Basic NEC Code Rules and Design Practice
BASIC NEC CODE RULES AND DESIGN PRACTICE
Wire Ampacity and Size
1. Maximum loading for any branch circuit is 80% of rating of circuit for ampacity of wire for any load. NEC 220-2, use NEC 310-16 for ampacity. This applies to not more than 3 phase conductors in 1 conduit.
Warning
Voltage Drop NEC 310-16 wire ampacity tables do not take into account voltage drop, only thermal limit. Overhead triplex, use 310-16 for wire ampacity.
Rule of Thumb
If circuit is loaded up to ampacity and distance to load is around 100 feet, then voltage drop will govern and will require larger wire size. If more than 3 phase wires are in one conduit, then wires must be derated according to Note 8, after Table 310-19 as follows:
TABLE 1
Number of Conductors Percent of Values in 310-16 *1-3 80 *4-6 80
7.24 70
25-42 60 43 and above 50
*If circuit is loaded up to 80% ampacity, then 6 phase wires can be put in one conduit. Neutral conductors supplying balanced loads do not have to be counted.
Note: Minimum Circuit size is #12 with 20 Amp CB.
Circuit Breaker Size 2. Maximum loading of any circuit breaker is 80% of rating for non-motor loads with exceptions as noted below. Breaker cannot be larger than ampacity of wire, except for motors and a few other loads.
TABLE 2
Load Maximum Circuit Breaker Size % of Current
Resistance Loads, Heat, Stoves, Toasters, Water Heater 125%
Lighting 125%
Motors except Hermetically Sealed 00-250% NEC 430-152
Motors Hermetically Sealed, Air conditioners 175%
And heat pumps
Welders 200%
MCP Breakers for Motors 125% or next larger size
Standard breaker sizes are listed Page 70-74, 240-6 NEC
Conduit Sizing
1. After circuits have been derated to get proper wire size use Table 3A if wires are all same size. Use Page 70-57"9, or Tables 4 and 5 if wires are all different size. Ground wires should be run with all branch
Wire Ampacity and Size
1. Maximum loading for any branch circuit is 80% of rating of circuit for ampacity of wire for any load. NEC 220-2, use NEC 310-16 for ampacity. This applies to not more than 3 phase conductors in 1 conduit.
Warning
Voltage Drop NEC 310-16 wire ampacity tables do not take into account voltage drop, only thermal limit. Overhead triplex, use 310-16 for wire ampacity.
Rule of Thumb
If circuit is loaded up to ampacity and distance to load is around 100 feet, then voltage drop will govern and will require larger wire size. If more than 3 phase wires are in one conduit, then wires must be derated according to Note 8, after Table 310-19 as follows:
TABLE 1
Number of Conductors Percent of Values in 310-16 *1-3 80 *4-6 80
7.24 70
25-42 60 43 and above 50
*If circuit is loaded up to 80% ampacity, then 6 phase wires can be put in one conduit. Neutral conductors supplying balanced loads do not have to be counted.
Note: Minimum Circuit size is #12 with 20 Amp CB.
Circuit Breaker Size 2. Maximum loading of any circuit breaker is 80% of rating for non-motor loads with exceptions as noted below. Breaker cannot be larger than ampacity of wire, except for motors and a few other loads.
TABLE 2
Load Maximum Circuit Breaker Size % of Current
Resistance Loads, Heat, Stoves, Toasters, Water Heater 125%
Lighting 125%
Motors except Hermetically Sealed 00-250% NEC 430-152
Motors Hermetically Sealed, Air conditioners 175%
And heat pumps
Welders 200%
MCP Breakers for Motors 125% or next larger size
Standard breaker sizes are listed Page 70-74, 240-6 NEC
Conduit Sizing
1. After circuits have been derated to get proper wire size use Table 3A if wires are all same size. Use Page 70-57"9, or Tables 4 and 5 if wires are all different size. Ground wires should be run with all branch