Main Bonding Jumper 10-32 ampacity

[tom baker]

A question came up in a grounding and bonding class. What is the ampacity of a 10-32 screw? The question was in relation to a main bonding jumper. My only reply is the panel mfg has the panel tested and listed by UL so the main bonding jumper is adequate for the application. Anyway the ampacity must be in the thousands of ampacity?

 

[jimwalker]

Re: Main Bonding Jumper 10-32 ampacity

might point out that all of that current does not travel thru the screw.Some is from metal to metal

 

[pierre]

Re: Main Bonding Jumper 10-32 ampacity

Tom
Don't you just love questions like that Just when you think you are getting through the presentation, someone will ask a question like that - something that you never really put much thought to before.

Pierre

 

[c-h]

Re: Main Bonding Jumper 10-32 ampacity

If you figure out the cross sectional area of the screw you can make a rough calculation. I looked up the 10-32 screw in a table and found that the outer diameter of the thread is about 4.8 mm. (Does that sound right? I've never seen a 10-32 screw) This means the cross section is about 18 mm2. From memory, the resistivity of steel is about ten times that of copper. If it is pure steel, it will have about the same resistance as a 2 mm2 (14 AWG) copper conductor.

 

[george t. everett]

Re: Main Bonding Jumper 10-32 ampacity

Jim.
In a main breaker panel the neutral bar is connected to the panel case thru the main bonding jumper. There is no other metal to metal contact.

 

[charlie]

Re: Main Bonding Jumper 10-32 ampacity

Tom, you have answered your own question. UL (and the other NRTLs) do not list a 10-32 for any ampacity. However, they do test the screw for its ability to withstand the maximum fault current for which the panelboard is listed for.

BTW, I always use the term panelboard since all load centers are panelboards but all panelboards are not load centers. Additionally, the Code doesn't know what a panel is and it is not defined.

 

[al hildenbrand]

Re: Main Bonding Jumper 10-32 ampacity

Tom,

That is an interesting question. I see three different areas of interest. </font>

• <font size="2" face="Verdana, Helvetica, sans-serif">The 10/32 screw and the conductivity of its metal,</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">The effective cross section area of the enclosure metal which is smallest at the perimeter of the screw hole, and the enclosure metal conductivity,</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">The quality of the metal to metal contact between the screw and the enclosure.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"> Item 3 is the wild card that the manufacturer can only hope to influence with the installation instructions. When correctly installed, the quality of the connection can still be degraded by environmental factors at any time during the life of the installation.

The successful behavior of the 10/32 bond will, IMO, be nonlinear, a curve very similar to a fuse curve, just at slightly lower current vs. time levels. The bond will work until the connection quality degrades. The real test of the bond occurs under high current, short duration pulses. The pulse can climb until just before the metal melts.

The idea of a 10/32 ampacity has the underlying concept of steady state current and heat handling ability which I would equate with a red herring. I think its more important to look at the installation instructions, trust the background engineering and lab testing, and do a solid mechanical assembly.

 

[charlie b]

Re: Main Bonding Jumper 10-32 ampacity

Article 100:

Ampacity. The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

By definition, therefore, the concept of ?ampacity? cannot be applied to a screw or to anything other than a conductor.

 

[tom baker]

Re: Main Bonding Jumper 10-32 ampacity

An interesting note to main bonding jumper. In Vancouver British Columbia, they use a wire in place of the screw. They found the screw corrodes and becomes loose.