ECN Forum Forums Code Related Discussion NEC & other Code issues Metal building bonding??

We`re wiring a metal buiding well it has metal skin on outside and the frame work is wood.The question is what is the proper way to bond the skin to ground. Why can`t the mounting screws that hold the meter base be the bond? ajh says no a separate bonding arrangement must be made. Thanks

Reel,
OK, you've got a metal skin, no structure whatsoever (metalically) goes to ground right? Then bonding it is not indicated by the code anymore than houses with aluminum siding have a separate bond strap going to them, nor the metallic face of foil insulation is bonded. No matter what you do, it's painted and will not conduct. Gonna create a heck uva capacitor though, PaulUK and I could create some danged interesting experiments with it

See the 2002 NEC, Section 250.116

(FPN) Where extensive metal in or on buildings may become energized and is subject to personal contact, adequate bonding and grounding will provide additional safety.

2002 NECH Commentary: Because metal siding on buildings is not electrical equipment, it is outside the scope of the Code [see 90.2(A)].

Therefore, the Code cannot require that it be grounded. Quite often, however, luminaires, signs, or receptacles are installed on buildings with metal siding that could become energized.

Grounding of metal siding reduces the risk of shock to persons who may come in contact with the siding.

NOTE: See 90.5


[This message has been edited by Joe Tedesco (edited 08-18-2002).]

Joe,
The only way to correctly apply a bond to metal siding would be to remove the protective coating from between the sheets. I would throw an electrician off my job for altering the structure. The (probably) metallic method of mounting those light fixtures should prove sufficient for the bond.

George,
you mean that each piece of siding would need to be 'bonded', or paint removed from them for them to bond?

that's sort of a stretch...

Negative Sparky, that is an exaggeration. The code does not allow us to bond to something unless we remove any protective coatings. We all know the reason is that it is NOT bonded if it goes through paint/powder coating/rustproofing etc.. Since these siding type panels are all rust coated/painted, and the BOCA/manufacturer would (properly) have a fit if 'sparkies' decided to remove that coating, the deal is "we CAN'T bond it properly". The only way it would properly be bonded is if each piece were attached to a metal structure, and we are certainly required to bond the metal structure. In this case, we have NO metal continuity from piece to piece, so bonding can not practically be accomplished. You would only succeed in bonding the one piece you attached your GEC to and likely really foul up the UL rating of the panels.

Hey George, I can just hear it now.

"Honey, I need to get the resonant frequency a few kilohertz higher. Would you go outside and pull off another sheet of siding?!"

On a more serious note, aluminum siding on houses is extremely rare over here, but a similar situation arises with mobile homes (or "caravans" as they are called here). We are required to bond the structural metalwork to the electrical ground, but not the individual sheets making the outside walls. As has been stated above, to securely bond every piece would require paint to be cleared and/or separate bonding to each sheet.

Paul,
EXACTLY what I was thinking I was going to come up with a resonant freq for the skin, could be one terrific experiment. I just tore down an old metal shop I've had for a long time and replaced it with a new wooden shop or I would be 'sperimenting.

There ARE words in various building codes to the effect that large amounts of exterior metal skins (siding, roofs...) need to be grounded. Problem I have found is that no one can show me a Listed device or system to do it. The problems cited in the other posts are valid. Never seen a metal skin system intended to be electrically continuous with any significant current carrying ability. Seen several with lightning strike damage and the results show that casual contact points blow clear. The lightning protection people might have something but it WILL be expensive!

Iv'e inspected a job where the contractor had a hot tub under a deck. The siding of the house was metal, (AL siding) so he bonded all the metal that was within 5 feet of the spa. That was quite a trick but if I remember correctly he ran a #8 cu up along side the corner channel and connected the indiviual pieces.