Separate Structure made of steel

[M. D.]

Dennis, this is a very good question.

250.104(C) requires that exposed structural metal that is not intentionally grounded and is likely to become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor or the grounding electrodes used. This also requires that the bonding jumper used to connect the building metal be sized in accordance with 250.66 and installed in accordance with 250.64(A),(B), and (E).

This section does not really address a detached structure.

IMHO, the easiest way to comply with this section is to intentionally ground the metal by connecting it to 2 ground rods.

Chris

I'm not sure that the equipment grounding conductor connected to the metal frame could be counted as being not "intentionally grounded" If so they need to change the name of it

Grounded(Grounding). Connected (connecting) to ground or to a conductive body that extends the ground connection

Ground. The earth

 

[raider1]

I'm not sure that the equipment grounding conductor connected to the metal frame could be counted as being not "intentionally grounded" If so they need to change the name of it

Grounded(Grounding). Connected (connecting) to ground or to a conductive body that extends the ground connection

Ground. The earth

That is a very good point, but if connecting the metal structure to an EGC is intentionally grounding the metal, why would that not be an accetable method in accordance with 250.104(C)?

According to 250.104(C) I must size the bonding conductor in accordance with 250.66 and install the conductor like a GEC.

Chris

 

[SEO]

Dennis it appears that you are using section 250.32(A) exception which eliminates the requirement for a grounding electrode and are using section 225.36 exception for your disconnecting means.(snap switches).

If you are required to bond the structural metal per section 250.104(C) your bonding jumper shall be sized in accordance with Table 250.66 and could be bonded to the service enclosure which is your metal box that houses your snap switches or install a disconnect that is suitable for service equipment. Using table 250.66 a #12 conductor is not listed a #2cu is the smallest and requires a #8. IMO the worst case would be to bond the steel with a #8cu to the service enclosure in the garage.

 

[electricmanscott]

I use this for gas piping so I'll use it here. Is the steel "likely to become energized"? If you answer yes, why and how? If you answer no, my answer, no bonding required.

 

[Dennis Alwon]

I use this for gas piping so I'll use it here. Is the steel "likely to become energized"? If you answer yes, why and how? If you answer no, my answer, no bonding required.

One could certainly argue that if any electrical equipment is in contact with the steel then it is "likely " to be energized. Likely is a bad work because I think it is very unlikely in almost every case.

The fact that the building is a separate structure seems to negate the GEC. Fortunately the carport is only a few feet (10) from the house but that really isn't the issue. Even if the steel were likely to get energized wouldn't the EGC cover that?

 

[Jim W in Tampa]

Ask them how this differs from a mobile home. Yes I beam is bonded but there is no solid contact from sideing to the I beam. And i very much see a problem likely. Also ask them how they deal with frame stuco. They are looking for a problem that simply is not there.

 

[electricmanscott]

This is what I was going to do but the inspection department wants a #4 back to the panel. I can't seem to argue it well enough to make my point.

Don't argue, turn the tables. Ask them why and what is this going to accomplish?

All you are trying to do is open an ocpd should the steel become energized. What's going to energize the steel? The circuit providing power to the structure. That's why you have an egc run with the circuit conductors. No reason for it to be giganto sized in relation to the circuit conductors.

Just mounting the disconnect to the steel will do the trick. At worst a bonding jumper from the disco to the steel sized the same as the circuit is more than enough.

That is of course if you can predict the future and know that this steel WILL become energized.

 

[SEO]

A #4 back to the main panel, what do they do with swimming pools? Must be that detached garage sure is a hazzard. Have they backed up their requirement with a code section?

 

[cadpoint]

NC has something like the 5th most lighting strikes verses the US, mostly due to rocks or puppy boulders buried four to 10 feet below the surface!

I know the TV weather man doesn't mind showing all of the strikes with a passing front.

I don't believe anyone has asked if there is a concrete pad and that you might need to tie the rod's or mat together with the other circuit? Aren't they in fact trying to creat an equipmental plain, and not saying so?

By Code the grounded wire in a box never has to bigger than the circuits that are made up in it. I don't get the additional up-sized wire (desired) or stated required back to the main panel.

Come on, this has gone every where...

 

[benaround]

Dennis,

I really don't see any other section that pertains to your situation except for 250.32, Its'

heading "Buildings or Structures Supplied by a Feeder or a Branch Circuit", all the other

sections are for other buildings and supplies. In 250.32 ( as you know ) the exception

for a single branch circuit relieves the requirement for a grounding electrode as long as the

circuit contains an equipment grounding conductor for grounding the normally non-current-

carrying metal parts of equipment.

Article 100 Definitions; Equipment. A general term including, materials,fittings, devices,

appliances, luminaires,apparatus,machinery, and the like used as a part of, or in connection

with, an electrical installation.

IMO, this covers your entire installation. The egc in a metal box mounted to the metal

supports of the carport will ensure it is not likely to become energized.

 

[M. D.]

.........

According to 250.104(C) I must size the bonding conductor in accordance with 250.66 and install the conductor like a GEC.

Chris

Only if the exposed structural metal has not been intetionally grounded .. once you satisfy this,. the rest is moot.

Can anyone show me where this can not be accomplished by use of the grounding conductor of the circuit??

 

[M. D.]

Dennis,

I really don't see any other section that pertains to your situation except for 250.32, Its'

I think it could be argued that section 250.104(c) applies to exposed structural metal as described whether or not there is even a branch circuit run to the building .

 

[benaround]

M.D.,

What do you think of 250.32(B)(1), IMO, it covers this aspect.

 

[M. D.]

M.D.,

What do you think of 250.32(B)(1), IMO, it covers this aspect.

I'd say I agree 250.32 (b) covers his situation.

 

[Pierre C Belarge]

Dennis
The equipment ground conductor installed with the supply conductors (12/3) is sufficiently sized.
Why 250.66 keeps being discussed I am not sure.


250.104(A)(3) Multiple Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).

The bonding jumper shall not be required to be larger than the largest ungrounded feeder or branch circuit conductor supplying the building.



This is the last sentence of the subsection.



If you are supplying wiring to the metal structure, the question is not will it become energized, the comment is I do not know if it will ever become energized. That is good enough to say it the bonding is necessary, as in our industry we try not to leave any safety aspect in question.


If at worst, use a short bonding jumper connected to the box and the steel structure as your required bonding jumper. Then you are done .

 

[M. D.]

Dennis
The equipment ground conductor installed with the supply conductors (12/3) is sufficiently sized.
Why 250.66 keeps being discussed I am not sure.


250.104(A)(3) Multiple Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).

The bonding jumper shall not be required to be larger than the largest ungrounded feeder or branch circuit conductor supplying the building.



This is the last sentence of the subsection.

Yah,.. only that section deals with bonding metal water piping systems.


250.104(c) requires his frame to be ,.. and 250.32 says the EGC can be

 

[electricmanscott]

Dennis


If you are supplying wiring to the metal structure, the question is not will it become energized, the comment is I do not know if it will ever become energized. That is good enough to say it the bonding is necessary, as in our industry we try not to leave any safety aspect in question.

I don't really agree. The NEC says "likely to become energized" . To me saying I don't know if it will become energized is allot different than saying it is likely to become energized.

Under normal conditions the building is likely NOT to become energized.

If the wording was "likely to become energized under xyz conditions" or "in the event of whatever" it might have more weight as far as an absolute requirement for bonding. In my view anyway.

 

[M. D.]

Two definitions ,.. niether of which are found in the NEC so take them for what they are I guess,... a couple more opinions

The NEC style manual

Likely to Become Energized: Failure of insulation on

by Jeffrey Sargent, Noel Williams
Likely to Become Energized:
Failure of insulation on energized parts or conductors........
..........likely in this sense does not address the probability that an insulation failure will occur.

 

[electricmanscott]

Two definitions ,.. niether of which are found in the NEC so take them for what they are I guess,... a couple more opinions

Those are great. :grin:

I guess the NEC just ignores the actual definitons of words and substitutes what they want something to mean. :roll:

 

[Dennis Alwon]

Well I called the nc state inspector because I was curious as to his interpretation. He absolutely agrees that the #4 bond is unnecessary and that the EGC of the circuit is all that is needed because it is a separate structure. I guess the rules change if this was an attached structure.