Quick Question re: AL Conduit

[charlie b]

I am at home, and have a short time before I have to leave for jury duty. Checking office email, I found the following question. I don't have time to try to look this up in the on-line NEC, so a quick reply would be appreciated.

A lab needs to route an antenna cable to the roof. It will be in conduit, and the conduit will run through a vertical shaft. At the top of the shaft is a grating, and above that is sky, so the shaft is exposed to the outdoor environment. The design called for (I seem to recall) rigid steel conduit. They want to use aluminum, because it is lighter and easier to install. Since the conduit is simply intended to protect the cables, and perhaps allow them to run more antenna cable later, does it matter what the material is? More of concern to me is the question of whether aluminum is rated for use in outdoor areas.

So, do you think aluminum is acceptable in this installation?

 

[Buck Parrish]

IMO, I would say that outside is fine. Its the lab that might be of concern...

 

[nhfire77]

I have installed antennas in conduit using the conduit as a redundant ground for the antenna. Would you be using the Aluminum as a Ground? If not, then it would seem fine. Like you said, protection only, then who cares if it corrodes.

 

[yanici]

I have installed antennas in conduit using the conduit as a redundant ground for the antenna. Would you be using the Aluminum as a Ground? If not, then it would seem fine. Like you said, protection only, then who cares if it corrodes.

Why should it corrode. I've seen aluminum conduit installed outdoors that has been in place for forty years with no signs of corrosion. Not placed in contact with the earth of course.

 

[charlie b]

Thanks to all. I will reply that I have no objections to the use of aluminum conduit.

(Aside to Buck: This is an electronics lab, so there is no concern over chemicals in that area.)

 

[nhfire77]

Why should it corrode. I've seen aluminum conduit installed outdoors that has been in place for forty years with no signs of corrosion. Not placed in contact with the earth of course.

Have you worked near the Ocean?

I was thinking worse case scenario.

 

[LEO2854]

Look at 344.10 2008 NEC AND 300.6 .
I say the use of Aluminum is good for this application.

 

[ike5547]

Have you worked near the Ocean?

I was thinking worse case scenario.

I use aluminum for my service risers. It's a selling point here near the beach. It does much better than steel.

 

[eric9822]

Have you worked near the Ocean?

I was thinking worse case scenario.

I am at a manufacturing plant about 5 miles from the ocean and we have miles of aluminum conduit outside It has held up just as well if not better then the RGS.

 

[steve66]

I also think Alum. is fine outdoors. Also, I don't see any reason it can't be used as a ground for an antenna.

I've had contractors tell me alum. is harder to pull conductors through, but that shouldn't matter for an antenna.

 

[Jim W in Tampa]

You left one serious detail out. Is this antenna for receiving or transmitting. For receiving i think it is fine. Transmitting changes some issues depending on wattage

 

[charlie b]

You left one serious detail out. Is this antenna for receiving or transmitting.

That I do not know. My inclination is to think it is for receiving. This is a seismology lab, so they will be receiving information from around the world. I think that if they need to transmit warnings or advisory information, they would use the Internet.

 

[K8MHZ]

You left one serious detail out. Is this antenna for receiving or transmitting. For receiving i think it is fine. Transmitting changes some issues depending on wattage

Really?

Please elaborate.

If the original design called for steel and Al was used instead, how would transmitting be affected?

I am going to guess that the antenna feed line is coax with a very good shield. It doesn't matter what you run it in, or how far it has to be from metal, that is why it is used instead of balanced feed line which is far more efficient than coax.

I have run Al outside, it works and looks great. I don't think it's any harder to pull wire through than steel, but we never stuffed our runs full so it was always easy.

 

[Jim W in Tampa]

Really?

Please elaborate.

If the original design called for steel and Al was used instead, how would transmitting be affected?

I am going to guess that the antenna feed line is coax with a very good shield. It doesn't matter what you run it in, or how far it has to be from metal, that is why it is used instead of balanced feed line which is far more efficient than coax.

I have run Al outside, it works and looks great. I don't think it's any harder to pull wire through than steel, but we never stuffed our runs full so it was always easy.

It is not likely an issue at low power such as a few hundred watts. If higher it might been speced for better shielding. Also we must look at what areas we are going thru such as plenums.
From back in my CB days i do know shielding was a major problem when running a few hundred watts as not to cause neighbors TV reception problems

 

[K8MHZ]

It is not likely an issue at low power such as a few hundred watts. If higher it might been speced for better shielding. Also we must look at what areas we are going thru such as plenums.
From back in my CB days i do know shielding was a major problem when running a few hundred watts as not to cause neighbors TV reception problems

That's because of crappy coax and poor antenna installation practices, coupled with the loads of spurii that illegal CB amps produce. Comparing a hacked up CB station to a properly engineered laboratory station is like comparing apples to oranges.

 

[Jim W in Tampa]

That's because of crappy coax and poor antenna installation practices, coupled with the loads of spurii that illegal CB amps produce. Comparing a hacked up CB station to a properly engineered laboratory station is like comparing apples to oranges.

Not always true. Back in early 70's many side banders were using maco amps that came with instruction of "do not move this wire to the soldered spot on the coil as it will result in illegal operation in the CB band "
We had some fine radio stations back in them days being CB was so close to the 10 meter band for hams.
The ones causing most of the trouble were simply DIY that lack skill.

 

[ericsherman37]

Have you worked near the Ocean?

I use aluminum for my service risers. It's a selling point here near the beach. It does much better than steel.

Maybe your beach is different than my beach, but here, steel and aluminum both get completely thrashed in just a year or three.

I recently did a job at one of the local fish processing facilities right on the bayfront, and when I opened up a cast aluminum LB condulet it was half-packed with oxide powder. Opening it up in the first place was no easy task either cause the screw heads both sheared partway off.

As far as the steel goes, I don't think I can even count how many regular steel meter bases I've replaced with stainless ones. There's some real rust-heaps, even on construction that only happened a few years ago.

 

[ike5547]

Maybe your beach is different than my beach, but here, steel and aluminum both get completely thrashed in just a year or three.

If you're talking about LBs, then, yeah, I've noticed too that those seem to get corroded for whatever reason. But aluminum rigid out performs steel rigid by a mile and a half. I imagine if they actually made steel LBs they wouldn't stand up very well beside aluminum LBs.

I haven't noticed aluminum getting "completely thrashed" in a year or three, though. Also, it's always a good idea to paint.

 

[ike5547]

Before anybody else mentions it I know there are some old steel condulets still in use floating around here and there.

 

[K8MHZ]

Not always true. Back in early 70's many side banders were using maco amps that came with instruction of "do not move this wire to the soldered spot on the coil as it will result in illegal operation in the CB band "
We had some fine radio stations back in them days being CB was so close to the 10 meter band for hams.
The ones causing most of the trouble were simply DIY that lack skill.

How many of them used a calibrated station monitor to verify spectral purity on this highly regarded Maco amp? In a lab setting, only the desired frequency with the desired bandwidth would be sent down the wire due to design, verification and adjustments. This would ensure that the shield of the feed line was not acting as an antenna and thus would not interact with any metallic surface nearby. Also, any high powered transmitter would have to use hard line coax for long runs which is nearly impossible to pull through pipe, as the coax itself is nearly rigid. The specs already allow for a ferrous conduit. If Al presented a problem, so would steel.

We had some fine radio stations back in them days being CB was so close to the 10 meter band for hams.

That insinuates that CB is no longer close to the Amateur 10 meter band.