How do I form an inductive loop?

Step 1: Select the Correct Loop Wire

1.1 Wire Jacket Type (Required)

• Use cross‑linked polyethylene (XLP) or equivalent low‑moisture‑absorption insulation.
• Recommended: XHHW
• Never use: THHN or similar wire types

1.2 Wire Gauge Selection

Choose wire gauge based on distance from loop to detector and roadway stress conditions.

Heavy‑duty applications (asphalt surfaces with heavy or stop‑and‑go traffic):

• Use 14 AWG or 12 AWG for added strength and longer loop life.

Step 2: Design the Loop Configuration

2.1 Determine Loop Size and Detection Height

• Usable detection height ≈ ⅔ of the shortest loop leg
• Example:
  • Loop size: 2.5 ft × 6 ft
  • Shortest leg = 2.5 ft (30 in)
  • Detection height = (30 × 2 ÷ 3) = 20 inches

2.2 Position Relative to Road Edge

• Motorcycles + vehicles: Loop within 1 ft of curb or road edge
• Vehicles only: Loop within 3 ft of curb or road edge

Step 3: Determine Number of Loop Turns

1. Calculate loop area:

Loop Area (sq ft) = Length (ft) × Width (ft)

1. Use the table below to select the number of turns.

Recommended Loop Turns

Step 4: Cut the Loop Slot in the Roadway

4.1 Saw Cutting

• Use a roadway‑appropriate saw blade.
• Slot width must be wide enough for wire + sealant encapsulation.
• Slot depth must allow:
  • Loop wire at bottom
  • Minimum ½ inch of sealant above the top wire
  • More depth is better, but avoid cutting too deep.

Avoid cutting roadway reinforcement, as this can weaken the surface.

4.2 Corner Cuts

• Crosscut corners at 45° angles
• Angled cuts should extend at least 9 inches back from the corner
• Only cut as far as necessary to maintain full slot depth

4.3 Home‑Run Exit Treatment

• Where the lead‑in meets the loop:
  • Either add an extra saw cut (Detail A)
  • Or chisel out the sharp inside corner (Detail B)
• This prevents insulation damage and stress on the wire.

Step 5: Clean the Saw Slot

1. Remove all loose debris.
2. Use high‑pressure air to:
   • Clear debris
   • Remove cutting dust from slot walls
3. Clean slots ensure better sealant adhesion and longer loop life.

Step 6: Install the Loop Wire

6.1 Wire Routing

• Use one continuous wire:
  • From detector → loop → all turns → back to detector
• Do not splice wire in the roadway.

6.2 Splicing (If Required)

• Allowed only in a junction box
• Connections must be:
  • Soldered
  • Fully weatherproofed
• Never use wire nuts

6.3 Twisting the Lead‑In

• Twist the wire at least 3 twists per foot
• More twists = better electrical noise rejection
• Keep twists tight
• Allow slack for wire shrinkage caused by twisting

Step 7: Secure Wire with Backer Rod

1. Place 1–2 inch backer rod pieces every 1–2 feet
2. Backer rod must fit snugly
3. Use a blunt tool (not a screwdriver) to push rod down
4. Purpose:
   • Keeps wire at bottom of slot
   • Allows full sealant encapsulation

Never use continuous backer rod over the loop wire.

Step 8: Seal the Loop Slot

8.1 Choose Proper Sealant

• Concrete: Epoxy or polyester sealants
• Asphalt: Polyester or polyurethane sealants
• Product choice may vary by environment and application.

8.2 Apply Sealant

• Fully encapsulate loop wire
• Eliminate air pockets
• Ensure sealant covers wire completely