Metal Enclosures in Carwashes, Amperage Calculations and More

If you have a problem related to the National Electrical Code (NEC), are experiencing difficulty understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie and he will let the Code decide. Answers to questions are based on the 2008 edition of the National Electrical Code. Questions and comments can be sent to

Metal box in a carwash
Is there anything in the Code that would allow the installation of a metal box in a carwash self-serve bay, the type that accepts the money and controls the wash, rinse, etc., to not have a ground wire attached to it even if it is on a Class 2 24-volt (V) system? Twice now, I have found where unused conductors in the multiconductor cable connected to it (no conduit) have been touching other higher voltage terminals in the pump control units, and the other end was touching the metal self service box, causing it to be energized.

There is no requirement that metal enclosures containing only Class 2 circuits be grounded unless they are likely to become energized by a higher voltage source [250.4(B)(4)]. In your instance, “where unused conductors in the multi-conductor cable connected to it (no conduit) have been touching other higher voltage terminals in the pump control units,” the problem is covered by 110.12—Mechanical Execution of Work.

Adjusting amperage for temperature
I have a dispute concerning the ampere (A) rating of wires in a conduit due to derating. Coming out from a three-phase, 120/208 panel, I ran nine wires, consisting of two sets of three-phase (black, red, blue and white) and a common green ground. The wires are marked THHN, T90 (temperature of 90°F). Looking in NEC 2008, Table 310.16, the ampere rating for 12 AWG THHN conductors, at 90°F is 30A. Considering the correction factor in the same table (living in South Florida, where the ambient temp is considered 87 to 95°F), the correction will be 30A 0.96 = 28.8A. Then in Table 310.15(B)(2)(a), where I have eight current-carrying conductors, and after I adjust my temperature rating, the percent will be 70, where 28.8A 70 percent = 20.16A. Those were my calculations.

I was told that I should not use the 90-degree amperage column, that I should use the 75-degree column because, in order to use the data in the 90-degree column, both ends of my wires must be connected to 90-degree terminals. The breaker is marked 40°C (104°F), but the receptacles don’t show any temp rating. And 25A 0.94 = 23.5A, so the calculation is 23.5 70 percent = 16.45A. Can you help me to understand where I am wrong?

The ampacity ratings shown in the 90-degree column of Table 310.16 are permitted by 110.14(C), Temperature Limitations, to be used for derating purposes where it tells us “conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both.” Using 30 (90-degree ampere rating) 0.96 (temperature correction factor) 0.70 [using the adjustment factor Table 310.15(B)(2)(a)] = 30 0.96 0.70 = 20.16A.

Penetrating ceiling with Romex
I read with interest a response you made regarding NEC 312.5(C) Exception (b) in your February 2010 article. I recently had the question of drywall on wood trusses in a residential garage. The inspector failed the job due to the raceway used in 312.5(C) that penetrated a structural ceiling in violation of Exception (b). The builder was present and said the drywall on the wood trusses in the garage is not structural, that the drywall could be removed with no structural implications. Would this interpretation change your answer? Why can’t a Romex sleeve penetrate a ceiling?

A sleeve (raceway) containing Type NM cable in accordance with 312.5(C) Exception (b) is not permitted to penetrate a structural ceiling. The reasoning for this is that access to the top of the sleeve must be provided to use the sleeve for the purpose for which it is provided and to seal the raceway [Exception (d)]. A structural ceiling would not permit ready access to the raceway where a nonstructural ceiling, such as a lay-in type ceiling, can be easily removed and replaced.

Branch circuit for microwave oven
Is it OK for a 15A-rated individual branch circuit to be used for a 12.3A-rated microwave oven? It is cord-and-plug connected, and some people think, because it is in the kitchen, it has to be a 20A circuit. I disagree because this circuit is not a small appliance circuit. I know that the circuit cannot be larger than the rating of the single receptacle. I also believe I could install a single 20A-rated receptacle and protect it with a 15A OCP. Also, it was stated that the receptacle had to be a single device. I disagree. Article 210.21(B) does not mandate a single receptacle.

A 15A-rated circuit cannot be used for a 12.3-rated load [210.21(B)(2)]. A 20A circuit is required. A 20A-rated receptacle must be used for a single receptacle on a 20A individual branch circuit. A single receptacle installed on an individual branch circuit must have an ampere rating not less than that of the branch-circuit rating [210.21(B)(1)]. A receptacle installed on an individual branch circuit must be a single receptacle. The definition of an individual branch circuit shown in Article 100 reads, “A branch circuit that supplies only one utilization device.” An individual branch circuit supplies only one single receptacle for the connection of a single attachment plug. A branch circuit that supplies one duplex receptacle for the connection of two cord-and-plug pieces of equipment is not an individual branch circuit.

Conductors in lugs
Can paralleled conductors or any other conductors be landed under the same mechanical lug? Where can I find it in the NEC?

Connection of more than one conductor on a single terminal is permitted only where the terminals are identified for more than one conductor [110.14(A)].

LFMC limitations
Is there a maximum length for liquidtight flexible metal conduit (LFMC) to a motor regardless of horsepower? Are there any exceptions?

LFMC can be used in any length, provided it complies with the uses permitted (350.10), uses not permitted (350.12) and other requirements, such as bends (350.25 and .26), securing and supporting (350.30) and grounding and bonding (350.60). Check the grounding and bonding requirements in 250.134(B) where grounding is obtained by connecting to an equipment-grounding conductor run within the same raceway or cable, and check 250.102(E) where the installation of bonding jumpers is permitted. When connecting to a motor, flexibility may be required, and an equipment-grounding conductor must be installed (350.60).

Low-voltage device in line-voltage box
We have a product in development, a (plastic) switch plate that needs a 3-watt (W) power supply. Since it would be difficult to mount the transformer on the outside of the plate, we would like to explore the possibility of having the transformer inside the box and have low-voltage wire leads going from the transformer to the switch plate. I have been searching the NEC for what it says on having a low-voltage device in a line-voltage box, but besides the box fill, I have not found any other info on the issue.

The only Code solution I can come up with for your problem is an outlet box with a barrier installed, such as is required in Class 2 installations. You didn’t say what voltage you are working with in this product, but take a look at Articles 411.3 and 725.136(B) and (D).

Receptacles orientation
Where does the Code state receptacles cannot be installed face-up? What about floor receptacles and show window receptacles?

NEC 406.4(E) states that, in dwelling units, receptacles cannot be installed in a face-up position in countertops or similar work surfaces. Boxes for floor receptacles are listed for this purpose, and show window receptacles, if floor-mounted, must be in boxes listed for this purpose. Floor boxes are provided with covers and gaskets and are intended to be installed in accordance with instructions provided by the manufacturer [110.3(B)].

TROUT answers the Code Question of the Day on the NECA Web site. He can be reached at

About the Author

Charlie Trout

Code Contributor
Charlie Trout is most known for his work with the National Electrical Code (NEC). He helped write the NEC Since 1990; he was a member of NECA’s National Codes & Standards Committee and chairman of the National Fire Protection Association (NFPA)’s Cod...

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.