Span Wire Across a 70-ft Crossing: Will the Conductor Carry It?
A crew is installing a new span-wire signal at a 70-ft-wide crossing. The controller cabinet is on the near-right corner, and the wire run from the cabinet terminal block to the farthest signal head on the span is 320 ft along the messenger. The connected load on that circuit is 9 A at 120 V - four vehicle heads and three pedestrian heads, all with LED inserts. The crew has #10 AWG copper on the truck and wants to pull it. The inspector asks for a voltage-drop check before any wire goes up. Conductor resistance is taken at 75°C. The site is in a high-wind area, and the messenger is 3/8-in. galvanized strand.
1. Compute the voltage drop for #10 and #8 copper and recommend a conductor. Show the formula, the resistance values, and the decision criterion.
Read the worked answer
Use the two-wire (out-and-back) formula:
Vd = 2 × L × I × R / 1000, with L in feet and R in ohms per 1000 ft.
L = 320 ft, I = 9 A.
- #10 AWG, R ≈ 1.24 Ω/1000 ft: Vd = 2 × 320 × 9 × 1.24 / 1000 = 7.1 V = 5.9% of 120 V.
- #8 AWG, R ≈ 0.778 Ω/1000 ft: Vd = 2 × 320 × 9 × 0.778 / 1000 = 4.5 V = 3.7%.
- #6 AWG, R ≈ 0.491 Ω/1000 ft: Vd = 2 × 320 × 9 × 0.491 / 1000 = 2.8 V = 2.4%.
Criterion: branch circuits are commonly designed for no more than 3% drop, with 5% total including the feeder. #10 fails clearly and #8 still exceeds 3%. Recommend #6.
Two cautions: (1) LED heads draw far less steady current than incandescent, but inrush and future head replacement make designing to the present LED load risky; (2) verify terminal and load-switch ratings and conduit/wireway fill before upsizing, and confirm the neutral is sized the same as the ungrounded conductor.
2. What mounting-height, lateral-placement, and span/messenger checks apply to this installation?
Read the worked answer
Mounting height: overhead signal faces must have the bottom of the housing at least 15 ft above the crown of the roadway at the center of the controlled lane, and generally no more than about 19 ft. Too low and trucks strike them; too high and the indication is hard to see. Post-mounted faces have their own lower requirement and should be checked separately.
Lateral placement: each approach needs at least one primary face, preferably two, positioned so the driver in the controlled lane can see a face without leaning or turning. Primary overhead faces are generally kept within about 40 ft of the center of the controlled lane, and no face should be so far off-axis that it reads as belonging to another lane.
Span and messenger:
- Confirm the messenger size and tension are adequate for the wind load plus the weight of heads, signs, and ice.
- Check that sag under design load still leaves every head above the minimum clearance.
- Use thimbles and proper clips at attachments; a sharp thimble edge against insulation is a future fault.
- Bond the messenger and all metallic supports to the equipment grounding system.
Document the measurements; clearance and lateral placement are among the most common inspection failures.
3. What grounding and bonding items would you verify before accepting the installation?
Read the worked answer
Walk the grounding system end to end, because a signal can operate perfectly on a dry day and fail in a storm if grounding is poor:
- Service grounding electrode system at the cabinet - ground rod(s), connections, and conductor size.
- Measured resistance to earth. A fall-of-potential or clamp-on test should show a low value; the NEC baseline is 25 Ω or less, or a supplemental electrode. Many agencies specify tighter. A high reading means added electrodes or repaired connections.
- Pole and messenger bonding. Metallic poles, the span messenger, and any metallic conduit must be bonded to the equipment ground. An unbonded messenger can become energized by a chafed conductor.
- Continuity of the equipment grounding conductor from cabinet to every head and pole, tested with a meter, not assumed.
- Neutral and ground separated except at the service point, with no neutral used as a ground path.
- Surge arrestor correctly installed, bonded, and rated for the service.
Record the readings. Grounding is the one item that cannot be visually verified after the fact.