DSP-22-1 User Manual
Page 14 of 22
DSP-22-1_MAN_D
Wiring:
Attach the loops to the appropriate loop screw terminals. Wire nuts should never be used at any point
in the loop circuit itself. All loop connections should be crimped or use screw terminals at a minimum and soldered
for best long-term reliability. Special attention should be paid to ensure that the loop wires remain tightly twisted
together. An air gap between the two wires for a loop may cause the detector to lock up if the wires are disturbed.
If connecting two loops to one channel of a detector, there are certain rules you should follow:
1.
Always connect multiple loops in series, not parallel. A failure of one loop may not be identified if the
loops are connected in parallel.
2.
The loops should be the same size, shape, and have the same number of turns. If any of these items are
different, the loops will have different sensitivities that cannot be adjusted for. The difference in
sensitivity could be very significant.
3.
Do not use a wire nut to connect the wire from each loop together. Wire nuts have a spring in them and
will expand and contract with temperature. Even the slightest change in resistance can cause detector
lock-ups.
Loop Installation
The reliability and overall performance of the detector are greatly dependent on the loop itself. Several factors
go into a good loop installation: type of wire used, loop configuration, and installation practices.
Type of Wire Used:
The wire used for wiring the loop should have a jacket of cross-linked polyethylene or similar
material that has very low moisture absorption properties. This would be a wire with an XLP jacket such as XHHW.
THHN or similar wire types should never be used for loop wire.
The gauge of the wire to use depends on two factors: Distance in cable feet from the loop to the detector and
stresses the wire may see. The gauge of the wire can be 20 AWG as long as the detector is within 50 feet of the
loop in cable distance. For 50 to 100 feet, use at least 18 AWG wire. At greater than 100 feet, use a 16 AWG wire
at a minimum. If the loop is installed in asphalt and there will be heavy vehicles or stopping and starting vehicles
in the loop area, a 14 AWG or 12 AWG should be used to provide additional strength to the loop. This helps
increase the life of the loop in areas where the asphalt may slowly move and/or deform due to wear and/or
temperature.
Loop Configuration:
The size and shape of the loop will determine what type of vehicles it can reliably detect.
There are many variables that come into play including loop dimensions, percent of coverage, length of lead-in
wire, number of turns in the loop area, and detection height to name just a few of them. One common rule is
that the useable field height of a loop is 2/3 of the shortest leg of the loop. So,
if you plan on using a 2.5’ x 6’ loop,
the expected useable detection height would be 20” (The shortest leg is 2.5’ or 30”, 30” x 2 = 60”, 60” / 3 = 20”).
If the installation requires the detection of motorcycles as well as vehicles, the loop should go to within one foot
of the curb or roadway edge, whichever is present. If only motor vehicle detection is required, within three feet
of the curb or roadway edge is all that is required.
The number of turns to use in a loop is dependent on the size of the loop and length of the lead-in. Rather than
dive into all of the calculations to arrive at a value, we will just give you a table of safe values based on the number
