Height for DIY pole mount

Steve961 · May 2012

I would like to mount a couple of panels on a top of pole mount for my small off-grid cabin. If I went with a commercial mount and 4" Schedule 40/80 pipe, it would cost upwards of $500. That doesn't even include the cost of concrete and digging the 6 foot deep hole. I normally wouldn't mind a ground mount, but I've had problems with theft at my place and would like to have the panels as high as possible.

I can build my own mount (metal and Unistrut) for about $150, and can also save some money if I attach it to a 6x6 treated post. A 6"x6"x16' treated post runs about $40 around here. The mount will have 22 square feet of panels and be at a 60 degree tilt.

My question is basically how high can the post go? I would like to go 10 feet, but I have my doubts.

Thanks.

Steve

themaxx69 · May 2012

Re: Height for DIY pole mount

What about attaching the post to some part of the cabin. Straps, bolts, screws etc. That's what I'm basically going to do. A couple of panels doesn't weight that much, even with mounting and everything. With enough metal straps and bolts and such, it wouldn't even move.

Steve961 · May 2012

Re: Height for DIY pole mount

themaxx69 wrote: »

What about attaching the post to some part of the cabin. Straps, bolts, screws etc. That's what I'm basically going to do. A couple of panels doesn't weight that much, even with mounting and everything. With enough metal straps and bolts and such, it wouldn't even move.

I'm pretty much set on a pole mount for my purposes even if that means I have to go the expensive route.

BB. · May 2012

Re: Height for DIY pole mount

Watch the wind too (if a problem in your area).

Perhaps you can find a scrap metal dealer or somebody that services signs and buy a used section of pipe from them.

-Bill

Cariboocoot · May 2012

Re: Height for DIY pole mount

For what it's worth, pipe is stronger than a post. Hollow tube has more resistance to lateral forces than a solid beam.

Jigme Urgyen · May 2012

Re: Height for DIY pole mount

put that 6x6 treated post into a good rcc concrete footing and it will be fine. id calculate something about 4 feet deep, 3 feet wide. being 10 feet of the ground, the airfoil effect will not be so great.

Photowhit · May 2012

Re: Height for DIY pole mount

OK, I'm NOT the guy for this question, but I wanted to interject, that 22 square feet of sail, is more than I had for a storm sail, that pushed my 2000lb sail boat along at hull speed in 30mph winds...

I might also say that what is behind your panels makes a difference, my ground mounted panels are @12 feet high and 15 wide and mostly just a triangle sitting on the ground, with my battery box on top, I had trees behind it though and when we have 60mph straight line winds, the panels are surpisingly stable and well protected from winds from the north. I think the the trees inhibit a vacum from forming behind the panels and pulling it over.

peakbagger · May 2012

Re: Height for DIY pole mount

The unistrut catalog has a basic section on the bending loads of unistrut. Usually its the deflection that limits what you use for a pole. Deflection is how much the pole will bend without permanent deforming when the load is taken off. If you have a pole that deflects a lot, certain wind conditions can start the array into whipping back and forth and self destructing. Look around on the Internet for the Tacoma Bridge Disaster to see what wind can do.

Note the deflection increases significantly the higher you go with no support. So you need to go with a lot bigger diameter pipe the higher you go up.

Steve961 · May 2012

Re: Height for DIY pole mount

I don't live in a windy area, but I was curious so I did some research into my regions winds and the potential wind load on the mount. I came up with an average annual speed of 8.3 mph, peak gust of 42.7 mph, and historical gust maximum (1971-1993) of 70.0 mph. These were extrapolated from the stations anemometer height of 33 feet to 10 feet using the wind profile power law.

I found a formula on the net for calculating wind load using L = A x .00256 x V x V x Cd where A is the area of the object, V is the wind speed in Mph, and Cd is the drag coefficient of the object.

Using this I get the following wind loads on the mount:

Average wind = 4.6 pounds
Peak gust = 124 pounds
Historical gust = 331 pounds

The question then becomes can a 10' high 6"x6" post withstand a lateral force of 331 pounds at its peak?

Unfortunately I can't find any tables on the net that will give me that answer. Any additional thoughts guys?

Thanks.

Steve

BB. · May 2012

Re: Height for DIY pole mount

In general, you would assume a factor of 10x safety factor (this assumes non-ideal building materials and aging)... So you would want the "typical" breaking strength of your support to be on the order ~3,310 lbs.

Would a trust a 6"x6" beam cantilevered out by 10' to lift a full sized car or pickup truck--Probably not.

Besides the safety issue--There is are the installation vs risk of failure costs too... Costs of "towers" tend to be much more than the costs of a wind turbine and cabling because of these strength of materials/safety/reliability issues.

-Bill

Steve961 · May 2012

Re: Height for DIY pole mount

BB. wrote: »

Would a trust a 6"x6" beam cantilevered out by 10' to lift a full sized car or pickup truck--Probably not.

Bill:

You make a good point here. But, in all fairness, I'm not sure I would even trust a 5' cantilevered length to hold a full sized car. Yet I would trust my panels on a 5' height post with no problem.

So, I have two options.

1. Lower the height of the 6x6 post to a safe level, which brings me back to my original question. How high can the post go?

2. Bite the bullet and get a 4" schedule 80 pipe. This then presents the question of "how high can I go"?

Thanks.

Steve

ywhic · May 2012

Re: Height for DIY pole mount

I just worked out single PV mounts using stuff from the HD.. 2" galv. steel pipe.. 10' long with 3 ft into the ground/contrete leaves me about 7' above ground to mount the 1 1/2" angle iron mount I came up with on to its side.. Gotta love U-bolts.... granted it may be overkill.. but for $50+/- per mount setup.. I rather spend $200 once for 4 panels and have something that can be for 190w or 280w panel sizes.. I based the build on upto 66" wide and 40" height (panel sideways on the mount).. this way I can use smaller 12v panels at 30 lbs. or 24v panels at about 50 Lbs...

vtmaps · May 2012

Re: Height for DIY pole mount

Are you willing to use guy wires? If so, that treated post could go up pretty high.
--vtMaps

solarvic · May 2012

Re: Height for DIY pole mount

Jigme Urgyen wrote: »

put that 6x6 treated post into a good rcc concrete footing and it will be fine. id calculate something about 4 feet deep, 3 feet wide. being 10 feet of the ground, the airfoil effect will not be so great.

I heard that treated post set in cement rot. Saw on this old house tv program that someone cemented treated wood in groubd for porch railing. All the wood rotted off where the cement stopped at the surface. :Dsolarvic:D

2manytoyz · May 2012

Re: Height for DIY pole mount

solarvic wrote: »

I heard that treated post set in cement rot. Saw on this old house tv program that someone cemented treated wood in groubd for porch railing. All the wood rotted off where the cement stopped at the surface. :Dsolarvic:D

Most pressure treated wood available today isn't very good since they removed the arsenic. The better quality material is readily available here in FL from places that sell materials for docks.

The "trick" is to put gravel in the bottom of the hole, place the post, then pour in the concrete. This allows water to drain out. If the post is set into a bed of concrete, it will rot out. This forms a concrete cup, keeping the wood wet all the time. Took years to learn this lesson.

solar_dave · May 2012

Re: Height for DIY pole mount

2manytoyz wrote: »

Most pressure treated wood available today isn't very good since they removed the arsenic. The better quality material is readily available here in FL from places that sell materials for docks.

The "trick" is to put gravel in the bottom of the hole, place the post, then pour in the concrete. This allows water to drain out. If the post is set into a bed of concrete, it will rot out. This forms a concrete cup, keeping the wood wet all the time. Took years to learn this lesson.

I put a cedar fence up using that method 30 years ago, the fence was still standing last time I drove by.

97TJ · May 2012

Re: Height for DIY pole mount

My Cabin is built on 6x6 posts. It wouldn't hurt to paint the post with tar before setting it in the ground. I did not set them in concrete because of reasons listed already. As far as a pole, I have an old portable basketball pole/backboard out back of the house. Kids are all grown and gone so for me that might make a good start. If I remember correctly, it is about 4" diameter. I also think I will sink two 2" posts in the ground in concrete that the 4" pole will sit between and through bolt in two places. That way I can tip the whole thing down if needed. I only have three panels so weight shouldn't be an issue.

65DegN · May 2012

Re: Height for DIY pole mount

If theft is a problem is a wooden pole really that much of a deterrent? I agree with the idea of pricing a 4" schedule 40 or 80 at a scrap metal dealer. Here they come quite cheap. Also you might look into some of the heavy industry in your area that uses metal poles. Here the oil industry has made many sizes readily available for cheap.
If it were me, I would use a metal pole with guy lines or braced to a building to get it up maybe 15' or so. It might also offer you better solar access.
GL

Steve961 · May 2012

Re: Height for DIY pole mount

GL:

You're probably right about a wood pole not being a theft deterrent. Where I'm located, everyone has a chain saw. It'd be easy to just saw the pole half way up and throw the whole darn thing in a pickup bed. I'm going to check some scrap metal yards to see what's available.

Thanks.

Steve

2manytoyz · May 2012

Re: Height for DIY pole mount

If someone is determined, a metal pole doesn't offer much more of a deterrent either. A cordless Sawzall can make short work of a pole, so can a cordless angle grinder with a cut-off wheel, or a torch if they're trying to be quiet. Heck, a pickup truck, a chain, pull the entire thing down, cut the brackets, leave the pole.

These are the reasons I have security cameras, an alarm system, and insurance if all else fails.

All my panels have S/Ns, all are recorded...

ywhic · May 2012

Re: Height for DIY pole mount

My input.. go low..

I'm putting mine low.. like 3' off the ground..(seeing how most panels are less then 36" in width).. then putting a fence just far enough away to make them invisible unless your peeking over the fence..

Where I'm out everything sticks out.. Heres a picture of my 12' x 24' cabin being put up..

niel · May 2012

Re: Height for DIY pole mount

i would not advise mounting pvs low where snow can be allowed to build and block the sunlight. it's also wrong to think that say if you get a 10in snowfall that all it would need is 11in as the snow that laned on the pv has to roll off of it and to the ground and can add far more to that which is already there. account for no snow melts for a month or 2 and it's buried for the season. wisconsin is not like texas weather wise.

Scottg4001 · August 2012

Re: Height for DIY pole mount

If you are worried about theft, use tamper resistant hardware to fasten the panels to the racking and the racking to the pole. The hardware requires a special "snake-eye" tool. If installed correctly it's practically impossible to loosen with any tool, especially if you use oversized washers that prevent any tool from getting a good grip. Our warehouse is in an industrial area and we have had many items stolen, but our R&D top of pole 3-panel array is still there.

Height for DIY pole mount

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