New Mexico Low Voltage Practice cheat sheet

Ohm's law: E = I × R; power P = E × I = I²R = E²/R. For three-phase, P = √3 × E_line × I_line × PF.

Series circuit: current is identical everywhere, resistances add, voltage divides in proportion to resistance. Parallel circuit: voltage is identical, conductances add (1/R_T = 1/R_1 + 1/R_2 + ...).

Conductor resistance: R = K × L / CM (K = circular-mil ohm per foot, L in feet, CM = circular mils). CM = d² for a solid round conductor in mils.

Voltage drop single-phase: VD = 2 × K × I × L / CM; three-phase: VD = 1.732 × K × I × L / CM. NEC recommends total drop ≤ 3% branch plus 2% feeder.

Ampacity adjustment: ambient temperature correction factors and bundling adjustment factors from NEC Table 310.15(B)(1) and 310.15(C)(1) are both applied multiplicatively to the table ampacity.

AC values: RMS = 0.707 × peak; average = 0.637 × peak. Power factor = true power (W) / apparent power (VA); reactive power in VAR.

Transformer relationships: E_p/E_s = N_p/N_s = I_s/I_p. A 480 V to 120 V transformer has a 4:1 turns ratio and a 1:4 current ratio.

Ground-fault current path: source winding to service equipment bonding jumper to equipment grounding conductor to the fault to enclosure bonding. High-impedance paths limit current and slow overcurrent clearing.

NEC Article 725 governs Class 1, 2, and 3 remote-control and signaling circuits; Class 2/3 circuits are power-limited and are classified by Table 725.4 based on source and load.

Class 2/3 circuit conductors must be separated from power conductors by a minimum 2 in. (50 mm) unless one of the exceptions applies (e.g., a barrier, raceway, or listed jacketed cable).

NEC Article 760 covers fire alarm systems: NPLFA (non-power-limited) and PLFA (power-limited) circuits. PLFA cable must be listed and marked per 760.179 or 760.176.

Class 2/3 cable substitutions: CL3 rated cable may substitute for CL2; CL2P/CL3P plenum rated cable is required in environmental air spaces per 300.22(C).

Low voltage does not mean no hazard: a 24 VDC 4 A circuit can still be a fire ignition source and a shock/burn source at available fault current.

Voltage drop on low voltage circuits is proportionally severe: at 24 V, a 2 V drop is 8.3%, so distance and conductor size must be designed, not assumed.

Power-limited fire alarm circuits require a listed power source and must be separated from Class 1 and non-power-limited circuits per 760.130 unless separated by a barrier.

Typical low voltage applications: 24 V HVAC controls, 0-10 V dimming, access control, CCTV (PoE 802.3af/at/bt), nurse call, and irrigation control.

Fire alarm initiating devices: smoke detectors spaced per NFPA 72 at 30 ft (900 sq ft per detector) on smooth ceilings; heat detectors at 50 ft spacing for 15 ft ceilings per the standard.

Notification appliance circuits (NAC): voltage drop must be calculated at the end device; a typical NAC is 24 VDC and must maintain device minimum operating voltage (often 16-17 V).

Fire alarm cable: FPL (riser), FPLP (plenum), and FPLR markings; FPLP is required in return air plenums per 300.22(C) and 760.179.

Addressable systems use a signaling line circuit (SLC) with individual device addresses; conventional zones report by zone only, not by device.

Structured cabling: T568A and T568B pinouts differ in pairs 2 and 3; the same standard must be used at both ends of a link, and a crossover is used only for direct device-to-device links.

Fiber optic: single-mode (9 µm core, long haul) vs. multimode (50 or 62.5 µm, building backbone); loss budget is calculated from connector, splice, and fiber attenuation.

Access control: a fail-safe lock releases on power loss (life safety egress), a fail-secure lock stays locked on power loss; egress doors must release per NFPA 101.

CCTV/PoE: 802.3af provides 15.4 W at the source, 802.3at (PoE+) 30 W, and 802.3bt up to 90 W; cable category and distance (100 m) limit usable power.