NCCER Telecom cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. Use these to size conductors, check voltage drop, and verify circuit loading before installing equipment.

Voltage drop in a copper pair: VD = 2 × L × I × R_per_length (the factor 2 accounts for the outgoing and return path). Keep drop within the equipment's specified tolerance.

Decibel relationships: dB = 10 log(P1/P2) for power and 20 log(V1/V2) for voltage; +3 dB doubles power, +10 dB is ten times power, +6 dB doubles voltage.

Characteristic impedance: 100 Ω for CAT5e/6/6A twisted pair, 75 Ω for coaxial video cable, 50 Ω for many RF and instrumentation coax runs. Mismatches cause reflections and return loss.

TIA-568 horizontal cabling limit is 90 m (295 ft) permanent link plus up to 10 m (33 ft) of patch cords, for 100 m (328 ft) total channel length.

Fiber basics: single-mode core ≈ 8–10 µm with 1310/1550 nm wavelengths for long haul; multimode is 50 or 62.5 µm using 850/1300 nm for shorter runs. Bend radius must respect the cable manufacturer's minimum.

Bonding and grounding: telecommunications bonding conductor (TBC) ties the TMGB to the TGB and then to the building service ground; grounding electrode conductor sizing follows NEC Article 250 tables.

Cable identification: tip is the positive/ring side convention; in a 4-pair cable, pair 1 is blue/white-blue, pair 2 orange, pair 3 green, pair 4 brown, with white-striped conductors as the tip side.

Safety and codes: treat all circuits as energized until tested; use lockout/tagout, verify with a tester rated for the category, and follow NEC Article 800 for communications circuits and Article 770 for optical fiber cables.

Copper certification sequence: verify tester calibration and reference, set the correct cable type and NVP, run a link test, then read the pass/fail summary for wire map, length, insertion loss, NEXT, and return loss.

Wire map acceptance: all eight conductors must map pin-for-pin per T568A or T568B with no opens, shorts, crossed pairs, or split pairs; a split pair shows continuity but fails NEXT.

Length measurement uses time-domain reflectometry with the cable's nominal velocity of propagation (NVP); set NVP per the cable datasheet rather than leaving a default.

Fiber end-face inspection: use an inspection scope at 200×–400×; a clean end face is free of scratches, pits, and debris. Clean with lint-free wipes and approved solvent, then re-inspect before mating.

Optical loss testing: use a light source and power meter matched to the wavelength (850/1300 nm multimode, 1310/1550 nm single-mode). Record reference power first, then measure and compare to the link loss budget.

OTDR use: set the correct range, pulse width, and index of refraction for the fiber; the trace shows splice loss, connector loss, and reflective events. A launch and receive cable are needed for accurate end measurements.

Grounding verification: measure the resistance from the telecommunications ground bar to the building ground with a ground resistance tester; a typical target is 5 Ω or less per site specification, and continuity must be confirmed before energizing equipment.

Documentation: label both ends of every cable with the same identifier, photograph the as-built, and record test results with tester serial number, date, and calibration due date so the report is traceable.

Performance verification of a link requires the correct adapter and reference cord set; a dirty or wrong adapter is a leading cause of false failures, so clean and check the reference before blaming the cable.