NEC & CEC Code Reference
Plain-English explanations of the most important electrical code sections. Written by electricians, linked to calculators, and built for exam prep.
NEC 310.16
Ampacity of Insulated Conductors
Table 310.16 is the single most-referenced table in the entire NEC. It tells you the maximum continuous current each conductor size can carry, organized by insulation temperature rating and material. Every wire-sizing decision starts here.
NEC 310.15(B)
Ambient Temperature Correction Factors
When the temperature around your conductors is higher (or lower) than the standard 86 degrees F, you must multiply the Table 310.16 ampacity by a correction factor. This section provides those multipliers based on conductor insulation type and actual ambient temperature.
NEC 310.15(C)
Conduit Fill Adjustment Factors
When you pack more than three current-carrying conductors into a single raceway or cable, each wire generates heat that the neighbors cannot escape. This section requires you to reduce (derate) the ampacity of every conductor in the bundle by a percentage that depends on the total count.
NEC 110.14(C)
Temperature Limitations at Terminations
Your circuit is only as good as its weakest thermal link. This section requires you to coordinate the conductor ampacity with the temperature rating of the terminals, equipment, and devices it connects to — using the lowest rating in the chain.
NEC 430.6
Determination of Motor Full-Load Current
This section establishes one of the most counterintuitive rules in the NEC: for sizing conductors and overcurrent protection, you must ignore the current stamped on the motor nameplate and instead use the full-load current values from NEC Tables 430.247 through 430.250. The one exception is overload protection, which does use the nameplate value.
NEC 430.22
Single Motor Branch-Circuit Conductor Sizing
Motor branch-circuit conductors for a single continuous-duty motor must have an ampacity of at least 125 percent of the motor full-load current from the NEC tables. This extra 25 percent accounts for the sustained heat generated during continuous motor operation.
NEC 430.52
Motor Branch-Circuit Short-Circuit and Ground-Fault Protection
Motor circuits get special overcurrent protection rules because motors draw a massive inrush current — often 6 to 8 times the running current — every time they start. Table 430.52 tells you the maximum percentage of the FLC you can use for each type of protective device, preventing nuisance trips while still protecting against genuine faults.
NEC 240.4
Protection of Conductors
This is the master rule for overcurrent protection of conductors. It says that every conductor must be protected by an overcurrent device that does not exceed its ampacity, with a handful of important exceptions for rounding to standard sizes, small conductors, tap conductors, and power-loss hazards.
NEC 240.4(D)
Small Conductor Protection
This subsection puts a hard ceiling on the maximum overcurrent protection device size for 14, 12, and 10 AWG conductors — the most commonly used wire sizes in residential and light commercial work. These limits cannot be exceeded regardless of the wire's ampacity rating.
NEC 240.6
Standard Ampere Ratings
This section provides the official list of standard fuse and circuit breaker sizes recognized by the NEC. Whenever you need to round up or round down to a standard overcurrent device size, this is the list you reference.
NEC Chapter 9, Table 1
Conduit Fill Percentages
This table sets the physical space limits for how much of a conduit's cross-sectional area can be occupied by conductors. The limits are 53 percent for one wire, 31 percent for two wires, and 40 percent for three or more wires. These are about preventing physical damage and pulling difficulty, not heat.
NEC 314.16
Box Fill Calculations
Every electrical box has a limited volume, and this section tells you exactly how to calculate whether your conductors, devices, clamps, and grounds will fit. Each component gets a volume allowance in cubic inches, and the total must not exceed the box's rated capacity.
NEC 314.28
Pull and Junction Box Sizing
When boxes contain conductors 4 AWG or larger, the box dimensions must be calculated using the raceway trade sizes entering the box. The formula differs for straight pulls (8 times the largest raceway) versus angle pulls and splices (6 times the largest, plus the others on the same wall).
NEC 250.66
Grounding Electrode Conductor Sizing
Table 250.66 tells you how large the grounding electrode conductor (GEC) must be based on the size of your service-entrance conductors. The GEC connects the electrical system to the grounding electrode (ground rod, water pipe, or concrete-encased electrode) and is a critical part of the grounding system.
NEC 250.122
Equipment Grounding Conductor Sizing
Table 250.122 sizes the equipment grounding conductor (EGC) — the green or bare wire that provides a low-impedance fault return path — based on the rating of the upstream overcurrent protective device, not the conductor ampacity.
NEC 220.12
General Lighting Loads
Table 220.12 provides the minimum volt-ampere (VA) per square foot values for general lighting loads based on building occupancy type. For dwellings, it is 3 VA per square foot. For offices, it is typically 2 VA per square foot. These are minimum values used in load calculations, not lighting design targets.
NEC 220.42
Lighting Load Demand Factors
Once you calculate the total general lighting load using 220.12, Table 220.42 allows you to apply demand factors that reduce the calculated load for feeders and services. The logic is simple: not every light in a building is on at the same time, so you do not need to size the service for 100 percent of the total lighting load.
NEC 220.82
Optional Method for Dwelling Unit Load Calculations
The optional method is a faster, simpler way to calculate the service load for a dwelling unit. Instead of applying individual demand factors to each load category, you lump most loads together and apply a blanket demand factor: 100 percent on the first 10 kVA and 40 percent on everything above that.
CEC CEC Rule 8-200
Dwelling Unit Service and Feeder Load Calculation
CEC Rule 8-200 is the Canadian equivalent of the NEC dwelling load calculation. It provides the method for determining the minimum ampacity of service or feeder conductors for single-family dwellings, starting with a basic load of 5,000 watts for the first 90 square metres, plus additional loads for the specific appliances and equipment installed.
CEC CEC Table 2
Ampacity of Insulated Conductors
CEC Table 2 is the Canadian equivalent of NEC Table 310.16 — it is the go-to reference for determining the maximum allowable current for copper conductors based on wire size, insulation temperature rating, and installation conditions. While the structure is similar, some ampacity values differ slightly between the CEC and NEC.
This is an educational resource, not the official code text. The NEC is copyright NFPA. The CEC (CSA C22.1) is copyright CSA Group. For the complete, official code, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.