NEC rules for the ends of a wire differ from those for the middle. (Adapted from Practical Electrical Wiring, 20th edition, © Park Publishing, 2008, all rights reserved).

The key to applying these rules, and the new NEC Example D3(a) in Annex D on this topic is to remember that the end of a wire is different from its middle. Special rules apply to calculating wire sizes based on how the terminations are expected to function.

Entirely different rules aim at assuring that wires, over their length, don’t overheat under prevailing loading and conditions of use. These two sets of rules have nothing to do with each other—they are based on entirely different thermodynamic considerations.

Some of the calculations use, purely by coincidence, identical multiplying factors. Sometimes it is the termination requirements that produce the largest wire, and sometimes it is the requirements to prevent conductor overheating.

You can’t tell until you complete all the calculations and then make a comparison. Until you are accustomed to doing these calculations, do them on separate pieces of paper.

Current is always related to heat.
Every conductor has some resistance and as you increase the current, you increase the amount of heat, all other things being equal. In fact, as is covered in Sec. 110 of Div. 1 and elsewhere, you increase the heat by the square of the current.

The ampacity tables in the NEC reflect heating in another way. As the reproduction of NEC Table 310.16 (see Table 18 in Div. 12) shows, the tables tell you how much current you can safely (meaning without overheating the insulation) and continuously draw through a conductor under the prevailing conditions—which is essentially the definition of ampacity in NEC Article 100: The current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

Ampacity tables show how conductors respond to heat.
The ampacity tables (such as Table 18 in Div. 1) do much more than what is described in the previous paragraph. They show, by implication, a current value below which a wire will run at or below a certain temperature limit.

Remember, conductor heating comes from current flowing through metal arranged in a specified geometry (generally, a long flexible cylinder of specified diameter and metallic content). In other words, for the purposes of thinking about how hot a wire is going to be running, you can ignore the different insulation styles.

As a learning tool, let’s make this into a “rule” and then see how the NEC makes use of it: A conductor, regardless of its insulation type, runs at or below the temperature limit indicated in an ampacity column when, after adjustment for the conditions of use, it is carrying equal or less current than the ampacity limit in that column.

For example, a 90 C THHN 10 AWG conductor has an ampacity of 40 amps. Our “rule” tells us that when 10 AWG copper conductors carry 40 amps under normal-use conditions, they will reach a worst-case, steady-state temperature of 90 C just below the insulation.

Meanwhile, the ampacity definition tells us that no matter how long this temperature continues, it won’t damage the wire. That’s not true of the device, however. If a wire on a wiring device gets too hot for too long, it could lead to loss of temper of the metal parts inside, cause instability of nonmetallic parts, and result in unreliable performance of overcurrent devices due to calibration shift.

Termination rules protect devices.
Because of the risk to devices from overheating, manufacturers set temperature limits for the conductors you put on their terminals. Consider that a metal-to-metal connection that is sound in the electrical sense probably conducts heat as efficiently as it conducts current. If you terminate a 90 C conductor on a circuit breaker, and the conductor reaches 90 C (almost the boiling point of water), the inside of the breaker won’t be much below that temperature.

Expecting that breaker to perform reliably with even a 75 C heat source bolted to it is expecting a lot. Testing laboratories take into account the vulnerability of devices to overheating, and there have been listing restrictions for many, many years to prevent use of wires that would cause device overheating. These restrictions now appear in the NEC.

Smaller devices (generally 100 amp and lower, or with termination provisions for 1 AWG or smaller wire) historically weren’t assumed to operate with wires rated over 60 C such as TW. Higherrated equipment assumed 75 C conductors but generally no higher for 600-volt equipment and below. This is still true today for the larger equipment. (Note that medium-voltage equipment, over 600 volts, has larger internal spacings and the usual allowance is for 90 C, but that equipment will not be further considered at this point.)

Today, smaller equipment increasingly has a “60/75 C” rating, which means it will function properly even where the conductors are sized based on the 75 C column of Table 18, Div. 1.


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