Delaware Low Voltage Practice cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. A 24 V circuit drawing 0.5 A dissipates 12 W.

Series circuit: current is common, resistances add (R_total = R1 + R2 + …), voltage divides in proportion to resistance.

Parallel circuit: voltage is common, conductances add (1/R_total = 1/R1 + 1/R2 + …), current divides inversely with resistance.

Voltage drop for DC or resistive loads: V_drop = 2 × L × I × R_per_length, using one-way length L and doubling for the return conductor.

AC impedance Z = √(R² + X²); inductive reactance X_L = 2πfL, capacitive reactance X_C = 1/(2πfC).

Balanced twisted pair rejects common-mode noise because equal and opposite currents create canceling magnetic fields; twist rate sets the rejection bandwidth.

Characteristic impedance (100 Ω CAT5e/6, 120 Ω RS-485/fieldbus) is a transmission-line property independent of cable length; mismatches cause reflections.

Signal bandwidth rule of thumb: a digital pulse needs roughly 5× the fundamental bit-rate frequency of bandwidth for acceptable edge fidelity.

Power over Ethernet: PSE supplies up to 15.4 W (Type 1), 30 W (Type 2), 60 W (Type 3), 90 W (Type 4) at the source; conductor heating and bundle derating apply.

Class 2 circuits are limited to 100 VA and Class 3 to 100 VA per circuit under Article 725 Tables 11(A)/11(B); Class 1 is unlimited but uses Chapter 3 wiring methods.

Separation rule: Class 2/3 cables must be separated from power conductors by at least 2 in. (50 mm) unless a barrier, raceway, or listed combination is used.

Plenum-rated cable (CMP) is required in air-handling spaces; riser-rated (CMR) is for vertical shafts between floors; CM/CMG for general horizontal runs.

Minimum bend radius: 4× cable OD for UTP during installation, 10× for fiber (or manufacturer spec, whichever is greater).

Maximum pull tension for 4-pair copper is typically 25 lbf (110 N); exceeding it elongates conductors and degrades return loss.

T568A and T568B pinouts differ only in pairs 2 and 3; both are acceptable if consistent end-to-end, but T568B is common in commercial US work.

Support horizontal cable at intervals not exceeding 4.5 ft (1.4 m) and no more than 12 in. (300 mm) from a box or jack.

Cable tray fill: NEC 392 permits 50% fill for multiconductor cables; conduit fill per Chapter 9 Tables 1 and 4, with 40% for 3 or more conductors.

Article 725 covers remote-control, signaling, and power-limited circuits; Article 760 fire alarm; Article 800 communications; Article 820 coaxial; Article 830 network-powered broadband.

Class 2/3 cable in a raceway with power conductors is permitted only if the power conductors are Class 1 or the raceway is divided by a barrier.

NEC 300.22(C): in environmental air spaces, only wiring methods specifically listed for plenum use may be installed.

NEC 250.94 requires an intersystem bonding termination accessible from the exterior for communications, CATV, and satellite systems.

NEC 800.100/810.21/820.93 require a listed primary protector and bonding to the grounding electrode system for outside-plant-originated conductive cables.

NEC 725.121 requires Class 2/3 power supplies to be listed; Class 2 limited to 100 VA and Class 3 to 100 VA per Table 725.121.

NEC 760.41/760.43: fire alarm power-limited circuits must use NPLFA or PLFA listed cable, and PLFA cannot share a raceway with non-power-limited conductors.

Box fill per NEC 314.16 counts each conductor, each device, and each cable clamp; low-voltage boxes are exempt but junction boxes still need accessibility (314.29).

Category ratings: CAT5e = 100 MHz, CAT6 = 250 MHz, CAT6A = 500 MHz; CAT6A supports 10GBASE-T to 100 m, CAT6 to ~55 m in high-alien-crosstalk environments.

Permanent link max = 90 m horizontal cable + 10 m of patch cords = 100 m channel; fiber backbone distances depend on type (OM4 multimode ~400 m at 10G).

Key tests: insertion loss, return loss, NEXT (near-end crosstalk), FEXT, ACR-F, propagation delay, delay skew, and wire map (opens, shorts, splits, reversals).

TIA-606 labeling requires unique identifiers for every cable, jack, patch panel port, and outlet; document moves, adds, and changes.

Single-mode fiber (OS2) uses 1310/1550 nm and reaches kilometers; multimode (OM3/OM4/OM5) uses 850 nm VCSELs for short building and campus runs.

Fiber connector types: LC (small form factor, duplex), SC (square), ST (bayonet); polish types PC, UPC, and APC (8° angle, green, lower return loss).

Fiber loss budget = total connector loss + splice loss + fiber attenuation × length; typical 0.3 dB per mated pair and 0.2–0.3 dB per splice.

Balanced twisted-pair pinout: 1-2, 3-6, 4-5, 7-8 pairs; a split pair (using conductors from different pairs) causes high NEXT and fails certification.

NFPA 72 defines initiating devices, notification appliances, and control equipment; circuits are either Class A (redundant path) or Class B (single path, supervised for open).

NAC voltage drop is calculated like any DC circuit: V_drop = 2 × L × I × R; the last appliance must remain within its listed operating range (often 16–33 V).

Smoke detector spacing on a smooth ceiling is 30 ft (9.1 m) nominal, with the first detector within 15 ft (4.6 m) of the wall; 900 sq ft max per detector.

Detector spacing must be reduced for ceiling heights above 10 ft per NFPA 72 tables; high ceilings require more detectors, not fewer.

Manual pull stations are required at each exit, within 5 ft (1.5 m) of the door, mounted 42–48 in. (1.07–1.22 m) above finished floor.

Audible notification must be at least 15 dBA above ambient or 5 dBA above maximum sound level, whichever is greater, measured at 10 ft; 75 dBA minimum in sleeping areas.

Survivability: NFPA 72 requires pathway survivability Levels 0–4 for in-building fire emergency voice systems; Level 2 or higher is common for high-rise voice evacuation.

Inspection, testing, and maintenance frequency: monthly for some supervisory devices, annual for most initiating and notification devices; sensitivity testing per manufacturer within one year of installation.

Wiegand interface: 26-bit format carries facility code (8 bits) and card number (16 bits); clock/data lines are 5 V open-collector, limited to about 500 ft.

RS-485 for access control readers and controllers: 120 Ω termination at each end of the bus, daisy-chain topology, up to 32 unit loads and 4,000 ft at low baud.

OSDP is the modern replacement for Wiegand: bidirectional, encrypted (Secure Channel), supports firmware updates and tamper reporting.

Fail-safe vs. fail-secure: fail-safe locks unlock on power loss (life safety egress); fail-secure locks remain locked and require a mechanical egress release.

Request-to-exit (REX) devices must release the lock; NFPA 101 requires free egress without keys or special knowledge at all times.

CCTV lens and sensor: 1/3 in. sensor with a 2.8–12 mm varifocal lens covers roughly 70–100° horizontal; IP cameras specify resolution in megapixels and bitrate in Mbps.

PoE budget: sum camera wattage plus 20% headroom; a 24-port PoE switch with 370 W total budget supports about 12 cameras at 25 W each.

IP camera bandwidth ≈ bitrate × 3600 ÷ 8 bytes per hour; 4 Mbps per camera yields about 1.8 GB per hour, so a 30-day retention needs careful storage sizing.

Constant-voltage distributed audio: 70.7 V and 100 V lines; total tap wattage must not exceed amplifier rated output, and line loss is minimized at higher voltage.

Speaker impedance on 8 Ω systems: parallel pairs halve impedance (two 8 Ω = 4 Ω); never drop below the amplifier's minimum rated load.

HDMI 2.0 supports 18 Gbps and 4K60 4:4:4; HDMI 2.1 supports 48 Gbps and 8K60 or 4K120; passive copper HDMI is reliable to about 5 m.

HDBaseT extends HDMI, Ethernet, power, and control over a single CAT cable up to 100 m using a proprietary chipset.

Control protocols: IR (line of sight), RS-232 (point-to-point, ~15 m at 9600 baud), IP (network), and CEC (consumer HDMI control).

Lighting control: 0–10 V analog dimming is unidirectional; DALI is addressable bidirectional digital; DMX512 uses 250 kbps over RS-485 with 512 channels per universe.

Structured wiring for automation: home-run CAT6 to a central enclosure, 14 AWG for keypads and sensors, and separation from line-voltage dimmers to avoid noise.

Audio power: SPL drops 6 dB per doubling of distance in free field; amplifier headroom of 3 dB (double power) is typical for transient peaks.

Grounding electrode system per NEC 250.50 includes metal water pipe, building steel, concrete-encased electrode, ground ring, and rod/pipe electrodes, all bonded together.

Grounding electrode conductor sizing per NEC 250.66 depends on the largest service conductor; a 2 AWG or smaller service uses 8 AWG copper GEC.

Single-point bonding: shield and drain wires are bonded at one end only to prevent ground loops; the other end is left floating or capacitively terminated.

Telecom primary protector must be bonded to the grounding electrode system with a conductor sized per NEC 800.100(B), typically 14 AWG copper minimum.

Surge protective devices are rated by MCOV, clamping voltage, surge current (kA per mode), and response time; Type 1 at service, Type 2 at panel, Type 3 at point of use.

Cascaded SPD coordination requires lead length under 10 ft (3 m) and a minimum impedance between stages, or the downstream device takes the full surge.

Ground loop noise appears as 60 Hz hum or common-mode voltage; isolation transformers, optical isolation, and baluns break the loop.

Bonding jumper for communications per NEC 800.100 must be no smaller than 14 AWG copper and as short and straight as practicable.

Delaware electrical licensing is administered by the Delaware Division of Professional Regulation, Board of Electrical Examiners; low-voltage work is regulated under the electrical statutes.

Delaware requires licensure for electrical contractors and masters; unlicensed contracting is a violation subject to fines and cease-and-desist action.

Delaware has no statewide general contractor license; contractors must register with the Division of Revenue and comply with local municipal permits.

Delaware has no state sales tax, but gross receipts tax applies to contractors; subcontractors must register and file with the Division of Revenue.

Delaware requires workers' compensation insurance for employers with one or more employees; proof is required for license renewal.

Continuing education: Delaware master and journeyperson electricians renew every two years and must complete required code-update hours.

Delaware contractor law requires written contracts over a threshold and a three-business-day right of rescission for certain home solicitation sales.

Bidding and permits: permits are issued by the local authority having jurisdiction (e.g., New Castle County, City of Wilmington); work must be inspected before concealment.