13 lightning rods on the roof of my house?! Guy says every 20'

threeapplesJanuary 25, 2012

Our house is 7,000 sq ft and the lightning rod guy says we need a 12" tall one every 20 feet, which makes 13 lightning rods! If we go to a 24" tall one we can have them every 25 ft. apart. This seems excessive, right?

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That spacing sounds about right.

The 'protection cone' for a rod is about 45 degrees from perpendicular (90 degree included angle) in the form of a cone around the tip of the rod.

A 12 foot rod cone is going to intersect the roof about 12 feet away.
The two feet of overlap is not bad at all, and helps ensure coverage.

Do you really need lightning rods at all?

Is you house going to be the tallest thing around on a prominent location?

Do you have that much lightning?

    Bookmark   January 25, 2012 at 11:57AM
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Brickeye asked the right question... Do you really need lightning rods at all? See the link to help you decide

Here is a link that might be useful: lightning risk assessment guide

    Bookmark   January 25, 2012 at 12:31PM
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I put lighting rods in after I built my house. If I had to do it again I would have done it while I was building my house. That way the wires can be hidden better.

I decided I needed the protection because lighting strikes took out our well pump three times. We are on top of a hill. After we put in (a) the rods and (b) the protection on the well, we never had an issue again.

How many rods you need and where they are placed can be determined using a number of methods. The method I understand is the sphere method. Imagine a big ball with a radious of about 50 feet. Now roll this all over your land and your house. Where ever it touches your house place a lighting rod. Move and add rods onto your house such that the ball never touches your house, just the lighting rods. When the guy said 20 feet, I think that is ball park right.

While you are at it, maybe you would also want to add some whole house sure protectors / suppressors at the main breaker panel of the house.

Hope this is useful.

Best, Mike.

    Bookmark   January 25, 2012 at 2:54PM
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Thanks everyone for the very helpful responses. Our house is the highest in our development on a wooded lot. We are in Northeastern Ohio if that makes any difference. I'm not sure we need them, but plan to check out the link included here to see if maybe we're going down the wrong path. I kind of feel like they will look silly all over the top of my roof.

    Bookmark   January 25, 2012 at 7:58PM
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"Our house is the highest in our development on a wooded lot."

How close are trees taller than the house?

They have the same 'protection zone' as a lightning rod (at least till they get hit the first time).

While installing rods so that the down leads "wires can be hidden better" is not always a good idea.

The down leads need to be as straight as possible, with as few bends as possible.

Any bends also need a relatively large smooth radius.
Tight bends lead to magnetic field crowding inside the bend and have the effect of 'ejecting' the current form the wire as a secondary flash.
Trees are good lightning rods, but since humans are much better conductors than the water in tree cambium, if you are standing near one as the current travels down it will jump to you to reach earth.

No down wire should enter into the house in any way.
Thew must be kept outside the building envelope and clear of plumbing vents and anything else conductive on the roof (like even flashing).

    Bookmark   January 26, 2012 at 12:44PM
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brickeye, you are so knowledgeable. thank you. the closest trees are about 50' from the house.

i have no idea how they will manage the bends or the down wires, but will bring all of this up with the installer. thanks again!

    Bookmark   January 26, 2012 at 3:12PM
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We installed them about 15 years ago, two years after we built and many lost appliances later. 20' spacing sounds right, and you really won't notice them. We also installed a whole-house surge protector but we know the lightning rods work to dissipate the charges. Before the rods we lost garage door openers, dryer, tv, vcr (this was 15 yrs ago)and all of our cordless phones. The plug box was literally melted into a blob. We were very lucky to not have a fire. We'd try to unplug things when a storm was brewing, but that was just about impossible. Since the lightning rods we haven't lost anything to electrical damage. We are on a hill, highest thing around. Our trees are just now reaching rooftop level, but I'd hate to lose them now! The lightning rods really do work!
Good luck!

    Bookmark   February 2, 2012 at 3:29PM
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Just to give you some idea, none of my bends in the wires where more than the bend of a British football. In the USA I mean a soccer ball. So all the bends have to be smooth.

Smooth bend are because (1) the copper wires are quite thick, about a thick pencil in thickness and (2) a sharp bend tends to increase the "flux" in the electric field. I.E. if you have a sharp point anywhere, the lighting bolt make shoot out at that point. Something that you do not want.

Visually it is not that bad I think. Depends on the color of your roof. I have charcoal colored GAF roofing tiles. Each rod is maybe a foot in height. My rods are more black right now. Maybe they will turn copper green eventually. But right now being black they are not that noticible.

best, Mike.

    Bookmark   February 2, 2012 at 3:56PM
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"the copper wires are quite thick, about a thick pencil in thickness"

Those are rather small down wires, and are unlikely to actually survive a direct hit.

Down wires to survive a hit are over an inch in diameter.

After losing all sorts of expensive electronic warfare test gear to hits on the roof of our 6 story poured concrete office building we determined that the rebar had not been properly bonded to provide a low impedance path to earth.

We then ran two 2,000 MCM down lines from our antenna 'bridge' (one each end) and never lost another piece of equipment.
2,000 MCM is about 1.6 inches in diameter from stranding.

Just make it a habit to look over the down wires after major storms.
You will know if they took a hit and you are home since the flash and the thunder will be at the same time (and louder than you can imagine until you hear a hit).

If you are not home, just look over the down lines and make sure they are there after the storm is well over.

Strikes from trialing edges miles form the cloud are not unheard of.

    Bookmark   February 2, 2012 at 4:58PM
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I think I have a class I system spec, as opposed to a class II. The wires I have are AWG 2 stranded copper. The bends are no more that 8 inches in diameter. The lines are stranded to help with the "Skin effect" and increased resistance at high frequencies. A lighting strike in effect is an impulse step function with high frequencies.

According to the lighting institute you can place downwires inside in conduit. Often PVC. This I think is correct because the building I am in right now has it done like that. And this ties in with what the instalation guy said when he put in my system at my home. I.E. if you put it in before the house is complete, the wires can be hidden inside the building. It is often prefeable to put down wires in conduit so as to protect the wires from damage, e.g. in a truck garage.

Best, Mike.

    Bookmark   February 3, 2012 at 7:26AM
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"The lines are stranded to help with the "Skin effect" and increased resistance at high frequencies."

They are stranded to make it easier to work with the larger wires sizes.
The stranding has no real effect on skin effect at the stranding sizes used.
You would need hundreds of strands.

"According to the lighting institute you can place downwires inside in conduit."

It is not a good idea to bring any down wires inside the house if it can be avoided.

You are inviting trouble to come in.

What you can do is not always what you should do.

    Bookmark   February 3, 2012 at 9:17AM
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