bond screw ampacity

[jimport]

An inspector told me today that connecting the ground from an SER cable to a lug on the disconnect enclosure was not the preferred method. He said I should have used the lug next to the neutral as it did not depend on the bond screw to clear the fault. The lug next to the neutral was not large enough for the ground so I used the supplied lug in the threaded hole per manufacturers instructions. He also said I should have grounded to the Line side of the neutral buss, not the bottom lugs on the buss. This was on a Square D QO disconnect for a 200 A service. As the buss is continuous I do not see how this would matter.

Could someone tell me how UL would list this? What is the ampacity requirement for a bond screw?

 

[paul]

Re: bond screw ampacity

I would think that manufacturers instructions take precedence.

 

[George Stolz]

Re: bond screw ampacity

An inspector told me today that connecting the ground from an SER cable to a lug on the disconnect enclosure was not the preferred method.

Then let him know that if he would prefer to change it, you'd prefer to watch.

 

[mc5w]

Re: bond screw ampacity

A better method of bonding is to run a #4 solid copper wire or large stranded wire from the neutral bus to a grounding bar. I have put as many as 3 grounding bars in a 40 slot panel so that I can keep the bare grounding wires short and away from the circuit breakers.

In a 40 slot panel you need a ground bar anyways so as not to run out of neutral terminals.

I usually do not use the factory holes for the ground bars in a 40 slot panel and instead drill 9/64ths inch holes in an area such as the top or bottom so that bare grounding wires do not have to be near circuit breaker terminals. Also, when changing a panel putting a ground bar in the upper left or right corner avoids having to splice the grounds.

The PK15GTAL through PK23GTAL ground bars have a terminal for 1/0 to 4/0 conductor. I have a PK15GTAL in a 200 amp meter main that I use for construction sites.

You can also get add-on terminals for ground bars and neutral bars that allow termination of large wires. Catalog numbers are LK100AN ( #6 to 2/0 ), LK150AN ( #2 to 3/0 ), and LK225AN ( #4 to 300 KCM ). Some of these attach in place of a setscrew and the LK225AN has 2 tabs that go into 2 terminals that normally would only accept #4 wire.

 

[mc5w]

Re: bond screw ampacity

The bonding screw is really only meant for bonding of the panelboard box, not for branch circuits.

A 10-32 or 12-24 steel screw is not going to have very much ampacity. Just take my word for it. In the case of a ground bar connecting a #4 or larger wire to the neutral bus will greatly outdo the 8-32 attachment screws.

[ May 20, 2005, 02:22 AM: Message edited by: mc5w ]

 

[bphgravity]

Re: bond screw ampacity

I don't think a screw has an "ampacity" or at least that function isn't really all that important. I mean, does a screw have a temperature rating? A screw does need to have a determined withstand rating, which would be determined by a NRTL. That rating would be of greater importance.

There is not a huge documented history of bonding screw failures, so I believe they are quite effective.

 

[charlie]

Re: bond screw ampacity

The reason a small 10-32 machine screw can be used is that it doesn't carry current except under fault conditions and then for a very short period of time. That is also why those screws do not have an ampacity rating.