Letter Codes, Feeder Conductor Size And More

If you have a problem related to the National Electrical Code (NEC), are experiencing difficulty in understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Questions can be sent to codefaqs@earthlink.net. Answers are based on the 2011 NEC.

Letter codes

Can you explain how to decipher the letter codes on conductor and cable insulations?

T is for thermoplastic, H is for 75°C, HH is for 90°C, N is for nylon jacket, W is for wet location, X is for crosslinked polyethylene, and R is for rubber. Therefore, THWN is thermoplastic insulation, 75°C rated, wet location and a nylon jacket. THHN is thermoplastic insulation, 90°C rated, dry location, nylon jacket. Type THWN/THHN is a dual-rated conductor.

Feeder conductor size

I want to run a feeder to three three-phase, 460-volt (V) squirrel-cage motors. The motors are 15, 30 and 40 horsepower (hp). What size feeder conductors must I run?

Section 430.24 requires the ampacity of the feeder to be not less than 125 percent of the full-load current rating of the largest motor, plus the sum of the full-load current ratings of all the other motors. The full-load current ratings shall be taken from Table 430.150, which gives the full-load current ratings of three-phase AC motors. For 15 hp, the full-load current rating is 21 amperes (A). For 30 hp, the rating is 40A. For 40 hp, the rating is 52A × 1.25 = 65A. Add the ratings together (21 + 40 + 65 = 126A). Table 310.16 shows 1 AWG THW CU conductors have an ampacity of 130A.

Motor-control circuit protection

Are motor-control circuits feeding remote-control devices required to have overcurrent protection?

Overcurrent protection for motor-­control circuits is covered in 430.72(B). The requirements for conductors that extend beyond the enclosure (remote) can be found in Table 430.72(B) in Column C. For example, if your motor branch-
circuit protective device is rated at 60A and you are using copper control-circuit conductors, then you find 60 in the copper column and move to the left to control-circuit conductor size where you will find 12. This means that you need to install control-circuit conductors not smaller than 12 AWG copper. Smaller conductors may also be used, but they would require supplemental overcurrent protection.

500 or 600 to the main?

For a 1,200A, three-phase, 120/208V service, can I run three sets of 500 kcmil underground from the transformer pad to the main 1,200A switch, or does it have to be 600 kcmil?

Table 310.15(B)(16) shows a 500 kcmil copper conductor with a 75°C rating as having an ampacity of 380A. Three sets run in parallel is 1,140A. NEC 240.4(B) permits the next higher standard overcurrent device rating above the ampacity of the conductors being protected to be used, if the next higher standard rating does not exceed 800A. Where the overcurrent device is rated over 800A, NEC 240.4(C) requires the ampacity of the conductors it protects to be equal to or greater than the rating of the overcurrent device. Therefore, you cannot run three sets of 500 kcmil copper conductors for a 1,200A system. Table 310.15(B)(16) shows 600 kcmil copper as having an ampacity of 420A. Three sets paralleled is 1,260A, which will satisfy the requirement.

Ground-rod connection

Does a connection to a ground rod have to be above-ground or made accessible by boxing around it to put it below or even with ground level?

Section 250.68(A) has an exception that states, “an encased or buried connection to a concrete-encased, driven, or buried grounding electrode shall not be required to be accessible.” Section 250.53(G) requires the upper end of the ground rod to be flush with or below ground level unless the upper end and the grounding electrode conductor attachment are protected against physical damage as specified in 250.10.


What does it mean in 200.7(C)(1) where it states, “but not as a return conductor from the switch to the switched outlet”?

As the wording indicates, when you use a cable assembly, such as a two-­conductor NM cable from a lighting outlet to a switch, the white conductor must be permanently re-identified to indicate its use as an ungrounded conductor. The re-identified conductor can only be used to supply the switch and not as a return conductor from the switch to the lighting outlet. I believe this wording is a throwback to a time when re-
identification of the grounded conductor was not a requirement. The purpose was to prevent a condition where two white conductors would be used to connect a lighting unit at the outlet.

Overcurrent protection requirement

Does a transformer installation require overcurrent protection on both the primary and secondary sides if the current is more than 9A?

For transformers rated 600V or less, Section 450.3(B) requires overcurrent protection to be provided in accordance with Table 450.3(B). Table 450.3(B) does not require overcurrent protection for the secondary of a transformer if the primary current is 9A or more and the primary overcurrent protection is limited to 125 percent of the transformer-rated current. Table 450.3(B) permits primary overcurrent protection to be 250 percent of the transformer-rated current if the secondary current is 9A or more and secondary overcurrent protection limited to 125 percent is provided. The requirement for overcurrent protection for the secondary winding of a transformer depends on the protection provided for the primary winding. The overcurrent protection of the feeder conductors for the transformer, if not larger than 250 percent of the transformer primary current, may serve as the overcurrent protection for the transformer primary winding. Protection of the transformer secondary conductors must be provided. Section 240.21(C)(1) through (6) provides the requirements for this protection.

How many terminals required?

I know that there is a reference in the Code that doesn’t allow more than one wire to be terminated under a screw terminal unless the terminal/device is marked to permit this. Is there a reference about not terminating more than one wire under a circuit breaker as well? Numerous panels at my workplace have this problem. I looked at a panel in particular that had 12 circuit breakers containing two wires. The panel in question is a 42-circuit, 200A MLO panel fed from a switchboard nearby.

Section 110.14(A) requires terminals for more than one conductor to be so identified. This applies to circuit breaker terminals. There are many circuit breakers of the type you’re referring to that have terminals listed for use with more than one conductor. But this must be marked on the breaker to allow this use. You can, if you wish, splice these conductors and pigtail a single conductor for use on the circuit breaker terminal in accordance with 312.8 where the splices or taps do not fill the wiring space to more than 75 percent of the cross-sectional area of that space. Apparently, the loads on these circuits are within the limits of the circuit breaker rating, and the combined loads do not exceed the rating of the overcurrent protection that is feeding the MLO panel you are referring to. Many times, it isn’t practical to splice lightly loaded circuits in a junction box outside of the panel and feed only two conductors to the panel. If the combined loads are properly identified in accordance with 408.4, then I don’t see a problem with combining the loads using a splice in the panel.

Sprinklers in an electrical room

Does the NEC address the issue of installing fire sprinklers inside an electrical room, especially above electrical equipment?

Section 110.26(E)(1)(c) addresses sprinkler protection in an electrical room. Sprinkler protection is permitted for the dedicated space where the piping complies with this section. If sprinklers are installed over equipment, they must be more than 6 feet above the equipment, and leak protection must be provided not less than 6 feet above the equipment. These Code requirements keep “other” equipment out of the dedicated electrical space.

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...

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