Burnt up lug

[krsparky]

I was called out to repair a burnt up lug and conductor on an solar inverter. The feeder was paralell 2/0 copper terminated to a two barrel lug with a 3/8 hole in the pad. The lug was terminated to the bus bar in the inverter with a 10-32 screw with a washer and a nut. Would this small screw have caused the excessive heat?

 

[ptonsparky]

If it looks fishy, smells fishy, it could be fishy.

Sounds like a field engineered connection to me.

 

[Ponchik]

It is usually a minimum 1/4--20 bolt and nut not a 10/32 screw.

 

[don_resqcapt19]

The bolt should be the same size as the hole in the lug.

 

[petersonra]

The bolt should be the same size as the hole in the lug.

based on what?

unless the manufacturer says so, I don't recall there being any such rule.

 

[don_resqcapt19]

based on what?

unless the manufacturer says so, I don't recall there being any such rule.

That is why I used the word should and not the word shall

I have seen a nubmer of articles that talked about where most of the current flows when you have a lug bolted to bus. It is mosly directly under the bolt head and washer. That is where the contact pressure is the greatest. Using a bolt that is smaller than the hole will reduce the area of high contact pressure.

 

[petersonra]

That is why I used the word should and not the word shall

I have seen a nubmer of articles that talked about where most of the current flows when you have a lug bolted to bus. It is mosly directly under the bolt head and washer. That is where the contact pressure is the greatest. Using a bolt that is smaller than the hole will reduce the area of high contact pressure.

I posed this question to panduit earlier today.

is there any requirement that a lug such as an LCA use a bolt or stud of the same size as the stud hole on the lug?

for instance, could one 'legally" use a 5/16" bolt on a lug with a 3/8" stud hole?

I was not able to find any guidance on this in the catalog or online.

I got this answer back.

You can use the 5/16 bolt with the 3/8 stud hole lug. There are not any issues as long as you are using the proper wire gauge. Please let us know if you need anything else.

 

[Zee]

5/16" bolt in a 3/8" hole is just one standard bolt size lower than the maximum allowable.

HOWEVER: a single #10-32 screw < #12 screw < 1/4" bolt < 5/16" bolt and FINALLY smaller than 3/8".

Certainly seems small.
Especially considering that this was a dual barrel lug w/ 2 @ 2/0 conductors!! Ampacity around 300-400 AMPS.........

 

[petersonra]

5/16" bolt in a 3/8" hole is just one standard bolt size lower than the maximum allowable.

HOWEVER: a single #10-32 screw < #12 screw < 1/4" bolt < 5/16" bolt and FINALLY smaller than 3/8".

Certainly seems small.
Especially considering that this was a dual barrel lug w/ 2 @ 2/0 conductors!! Ampacity around 300-400 AMPS.........

There is probably some practical limit. I don't know what it is though. I have not been able to find a defintive answer to this particular question.

 

[BillK-AZ]

The other consideration is having the mechanical integrity to stay in place during a fault while the OCPD opens. If the cable is the feed this is real important.

 

[hurk27]

In what I was always taught is the maintaining of the connection of the two parts is the most important factor, the bolt is not the connection point, it is there to maintain it, the problem happens when we put steel bolts in to hold together aluminum buss or lugs, as aluminum will expand faster then the steel, and when it cools it will be loose because of cold flow of the aluminum, this is where bevel washers come into play, bevel washers act as a spring to allow the aluminum to expand and contract without the cold flow, this keeps the connection between the two parts for loosening up.

With that said, I found a few manufactures that stopped or never did use any kind of expandable connection, many of square D panels will have the bevel washers while T&B never had them, T&B were always failing on the buss connections, I still have a small selection of bevel washers in my truck, from 1/4"-3/8" that I use when I run across aluminum busses or lugs.

 

[petersonra]

I certainly did not expect this answer. I sent panduit another email.

is there some lower limit? could I use a 4-40 screw? as a practical matter i know that won't work, but it would seem there must be some kind of guideline for the bolt sizes that are acceptable for a given lug.

This is their response.

The suggested sizes are the bolt hole sizes. You can use any size smaller that you would like. You may have to use a washer to secure properly. We do not recommend using larger bolts for the application. Please let us know if you need anything else.

 

[petersonra]

In what I was always taught is the maintaining of the connection of the two parts is the most important factor, the bolt is not the connection point, it is there to maintain it, the problem happens when we put steel bolts in to hold together aluminum buss or lugs, as aluminum will expand faster then the steel, and when it cools it will be loose because of cold flow of the aluminum, this is where bevel washers come into play, bevel washers act as a spring to allow the aluminum to expand and contract without the cold flow, this keeps the connection between the two parts for loosening up.

With that said, I found a few manufactures that stopped or never did use any kind of expandable connection, many of square D panels will have the bevel washers while T&B never had them, T&B were always failing on the buss connections, I still have a small selection of bevel washers in my truck, from 1/4"-3/8" that I use when I run across aluminum busses or lugs.

The panduit catalog does say to use Belleville washers with aluminum lugs. Also suggests antiox comppound. Does not say anything special about aluminum bus.