Code Quandaries

Aug 1, 2003 12:00 PM, By Mike Holt, NEC Consultant

Q.

My question concerns the termination of home automation control conductors that operate at less than 30V. Currently we run twisted-pair, Class 2 cables from the control panel to the 125V home automation switches. Some inspectors prohibit the Class 2 cable from entering the switch box. Others say it's okay, but they insist we maintain .25-in. clearance between the power and control wiring. Then there are those who want us to install a barrier between the Class 2 wiring and the line voltage wiring. What rules apply to this installation?

A.

The rule you're looking for is 725.55(D), which permits you to mix Class 2 conductors with power conductors (without a barrier) if the power conductors are introduced into the enclosure solely to connect to the Class 2 controlled equipment (your switch in this case). But this is only permitted if a minimum of .25 in. of separation is maintained between the Class 2 conductors and the power conductors.

Q.

When is an expansion fitting required for rigid nonmetallic conduit?

A. An expansion fitting is required to compensate for thermal expansion and contraction where the length of the raceway change is expected to be .25 in. or greater in a straight run between securely mounted items, such as boxes, cabinets, elbows, or other conduit terminations (352.44) (Figure).

Listing instructions that are provided with expansion fittings indicate that you should add 30°F to the ambient temperature when you're installing the raceway in direct sunlight. Assuming a high ambient temperature of 90°F (plus 30°F due to solar heating) and a low temperature of 0°F (no solar exposure), the temperature change will be 120°F. Table 352.44(A) indicates that the total expansion and contraction length change for 120°F would be 4.9 in. for a 100-ft run.

Be careful, because if the ambient temperature during installation is high, you should realize that the conduit is at its expanded range and will contract when the temperature drops. Of course, the opposite applies if the ambient temperature is low.

Where direct-buried raceways are subject to movement by settlement or frost, raceways shall contain expansion fittings to prevent damage to the enclosed conductors [300.5(J)].

Q.

What is the minimum mounting height for panelboards, disconnects, or meter enclosures?

A.

The NEC doesn't mandate a minimum mounting height for panelboards or disconnects. However, per the requirements of 404.8(A), switches and circuit breakers used as switches shall be installed so the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, isn't more than 6 ft, 7 in. above the floor or working platform.

Exception No. 2 to 404.8(A) permits switches and circuit breakers used as switches to be mounted higher than 6 ft, 7 in. if they're located adjacent to the equipment they supply and if accessible by portable means.

Note that 550.32(F) requires the outdoor mobile home disconnect to be installed so the bottom of the enclosure isn't less than 2 ft above finished grade or working platform.

And what about that meter enclosure? There is no rule in the NEC pertaining to the mounting height of a meter enclosure. However, the local electric utility will have a requirement in their specifications manual.

Q.

Is GFCI protection required for a 125V, 15A or 20A receptacle supplying power to a cord-and-plug water cooler?

A.

GFCI protection isn't required for the receptacle supplying a water cooler unless the receptacle is located in an area where GFCI protection is required. The Table above notes the sections of the Code that call for the use of a GFCI.

Q.

I'm an electrical contractor in New Jersey. The local inspector wants me to drive two ground rods on a residential 200A service. I can't find anything in the NEC that says I have to drive a supplemental ground rod, unless the first ground rod has a ground resistance of 25 ohms. The inspector isn't measuring the ground resistance, so why is he requiring me to drive two ground rods?

A.

I'm not sure why he's asking you to do this. The NEC permits one ground rod to be used as the required grounding electrode if the ground resistance of that single rod doesn't exceed 25 ohms (250.56). Since it's not the electrical inspector's responsibility to verify the ground resistance, I recommend you drive two ground rods unless you can prove that the resistance of a single rod is not greater than 25 ohms.

Q.

In our standard grounding detail for a 1,200A service, we show a separate grounding electrode conductor to the building steel, water pipe, and ground rod. Some individuals in our office insist that we only need a 6 AWG to the ground rod. I say it needs to be full sized in accordance with Table 250.66. Who's right?

A.

Not you. Where the grounding electrode conductor is connected to a ground rod, that portion of the grounding electrode conductor that is the sole connection to the ground rod need not be larger than 6 AWG copper [250.53(E) and 250.66(A)].

The resistivity of the soil prevents the possibility of a current flowing that is higher than what 6 AWG can handle. The purpose of this conductor is to carry lightning discharge current to the earth, not help clear a fault. Lightning doesn't know the size of the service on a building, so a 6 AWG to a ground rod for a 200A or 1,200A service is fine.

Still confused by the Code? For additional information on Code-related topics please visit www.mikeholt.com or send an e-mail directly to the author at mike@mikeholt.com.