Codemakers gone wild: protection from physical damage

WillK · 06-14-2011, 08:11 PM

One of my inspector's issues has been my use of schedule 40 PVC conduit for 3 short runs in my garage, he is requiring that I use conduit rated for protection from physical damage. One of these runs is in a corner, behind an EMT run where it would be very difficult to hit. The other 3 runs are shoulder height and up, going to the ceiling.

I understand exactly what code says for what types of conduit are and are not rated for protection from physical damage. This is not a thread where I ask a question. This is a thread where I express an engineering disagreement with code.

I decided to perform a test. As I am replacing these schedule 40 PVC runs with schedule 80, and they are all short runs, the 1 stick I used had material left over. So I have a schedule 80 piece, a schedule 40 piece and a piece of EMT. All 3/4" electrical conduit.

For this test, I drop a sledgehammer on the conduit from about 24". The conduit rests on my concrete driveway unsecured.

Results: See attached photo. IMO, schedule 80 PVC performed the worst.

You see, mechanical engineers understand that stronger is not always better. Higher hardness materials have greater tensile strength, true, but typically they also gain strength while loosing elongation - so they will tend to fail as brittle fractures.

That is what happenned with the schedule 80. The thinner wall of the schedule 40 allowed it to deflect and rebound to its original shape, but the schedule 80 was too rigid and it shatterred. In an impact, the item making the impact would have actually come into contact with wire inside the conduit. Probably not with the schedule 40.

I was expecting to see schedule 40 crack but not fracture while EMT would have pinched and taken a permanent set. The EMT dented, but not enough to affect a legal conduit fill... I suspect that a sufficient impact to have pinched wires in EMT would have shatterred schedule 40, so I would endorse metal conduit for physical protection.

Schedule 80 leaves me skeptical after this test. But because I believe the application of the physical damage requirement for the locations in question is questionable, I'm inclined to go with it for the sake of humoring the code.

Jim Port · 06-14-2011, 08:36 PM

Interesting test results Will.

I think I would have lost patience with your inspector. Come on now, I know the damage issue is subjective and not spelled out, but above shoulder height? Give me a break.

electures · 06-14-2011, 09:44 PM

Quote:

Originally Posted by WillK

One of my inspector's issues has been my use of schedule 40 PVC conduit for 3 short runs in my garage, he is requiring that I use conduit rated for protection from physical damage. One of these runs is in a corner, behind an EMT run where it would be very difficult to hit. The other 3 runs are shoulder height and up, going to the ceiling.

I understand exactly what code says for what types of conduit are and are not rated for protection from physical damage. This is not a thread where I ask a question. This is a thread where I express an engineering disagreement with code.

I decided to perform a test. As I am replacing these schedule 40 PVC runs with schedule 80, and they are all short runs, the 1 stick I used had material left over. So I have a schedule 80 piece, a schedule 40 piece and a piece of EMT. All 3/4" electrical conduit.

For this test, I drop a sledgehammer on the conduit from about 24". The conduit rests on my concrete driveway unsecured.

Results: See attached photo. IMO, schedule 80 PVC performed the worst.

You see, mechanical engineers understand that stronger is not always better. Higher hardness materials have greater tensile strength, true, but typically they also gain strength while loosing elongation - so they will tend to fail as brittle fractures.

That is what happenned with the schedule 80. The thinner wall of the schedule 40 allowed it to deflect and rebound to its original shape, but the schedule 80 was too rigid and it shatterred. In an impact, the item making the impact would have actually come into contact with wire inside the conduit. Probably not with the schedule 40.

I was expecting to see schedule 40 crack but not fracture while EMT would have pinched and taken a permanent set. The EMT dented, but not enough to affect a legal conduit fill... I suspect that a sufficient impact to have pinched wires in EMT would have shatterred schedule 40, so I would endorse metal conduit for physical protection.

Schedule 80 leaves me skeptical after this test. But because I believe the application of the physical damage requirement for the locations in question is questionable, I'm inclined to go with it for the sake of humoring the code.

So what exactly did you do to piss this guy off?

TEIN · 06-14-2011, 10:34 PM

Well this is my 1st post,Hello everyone.

I just had to reply to this. After 30 plus years in the trade I can honestly say I have never seen anything like this.

Great demonstration.

WillK · 06-15-2011, 12:02 AM

Now don't get me wrong, this one test isn't enough to discredit code by itself nor is it necessarily based upon the same performance criterea that code or the UL listing applicable to PVC conduit would require. It is interesting, it is informational, but as someone who has done testing of hospital beds to UL test requirements, I must point out that my test would not have any meaning in any official context. For one thing, no matter how accurately my test might reflect any UL spec for testing damage protection, my driveway is not a UL certified test lab.

There will without question be types of damage where the increased wall thickness of sch. 80 protects wires better than the less thick wall of sch. 40, such as any sort of abrasion. For all I know, because I haven't made any effort to look up the UL specs on the pipes. It may be that abrasion is the type of damage being protected from while impact is subject to a lesser requirement than what I used.

gregzoll · 06-15-2011, 12:39 AM

So what did he/she exactly state, or mark down on the report as to why they failed you. Sometimes you catch them on a bad day, or due to they get tired of going back and forth to the same place, they feel that they are going to treat you like the codemaker on The Money Pit.

frenchelectrican · 06-15-2011, 03:29 AM

Did the inspector ever give you the art #'s or not ?? if they did give you the art numbers then we can able help you on this one but no number some case you can challange them but make sure you have something to back it up and do it in nice way.

Merci,
Marc