Conductor sizing and ampacity

Choosing the right conductor size is the calculation electricians make most often. Too small, and the conductor overheats: insulation ages quickly, connections loosen, and in the worst case a fire starts inside a wall. Too large, and the job costs more and the conductors may not fit the terminals. The code gives a clear procedure: find the load, choose the overcurrent protection, select a conductor whose ampacity suits both after allowing for its installation conditions, check the terminations, and confirm the voltage drop. This chapter walks through each step with worked examples.

The ampacity rules are in Section 4 of the CEC (Rule 4-004 and the related rules), with the ampacity values in Tables 1 to 4 and the correction factors in Tables 5A to 5D. Overcurrent protection of conductors is in Section 14 and circuit loading in Section 8.

What limits a conductor's current

A conductor carrying current heats up because of its resistance (\(P = I^2R\), from ELEC-110). It reaches a steady temperature when the heat generated equals the heat it can shed to its surroundings. Ampacity is the current at which that steady temperature equals the maximum rating of the insulation. It therefore depends on:

• The conductor size and material: larger conductors have less resistance; aluminum has about 1.6 times the resistance of copper of the same size, so it needs a larger size for the same current.
• The insulation temperature rating, usually 60 °C, 75 °C or 90 °C.
• How easily heat escapes: a single conductor in free air cools better than several bundled in a raceway or cable.
• The ambient temperature: a hot attic or a boiler room leaves less margin before the conductor reaches its limit.

Common insulation types include RW90 (cross-linked polyethylene, 90 °C, wet and dry), T90 Nylon (90 °C dry, thermoplastic with nylon jacket, used in conduit), TWU and RWU (underground), and cables such as NMD90 (non-metallic sheathed cable for dwellings), AC90 (armoured cable) and TECK90 (armoured, jacketed cable for industrial use).

The ampacity tables

Table 2 gives the ampacity of copper conductors with not more than three current-carrying conductors in a raceway or cable (or directly buried), based on an ambient temperature of 30 °C, with separate columns for 60 °C, 75 °C and 90 °C insulation. Table 1 covers single copper conductors in free air, and Tables 3 and 4 do the same for aluminum. A few representative values for copper in the 75 °C column of Table 2 are shown below; always use the table in your adopted edition.

Small conductors

For the small copper conductors used in most branch circuits, the code limits the overcurrent protection regardless of the higher table values: 14 AWG copper to 15 A, 12 AWG to 20 A and 10 AWG to 30 A (Rule 14-104, with limited exceptions such as some motor and heating circuits). So a 12 AWG conductor rated 25 A at 75 °C is still protected at 20 A.

Which column?

A conductor with 90 °C insulation does not automatically get the 90 °C ampacity. The whole circuit is only as good as its weakest point, and the terminations on breakers, panels and devices are usually rated 60 °C or 75 °C. The rule of thumb used by both codes:

• Equipment rated 100 A or less, or marked for 14 to 1 AWG conductors, generally uses the 60 °C column unless the equipment is marked for 75 °C.
• Equipment rated over 100 A, or marked for conductors larger than 1 AWG, generally uses the 75 °C column.
• The 90 °C column can be used as the starting point for correction and adjustment calculations, as long as the final ampacity does not exceed the termination column.

These limits come from Rule 4-006 (temperature limitations at terminations) and the equipment markings. Most modern breakers and panelboards are marked for 75 °C, but check.

Correction and adjustment factors

When the conditions are worse than those assumed by the table, the ampacity must be reduced:

• Ambient temperature above 30 °C: multiply by the correction factor from Table 5A. For 90 °C insulation, the factor is about 0.91 at 40 °C and 0.87 at 45 °C; for 75 °C insulation, about 0.88 at 40 °C.
• More than three current-carrying conductors in a raceway or cable: multiply by the factor from Table 5C: 80% for four to six conductors and 70% for seven or more up to the next step in the table. Neutrals that carry only the unbalanced current of a balanced circuit, and bonding conductors, are not counted.

Both factors apply together when both conditions exist. Start from the column of the insulation rating (usually 90 °C), apply the factors, then check the result against the termination column and the small-conductor limits.

Overcurrent protection and standard sizes

Overcurrent devices come in standard ratings (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225 A and so on). The general principle is that a conductor must be protected at no more than its ampacity. When a conductor's ampacity falls between standard ratings, the code (Rule 14-104 and related rules) allows, under specific conditions, the next higher standard rating; check the conditions carefully before using it.

Parallel conductors

For large feeders and services, a single conductor can become unmanageably large. Both codes allow conductors of 1/0 AWG and larger to be run in parallel, with each phase made up of two or more conductors that are the same length, material, size, insulation type and termination method, so that the current divides equally. A 600 A feeder might use two sets of 350 kcmil copper, each set in its own raceway with all phases and the neutral together, to avoid heating from induced currents in metal raceways.

Voltage drop

Ampacity keeps conductors cool; it does not guarantee that the load receives adequate voltage. On long runs, the voltage drop calculated in ELEC-110 may require a larger conductor than ampacity alone.

Rule 8-102 limits the voltage drop in an installation to 5% from the supply side of the consumer's service to the point of utilization, and 3% in a feeder or branch circuit. These limits are mandatory in the CEC.

Lesson 5 continues with the overcurrent devices that protect these conductors, including interrupting ratings, selective coordination and tap rules.