The NEC requires that unless permitted elsewhere in the NEC, all conductors be insulated, 310.106(D). There are a few exceptions, such as the permission to use a bare neutral conductor for services, and bare equipment grounding conductors.

NEC Table 310.104(A) shows many types of conductors and their applications and insulations. The most commonly used insulations are of the thermoplastic and thermoset type.

What Is Thermoplastic Insulation?
Thermoplastic insulation is the most common. Like chocolate, it will soften and melt if heated above its rated temperature. It can be heated, melted, and reshaped.

Thermoplastic insulation will stiffen at temperatures colder than 148F. Typical examples of thermoplastic insulation are Types THHN, THHW, THW, THWN, and TW.

What Is Thermoset Insulation?
Thermoset insulation can withstand higher and lower temperatures. Like a baked cake, once the ingredients have been mixed, heated, and formed, it can never be reheated and reshaped.

If heated above its rated temperature, it will char and crack. Typical examples of thermoset insulation are Types RHH, RHW, XHH, and XHHW.

Wire Selection
NEC Table 310.15(B)(16) places the wires available for building construction into two categories: copper and aluminum or copper-clad aluminum.

Copper-clad aluminum wires have not been produced for many years. Copper wire is usually the wire of choice partly because the allowable ampacity of a given size of conductor is greater for copper than for the aluminum wire.

For example, the allowable ampacity of a 12 AWG conductor with Type THHN insulation is 30 amperes for copper and 25 amperes for the aluminum. These ampacities are suitable for applying adjustment or correction (derating) factors, but not for terminations.

In addition, serious connection problems exist with the aluminum conductors:

• If moisture is present, there may be corrosive action when dissimilar metals come in contact, such as an aluminum conductor in a copper connector.

• The surface of aluminum oxidizes when exposed to air. If the oxidation is not broken, a poor connection
will result. Thus, when installing larger aluminum conductors, an inhibitor is brushed onto the conductor and then it is scraped with a stiff brush.

The scraping breaks through the oxidation, thus allowing the inhibitor to cover the wire surface. This prevents air from coming in contact with the aluminum surface. Pressuretype aluminum connectors usually have a factory- installed inhibitor in the connector.

• Aluminum wire expands and contracts to a greater degree than copper wire for a given current. Referred to as “cold flow,” this action can loosen connections that were not correctly made.

Thus, it is extra important that an aluminum connection be tightened properly in compliance with the manufacturer’s instructions.

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