Grounding separate structure steel

[petersonra]

Bonding building steel to the EGC is a good idea because then there is a path back to the main bonding jumper and ground fault current can find it's way home. Earth is not a good conductor.

The thing is unless the steel gets energized somehow, it just does not matter. If it gets energized, the steel should be bonded by whatever circuit is energizing it. So any extra bonding serves no real purpose.

 

[Anderman123]

So it seems that 250.32(B)(1) says to use the EGC sized in 250.122, in this case #12 wire for 20A circuit, to bond to equipment and structures.

But 250.104(C) says I need to use a bonding jumper sized by 250.102(C)(1) for the steel building, which would be #8. In this case, is it asking me to bond the building with a #8 to the #12 GEC of the only branch circuit to the building?

 

[petersonra]

and what is your point?

 

[jap]

So it seems that 250.32(B)(1) says to use the EGC sized in 250.122, in this case #12 wire for 20A circuit, to bond to equipment and structures.

But 250.104(C) says I need to use a bonding jumper sized by 250.102(C)(1) for the steel building, which would be #8. In this case, is it asking me to bond the building with a #8 to the #12 GEC of the only branch circuit to the building?

250.104(C) Structural Metal. Exposed structural metal that is inter‐connected to form a metal building frame and is not intentionally grounded or bonded and is likely to become energized shall be bonded to any of the following:
(1)Service equipment enclosure
(2)Grounded conductor at the service
(3)Disconnecting means for buildings or structures supplied by a feeder or branch circuit
(4)Grounding electrode conductor, if of sufficient size
(5)One or more grounding electrodes used, if the grounding electrode conductor or bonding jumper to the grounding electrode is of sufficient size


Since you are only running a single 20 amp circuit to feed the outbuilding with an EGC the EGC is going to get you where you need to be.
You're saying that there's no overload protection, but, there is where the 20 amp branch circuit originates. or at least I hope there is.
There is no need to us a bonding jumper that is larger than the EGC ran with the branch circuit to the out building.
The rule above indicates the metal building shall be bonded by "ANY" of 1,2,3,4 or 5,,, not all of them.
The #12 EGC you are pulling with the branch circuit to the outbuilding (if wired correctly) already makes the connection to #1 and#2.
When you connect the EGC from the branch circuit to what I'm assuming is going to be a metal 4 square box or handy box you take care of #3.
You probably wont have a situation #4 or #5 at the out building.
I don't think it's required if you are only running a single 20a branch circuit out there, but I'd have to look it up.

If a ground rod out there was required, I'd treat it like the rules of a separate structure and bonding requirements, and, that's where I'd start considering using a #8 since I never bring anything smaller than that to a ground rod anyway.

 

[suemarkp]

I think the point is that water piping and building steel use 250.102 to size their bonding jumpers, not 250.122. The minimum size in that table is #8. He only ran a #12 branch circuit to that building. It sounds like he needs to run a jumper from building steel to the building disconnect with #8 as none of the other choices in 250.104(C) are available or have a conductor that large run to them from the originating service.

Seems kind of silly to run a #8 jumper from steel to the disconnect when the branch circuit only has a #12 back to the service. He may need to put a small ground bar or lug in that 4 square or whatever the building disconnect is in order to land that #8.

 

[LarryFine]

I would just mount a grounded metal box directly to the building steel.

 

[Anderman123]

I think the point is that water piping and building steel use 250.102 to size their bonding jumpers, not 250.122. The minimum size in that table is #8. He only ran a #12 branch circuit to that building. It sounds like he needs to run a jumper from building steel to the building disconnect with #8 as none of the other choices in 250.104(C) are available or have a conductor that large run to them from the originating service.

Seems kind of silly to run a #8 jumper from steel to the disconnect when the branch circuit only has a #12 back to the service. He may need to put a small ground bar or lug in that 4 square or whatever the building disconnect is in order to land that #8.

This is my point exactly.

While there are provisions in some cases to not have to use a ground wire larger than the EGC, I don't see that provision in this part of the code.

 

[JoeStillman]

The thing is unless the steel gets energized somehow, it just does not matter. If it gets energized, the steel should be bonded by whatever circuit is energizing it. So any extra bonding serves no real purpose.

Keywords here: "unless the steel gets energized". You are right, it doesn't matter. But only until it does. If a hot wire faults to the steel, there needs to be a path back to the service neutral point so that there can be enough current to trip a breaker. Earth has too high of a resistance and you won't get 20A.

What do you mean by "bonded by whatever circuit is energizing it"?

 

[jap]

Keywords here: "unless the steel gets energized". You are right, it doesn't matter. But only until it does. If a hot wire faults to the steel, there needs to be a path back to the service neutral point so that there can be enough current to trip a breaker. Earth has too high of a resistance and you won't get 20A.

What do you mean by "bonded by whatever circuit is energizing it"?

The OP is not disputing this fact.

He's simply bringing up the irony of the size of the bonding jumper for building steel in the table being a minimum size of #8, which he's going to also tie to his #12 EGC he pulled to the metal building.

This table was mostly tailored for Service Entrance conductors where a #8 would make sense, but, by rights, since the coder refers to this table for bonding evidently a #8 it is.

Some would let the #12 EGC in the branch circuit bond the building by making a connection of a metal box to the metal building.
The OP's interpretation is to take a #8 bonding jumper to the #12 EGC.

Nothing wrong with either,,,,different strokes for different folks.

Others are saying the building will be bonded by the EGC pulled with the branch circuit (the circuit that is energizing it) because that #12 EGC of that circuit is connected to the metal building.


JAP>