HOW TO SIZE ELECTRICAL CIRCUITS ICT BASIC ELECTRICAL TECH 100 HOW TO SIZE ELECTRICAL CIRCUITS #2 Wire Sizing and Materials

ACITY AMPACITY AMP Ampacity is: The AMPerage handling capACITY of wire, fuse, or CB It is the amperage draw PLUS a 25% safety factor Or, the amperage load x 1.25 = Ampacity Example: Name plate amperage is 12 amps 12 amps load x 1.25 = 15 amps ampacity Ampacity is the value used to determine the correct size of wires, fuses, and circuit breakers for inductive (motor) loads

Wire Insulation The insulation on a wire is a factor in its rated ampacity Examples of wire insulation descriptions: TW Thermoplastic, Wet (and dry) locations THHN Thermoplastic, High-Heat resistant, Nylon

Wire sizing Wire is rated in AWG (American Wire Gauge) The lower the number, the thicker the wire The lower the number, the higher the amperage capacity

Ampacity and Wire Insulation Wire size AWG Ampacity for Type TW & UF Ampacity for Type THHN Maximum Amp Ratings* # 14 20 25 15* # 12 25 30 20* # 10 30 40 30* *According to the electrical code, the overcurrent protection shall not exceed 15A for #14, 20A for #12, or 30A for #10 * (However, insulation is a factor in locations above 86 F)

Wire Marking Marking on the cable usually indicates wire type, insulation, conductor size, and application. Following are some examples: NM: Non Metallic casing enclosing the wire MC: Metal Clad casing enclosing the wire UF: Underground Feeder SEC: Service Entrance Cable #12 AWG: Size 12 American Wire Gauge

Some Examples of Common Indoor Wiring Materials Non Metallic cable (NM, Romex®) Armored cable (AC, MC, BX) Rigid Metallic Conduit (EMT, “Thinwall”)

12-2 WITH GROUND TYPE NM-B 600 V (UL) “Romex” Type TW insulation 12-2 WITH GROUND TYPE NM-B 600 V (UL) Two #12 wires with a bare ground wire Non-metallic cable

MC (Metal Clad) or Armored Cable (AC) Insulating Bushing

Electric Metallic Tubing (EMT) Coupling Connector

Outdoor Wiring For outdoor condensing units, the wiring materials must be flexible, liquid tight, and damage resistant Following is one example of the wiring used

Flexible Liquid Tight Conduit Coiled metal for flexible strength 90 ° Connector Plastic sheathing to make it “liquid tight”

Unit Ratings The unit data plate contains important electrical information Based on the information on the following slide, wire and circuit breaker sizes can be determined

Sample A/C Electrical Data Unit Rating Voltage Compressor RLA Compressor LRA OFM FLA MCA MOCP 7 ½ Tons 208/230-3-60 19.0 142 1.9 25.6 35 What is the wire size and circuit size for the above unit? A unit with 25.6 ampacity (MCA) requires: a minimum 30 amp circuit # 10 wire The circuit may be fused at a maximum of 35 amps (MOCP) Note: For ampacity calculations of a multi-motor assembly, 25% is added only to the largest motor. Example: 19.0 x 1.25 = 23.75 + 1.9 for OFM = 25.6 MCA LEGEND RLA – Rated Load Amps FLA – Full Load Amps LRA – Lock Rotor Amps MCA – Minimum Circuit Amps OFM – Outside Fan Motor MOCP – Max.*Overcurrent Protection Note: The overcurrent protection device must be a fuse or HACR breaker. *

Common HVACR Ratings The following table is a useful guideline for determining the wire and circuit breaker sizes for the most common motor amperages

Wire and Circuit Breaker Sizing for Motor Loads Total Unit Amps Circuit Breaker Amps Wire Size 0 – 12 15 # 14 13 – 16 20 # 12 17 – 24 30 # 10 25 - 32 40 # 8 33 - 48 60 # 6