Determining the Minimum Size Conductor


When determining the minimum size conductor for an installation, the temperature rating of everything involved in the circuit must be taken into account. This includes the conductor’s insulation, terminations, and devices. The lowest associated temperature is then used for the selection of the minimum size conductor.

For example: You are installing a piece of equipment on a new branch circuit. The terminals on the breaker and the equipment as well as the conductor’s insulation have temperature ratings. Let’s say that the circuit breaker is listed and marked for use with 75° conductors and the equipment is listed and marked for use with 60° conductors. If you were using a 90° insulation like THHN, you can’t use the 90° column in the ampacity table for selecting the minimum size. You will have to use the 60° column because it is the lowest associated temperature. The minimum size conductor required for the circuits load will be based on the 60° column of the ampacity table.

Using a conductor with a higher temperature rating does not allow you to go below the size necessary for the terminations. That would exceed their rating. What it will give you is a higher capacity for adjustment and/or correction under various conditions of use. Like installing more than three current carrying conductors in a raceway or cable or in an ambient temperature above 86° F or both. At best you won’t have to increase the size of the conductor.

If you don’t know the temperature rating of the terminals and they aren’t marked. You must use the 60° column for selecting conductor size for circuits of 100 amps and less. That includes 100 amps. You must also use the 60° column ampacities if the terminals are marked for use with conductor sizes 14 AWG – 1 AWG without temperature markings.

For circuits over 100 amps or marked for conductors larger than 1 AWG you will default to the 75° column of the ampacity table for conductor size selection when temperature rating isn’t evident.

The 90° ampacity column will rarely be used for size selection in most applications. All associated terminations, devices, and the conductor must be listed and marked for use at 90° for that to happen.

The conductor ampacity table is 310.16 in the 2008 NEC (and prior) and 310.15(B)(15) in the 2011.

Determining a conductor’s ampacity because of conditions of use and selecting the minimum size required for a circuit have little to do with one another. That is where the confusion is and why terminal temperatures are often exceeded. Some believe that using a higher rated insulation allows the use of smaller wire and it does not.

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