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

2. 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

3. 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

4. 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

5. 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)

6. 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

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

8. 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

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

10. Electric Metallic Tubing (EMT)
Coupling
Connector

11. 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

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

13. 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

14. Sample A/C Electrical Data
Unit Rating
Voltage
Compressor RLA
Compressor LRA
OFM FLA
MCA
MOCP
7 ½ Tons
208/
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 = 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. *

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

16. 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 40
# 8 60
# 6