Residential Lightning Protection
The following information describes how to install lightning protection on residential structures which includes houses, barns, garages and sheds. It doesn’t include all options and requirements. It gives you a general understanding of the components and installation. Blackbird Lightning Protection provides free designs and assistance. We will provide you with a drawing and answer any questions on the installation.
Structure Class- If the structure is 75’ and below then it’s class 1. If it’s above 75’ then it's class 2. You need to know the class so you order the correct parts.
Metal Compatibility- If the roof is steel or aluminum, you should use aluminum parts and cables due to corrosion rates. Copper can be used on most other normal roofing surfaces. Review a metal compatibility chart if you are not sure of material compatibility.
Air Terminals (lightning rods)- Measure the length of the roof. 12” air terminals are placed no further than every 20’. All air terminals 24” or greater in height can be spaced every 25’. An air terminal greater than 24” requires a tripod for stability. An air terminal must be placed within 2’ of the roof peak edges (sides of the roof peak) then you can evenly space the remaining air terminals but no more than the above stated distances. Your Blackbird Lightning Protection designer will determine where all the other air terminals will be placed (chimney, dormers, etc).
The diameter of a class 1 copper air terminal is 3/8” (part number 24A-BT) and class 2 copper is ½” (part number 24-BT). Class 1 aluminum is ½” diameter (part number A24-BT) and class 2 aluminum is 5/8” (part number A24B-BT). The above part numbers are for 12” blunt tip air terminals. Blunt tip and pointed tip air terminals are the same electrically. We recommend you buy the blunt tip so you don't stab yourself accidentally while installing the pointed tip.
Air Terminal Bases (mounts)- The most common way to mount an air terminal on a sloped roof is to use a base with an adjustable adapter. Air terminal base 637AB3 with adjustable adapter 836-3F3M are copper parts for class 1 structures. For class 2 copper, use 637AB2 with adjustable adapter 836-2F2M. For aluminum parts use A637AB with A836-2F2M for class 1 structures. For class 2 aluminum use A637AB with A836-2M5F. The bases are installed on the backside of the house roof peak so the cables are not visible from the street. The bases are secured with screws a few inches from the peak of the roof and then the adapter necks are adjusted so the air terminals point up. On a flat roof you can paste down the bases. The air terminals screw directly into the bases. You can use Chem Link's construction adhesive (part number M1-black) to secure the bases. One 10 fluid ounce bottle can paste down 20 air terminal bases. It adheres to wood, concrete, brick, drywall, metal, fiberglass panel and wallboard. It does not adhere to asphalt shingles. Asphalt shingles have to be screwed through then a sealant is applied. M1-black can be used as the sealant.
Cables- Main conductor and bonding are the two types of cable. Main conductor cable routes through the bases and then down the structure. When the cable is going down the side of the structure it is called the down conductor. It is still main conductor size cable but just called down conductor for reference purposes. Bonding cable is smaller and is used to bond metal objects with a potential to ground and less than 3/16” thick. If the metal object with a potential to ground is 3/16” or thicker (water pipe, conduit, etc) than you need to bond to it with the main conductor size cable. To keep things simple, most installers use the main conductor cable for the main line and bonding.
Blackbird Lightning Protection sells the following sizes for main conductor cable:
Copper cables- Bare stranded copper #2 gauge (part number is 1) for class 1 main conductor cable. Bare stranded copper 1/0 gauge for class 2 main conductor cable (part number 604).
Aluminum cables- Bare stranded aluminum 1/0 gauge for class 1 main conductor cable (part number A1). Bare stranded aluminum 4/0 gauge for class 2 main conductor cable (part number A604). Make sure the cables are not bent more than an 8” bend radius anywhere on the structure. 90 degree bends are not allowed.
Cable Holders & Fasteners- Cables are required to be secured to the roof and the side of the structure every 3’. It’s every 3’ because when the lightning strikes, it causes the cable to jump and move. Lightning protection systems are designed to take hit after hit without adjustments or overhauls. For flat roofs, you will use cable holders. Part number 683 is used with copper cable and A683 with aluminum cable. They can be nailed or screwed down but are usually pasted down with adhesive. You can use Chem Link's construction adhesive (part number M1-black) to secure the cable holders. One 10 fluid ounce bottle can paste down 20 cable holders. It adheres to wood, concrete, brick, drywall, metal, fiberglass panel and wallboard.
Cable fasteners are used on sloped roofs and walls. Cable fasteners are nailed or screwed down. Part number 72A secures 1/4" to 1/2" copper cable (class 1 cable). Fastener part number 72 secures 3/8" to 5/8" copper cable (class 2 cable).
Fastener part number A72A fits 1/4" to 1/2" aluminum cable (class 1 cable). Part number A72 fits 3/8" to 5/8" aluminum cable (class 2 cable). M1-black can be used as a sealant for these fasteners if needed.
Down Conductors- Every structure has to have at least 2 down conductors regardless of size. Down conductors are main conductor size cables that run down the sides of the structure. Down conductors are normally placed at opposite sides of the structure when possible. Aluminum down conductors will transition to copper cable no lower than 18” from the ground. You don’t want aluminum in the ground due to the high corrosion rate. Use bi-metal transition connectors (A341) to connect the aluminum cable to the copper cable. The copper cable then connects to the ground rod.
Ground Rods and Plates- All structures are required to have at least two ground rods. Ground rods have to be installed at least 2 feet away from the structure. The bottom of the ground rod should be at least 10’ deep. The most common size used is 5/8" x 10' (part number 94-10). All ground rods have to be at least 20 feet apart from each other.
A routine mistake made is using the smaller “acorn” ground rod clamps. The acorn clamp is too small and not UL listed for lightning protection. You have to install the thicker longer UL listed lightning protection clamp. The clamp has to be at least 1.5” long parallel to the ground rod and have two bolts. Part number 98A (UL listed) fits 5/8” diameter ground rods.
You can install a ground plate if bedrock conditions don’t allow ground rods. The plate shall be at least 18” below the surface when feasible. Blackbird sells a .032” x 12" x 12" copper plate (part number 88X). This plate includes a clamp to secure the ground cable. A ground plate electrode must have a minimum surface contact area of 2 square feet (both sides of the plate can be counted as surface area). The 88X meets this requirement.
General Grounding and Bonding- The lightning protection system will be grounded separately from the house grounding system. The house ground rods can’t be used as the primary lightning protection system ground rods.