Ampacity Determination, Retrofitted Signs and More

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iStock/ Emiphoto
Published On
Dec 13, 2019

Jim Dollard has an extensive background in codes and standards. If you have a query about the National Electrical Code (NEC) , Jim will help you solve it. Send questions to Answers are based on the 2017 NEC .

Ampacity determination

An inspector failed a small job we had where a 12/4 type NM cable was installed. All four conductors were current-carrying, and all were protected at 20A. He told us it was due to over three current-carrying conductors. He had us remove the cable and replace it with 10/4. Is that right?

No, the installation with 12/4 type NM cable protected at 20A, as described in your question, is NEC -compliant. Section 334.112 requires conductor insulation of Type NM cable to be rated at 90°C. The ampacity of Type NM cable is addressed in 334.80. The general rule is that the allowable ampacity cannot exceed that of a 60°C rated conductor. However, this section permits the 90°C ampacity rating to be used for ampacity adjustment and correction provided the final calculated ampacity does not exceed that of a 60°C rated conductor. Table 310.16 would apply for the conductor ampacity resulting in 30A in the 90°C column. The ampacity reduction for more than three current-carrying conductors according to Table 310.15(B)(3)(a) is 80%. The final calculated ampacity is 24A (30 × 0.8 = 24). The 60°C column rating in Table 310.16 is 20A. 334.80 requires that the 60°C rating of 20A be applied.

Retrofitted signs

We are retrofitting some LED-tube-type lamps into existing signs. The NEC requires a warning label. What text must the label contain?

Existing signs that have the illumination system retrofitted must contain multiple markings as required in 600.4(B). Where tubular-type LED lamps are installed as part of the retrofit and will be powered by existing sockets in the sign, a label must be installed to alert people servicing the sign. This label must also include a warning that is visible during lamp replacement to not install fluorescent-type lamps. Additionally, retrofitted signs must be marked with the name, logo or identifier of the entity that manufactured the retrofit kit used to upgrade the sign’s illumination system. It is important to note that Section 600.3 requires that retrofit kits used to upgrade signs be listed. Retrofit kits must also be provided with installation instructions and must be installed in accordance with the instructions and listing.

OCPDs in parallel?

Why does the NEC prohibit the use of circuit breakers in parallel? It is permitted in equipment, so why can’t we do the same? Should the inspector fail these products?

Section 240.8 addresses fuses and circuit breakers installed in parallel. Fuses and circuit breakers are permitted to be connected in parallel only where they are factory- assembled and listed as a unit. As stated in your question, the NEC, in general, prohibits fuses, circuit breakers or any combination of the two connected in parallel. There are many safety-driven reasons to prohibit the connection of overcurrent protective devices (OCPDs) in parallel. It is imperative that a single action is all that is needed to disconnect the source from service, feeder or branch- circuit conductors. If the NEC were to permit OCPDs to be installed in parallel, more than one device would be required to be open to remove the source of supply to equipment, which would create serious safety concerns. More than one device supplying the same conductors from the same source would also need to be locked out and tagged as part of the creation of an electrically safe work condition. In some cases, listed equipment may contain OCPDs in parallel. This is recognized and permitted in 240.8 where the equipment is factory-assembled and is also listed as a unit. See Section 90.7, which clarifies factory-assembled internal wiring is not subject to an electrical inspection where the equipment is listed by a qualified electrical testing laboratory.

GFPE performance requirements?

So, I’m looking at some changes in 2020 NEC and I see how, in 240.87 and 240.67, we have to make sure that the arcing current will actually trigger the reduction technologies used to meet these Code requirements. Why isn’t there a requirement in 230.95 to make sure that the ground-fault current is high enough to trigger the ground-fault protection of equipment (GFPE)? Doesn’t anyone have to show that GFPE will actually prevent the burning down of the service equipment, or is this just another ploy of the manufacturer to sell a product?

In order to answer your question, we must first look at the revision you mentioned and the existing requirements. The revision in the 2020 NEC that you are referring to has been added to the documentation requirements in both 240.67(A) and 240.87(A). Each of these sections is titled "Arc Energy Reduction." Section 240.67 addresses installations of fuses rated 1,200 amps (A) or more, and 240.87 covers circuit breakers where the highest continuous-current trip setting for the overcurrent device installed in a circuit breaker is rated or can be adjusted is 1,200A or higher. Each of these documentation requirements now mandate that the installer demonstrate the method chosen (to achieve arc-energy reduction) to reduce the clearing time is set to operate at a value below the available arcing current.

This is a very significant revision. While requirements for arc-energy reduction are not new, installers must now document that the method [e.g., see 240.87(B)] they choose to reduce clearing time will operate at a value below the available arcing current. The requirements of 230.95 for GFPE do not contain a requirement that the system will operate at a prescriptive value of current but there are maximum GFPE setting values provided in 230.95(A). These values include a maximum setting of 1,200A and a maximum time delay of one second for ground-fault currents equal to or greater than 3,000A. These requirements are dynamically different in that 230.95 is directed at protecting electrical equipment, and 240.67 and 240.87 are focused on reducing arc-energy exposure to installers and maintainers. Additionally, 230.95 applies only to systems over 150 volts (V) to ground but not over 1,000V phase-to-phase for each service disconnect rated 1,000A or more. To answer your question, 230.95 does provide parameters for the activation of the required GFPE protection but not a prescriptive value of arcing current to ensure operation. Allowing ground faults over 1,200A to sustain for one second before activation will not protect the equipment; it will only ensure that the arcing fault is cleared in at least 60 cycles. The methods listed in 240.67 and 240.87 to reduce clearing time include technologies that were not available when 230.95 was created. These new methods, if used as GFPE protection, would provide significantly more equipment protection through reduced clearing times and resulting in significantly less arc energy. Perhaps future editions of the NEC will recognize these newer technologies to provide GFPE along with a prescriptive requirement for the activation of GFPE in 230.95.

Article 517 and exam rooms

Does Article 517 apply to a doctor’s examining room located in a large commercial facility? This is an insurance company and the doctor is there during the normal work week to handle injuries, anyone feeling ill, general physicals, immunizations for travel and flu shots, etc.

Yes. See the scope of Article 517 in Section 517.1. This article covers all electrical construction and installation criteria in healthcare facilities. Clarity is provided in 517.1 explaining that parts II and III of Article 517 apply in single-function buildings, such as a hospital, but also apply within a multifunction building as described in your question. This would include a medical office or doctor's examining room as described. A medical office as defined in 517.2 is a building or part of a building where (1) examinations and minor treatments or procedures are performed under the continuous supervision of a medical professional, (2) only sedation or local anesthesia is involved and treatment or procedures do not render the patient incapable of self-preservation under emergency conditions, and (3) overnight stays or 24-hour operations are not provided.

Busway lighting permitted?

Is it permitted to supply lighting fixtures directly from a busway protected at 60A? Are the fixtures permitted to be attached to the busway?

Section 368.17 contains requirements for busway overcurrent protection. A busway is permitted to be used as a feeder with devices or plug-in connections used for tapping off of the feeder. This section requires that the device or plug-in connection making the tap off of the busway must contain overcurrent protective devices required for the branch circuit. The general rule is that the plug-in device must contain an externally operable circuit breaker or externally operable fusible switch. Exception No. 3 to 368.17 permits luminaires to be plugged directly into the busway with the overcurrent device mounted on the luminaire. The requirements of 240.21(B) for feeder taps would apply and would be used to determine the size of the tap conductors to the busway feeder and the rating of the required overcurrent protection. Luminaires are permitted to be supported from the busway when this requirement is applied. See Section 410.36(F).

About the Author

Jim Dollard

Code Columnist

Jim Dollard is the safety coordinator for IBEW Local 98 in Philadelphia. He is a member of the NEC Correlating Committee, NEC CMP-10, NEC CMP-13, NFPA 70E, NFPA 90A/B and the UL Electrical Council. He can be reached at

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