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.
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.
