DSP-10 User Manual
Page 16 of 26
DSP10_MAN_C
5. Installation
General Rules and Best Practices for Inductive Loops
Before beginning the installation, it is important to make that the loop you are about to use is appropriate for
this installation. Here are some general rules and best practices that will help you ensure that the loop you use
will have the desired performance and a long life.
1. The preferred insulation material for loop wire is cross-linked polyethylene (XHHN or XHHW are types of
this wire). Wire with PVC insulation should never be used. This includes THHN. The cross-linked
polyethylene is a much more durable insulation and, more importantly, has a much lower moisture
absorption rate. Moisture absorption can affect loop stability.
2. Wire gauge of the loop wire has little to no impact on the sensitivity of the loop. A larger wire gauge
may help extend the life of the loop, especially in asphalt installations where heavy truck traffic is
expected.
3. Wire nuts should never be used in loop connections. At a minimum wire to wire connections should be
crimped or use a screw terminal. Ideally, solder the connections.
4. Loop saw cuts should be mitered to reduce the loop wire bend angle to not exceed 45 degrees at any
location.
5. Use backer rod to hold the loop wire at the bottom of the saw cut before sealing the loop. The backer
rod should be cut into 1-inch pieces and one placed every foot or so, as needed.
6. If a loop installation will cross an expansion joint in the roadway surface some method of leaving some
slack wire at the joint should be implemented. This can be done by core drilling a 1” or larger hole
where the saw cut crosses the expansion joint and leaving a loop of wire in the hole each time you go
through it when installing the loop wire. The other option is to dip the saw at the expansion joint to
make the saw slot at least two inches deeper at the joint. Then when installing the loop wire, ensure
that the wire lays all the way at the bottom of the saw slot at the expansion joint.
7. Useable detection height of a loop is 2/3rds of the shortest leg of the loop. Therefore, if you need to
detect semi-truck trailers, no side of the loop can be shorter than 6’.
8. The approximate inductance of the loops can be calculated using the formula:
L = (P / 4) x (N + N2)
Where: L = Loop Inductance in microhenries (µH)
P = Perimeter of the loop in feet
N = Number of turns (wraps) in the loop
For example, a 6’ by 14’ loop with 2 turns would be:
L = ( (6 + 14 + 6 + 14) / 4) x (2 + 2
2
)
L = (40 / 4) x (2 + 4)
L = 10 x 6
Loop Inductance is approximately 60µH.
