/images/PowerCableUpdate.jpg/images/southwire_100x70.jpgParalleling Conductors? Check the NECParallelingConductorsChecktheNEC.htm"Some people aren't aware of what the NEC requires to parallel conductors, and some people think the NEC calls for more than it really does," says Dave Mercier, technical director for Southwire's Electrical Division.
ENU
Register Now Forgot Password?

Paralleling Conductors? Check the NEC

"Some people aren't aware of what the NEC requires to parallel conductors, and some people think the NEC calls for more than it really does," says Dave Mercier, technical director for Southwire's Electrical Division. To make it more complicated, the 1999 NEC contains a paralleling option not everyone is aware of."

Grounds must be oversized or GF-protected

"The size of the ground conductor is a key issue when you parallel conductors," says Mercier. "Historically, the NEC required you to size each of the paralleled ground conductors to be big enough to handle the total current allowed by the device protecting the circuit. If you've got two cables in parallel feeding from a 200A circuit breaker, both ground conductors have to be able to handle 200A of fault current."

Think of it this way: When the fork lift runs into the cable tray and one of the two paralleled cables breaks, the ground conductor in the remaining cable has to handle all the fault current in order for the circuit breaker to operate properly.

The 1999 NEC does offer an optional approach to the oversized-ground strategy using ground fault protection equipment, but there's a catch. The option depends on equipment that may not be available.

GF options may be limited by equipment availability

NEC Section 250-122 (f) says that where only qualified personnel will service the installation, you can use a "normal" size ground as listed in Table 250-122, if the ground conductors are protected by listed ground-fault protection equipment (GFPE). The GFPE has to be set to trip on a current flow no more than the ampacity of one of the paralleled grounding conductors. It's another approach to the same principle: A single grounding conductor has to be able to carry all the current the circuit equipment will allow.

"The problem is that although NEC 250-122 (f) theoretically allows ground fault protection in lieu of oversized grounds, as far as we know equipment for this application isn't available," Mercier says.

What needs to match in parallel circuits?

The other historical misunderstanding about paralleled conductors that still crops up is whether all the paralleled conductors have to be identical. As of 1999, NEC Section 310-4 explicitly states that, "Conductors of one phase, neutral, or grounded circuit conductor shall not be required to have the same physical characteristics as those of another phase, neutral, or grounded circuit conductor to achieve balance."

Mercier points out, "Notice that conductors within each phase still need to have the same physical characteristics." Section 310-4 defines these physical characteristics as: length, conductor material, circular mil area, insulation type, and termination. So you wouldn't be able to mix type MC and type TC cable conductors within an individual phase circuit, for instance.

"Paralleling conductors can be a useful design strategy if it's done right," says Mercier. "Southwire's Type MC cables are frequently used in paralleled circuits, and Southwire can manufacture it with almost any combination of ground sizes."