to be bonded though.

 

[petersonra]

The circuits come off a class 1 power supply in the PLC which my understanding is they automatically default to class 1 but they are all 24VDC


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I think if you look very closely at what the code says, they are still class 2 circuits.

 

[jaggedben]

I think if you look very closely at what the code says, they are still class 2 circuits.

No, that depends on the power supply.

 

[petersonra]

 

[jaggedben]

View attachment 2576718

And?
OP said it was class 1. If it's not part of a listed assembly that's marked as something other than Class 1, then it's still Class 1. Also see chapter 9 Tables 11(A) and (B).

 

[petersonra]

And?
OP said it was class 1. If it's not part of a listed assembly that's marked as something other than Class 1, then it's still Class 1. Also see chapter 9 Tables 11(A) and (B).

Look at exception 2. Then look at the spec on power limits for this ain card.

 

[jaggedben]

We haven't established that his power source has those limits. I agree there's a good chance you're right, but the code does not say that a Class 1 power source becomes Class 2 just because it's part of a PLC.

 

[Isaiah]

If your 24VDC is separately derived and ungrounded then the only thing accomplished by bonding the box is to ensure that if there's a fault in the box then everything else bonded to the box becomes energized at 24V to the other DC conductor. Seems low stakes and pretty ticky-tack on the part of the inspector.

I suppose 250.4(A)(3) still requires the box to be bonded though.

I don’t think a PLC power supply would be considered a separately derived system


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[jaggedben]

I don’t think a PLC power supply would be considered a separately derived system

That basically depends if it has transformer.