Since the 1965 NEC^®, electricians have been required by the code to consider things like short-circuit current ratings (SCCR’s), circuit impedance, the speed at which overcurrent devices operate and other circuit characteristics in order to ensure that any installed overcurrent device can eliminate an overcurrent condition before the equipment being protected sustains extensive damage.

The code language found in 110.10 is a general requirement and is not trying to target the electrical engineer specifically. There are many places in the NEC^® where rules are directed towards an electrical engineer rather than the electrician. For examples, see 310.15(A) first paragraph referencing 310.15(C). See also: 310.60(A), 430.225(B)(1), 645.25, 705.12(D)(2)(3)(d), Annex B, and many other locations in the NEC^® where engineering supervision is required.

It is up to the electrician to understand basic electrical theory in order to comply with 110.10. Similar to other items in the NEC^® such as transformer calculations which are often performed by the electrician. In Article 450 for transformers, Tables 450.3(A) and (B) simply provide percentages to use when determining the overcurrent protection requirements for a transformer. The electrician must know things beyond the scope of the NEC^® in order to use those transformer tables.

Example: (75 kVA transformer calculation) 75,000 ÷ 831 (480 X 1.732) = 90.25 amps.

This calculation is not found in article 450 but the electrician must know the calculation in order to apply the percentages in the tables.

In the 2014 NEC^®, an important sentence was added to the scope of the NEC^® in Section 90.1 as highlighted below:

90.1(A) Practical Safeguarding. The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity. This Code is not intended as a design specification or an instruction manual for untrained persons.

The NEC^® does not mention how to calculate 3 phase circuits or how to figure voltage drop or how current adds when wiring in parallel while voltage adds while wiring in series. Often, the NEC^® only provides the rule regarding the actual installation and it is expected that the qualified electrician will learn about basic electrical fundamentals through proper electrician training.

All too often, the code reader just reads sections like 110.9 and 110.10 but doesn’t take the time to learn how to apply and understand their intended meaning or even how the rules can have an effect on an installation which can create a dangerous situation when misapplied or in many cases, ignored.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2017-2023 Code Language:

110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics. The overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the equipment grounding conductor(s) permitted in 250.118. Listed equipment applied in accordance with their listing shall be considered to meet the requirements of this section.