Prevent Nuisance Tripping – Proper Conductor Sizing Techniques per 2014 National Electrical Code

 

 

Contributed by Justin Grubbs, P.E. – I&E Department Manager

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Prevent Nuisance Tripping – Proper Conductor Sizing Techniques per 2014 National Electrical Code

Prevent Nuisance Tripping - Proper Conductor Sizing Techniques per 2014 National Electrical Code - H+M Industrial EPC

I have gone skydiving more than a few times in my life. As an active and licensed skydiver, the safety requirements involved are clear and have been established as the cornerstone of the sport. The requirements range from medical considerations to equipment specs, each one extensively thought through and put in place to ensure safety from the time you put on your rig to the time you land under your parachute. Coincidentally, these type of regulations are also common in my line of work. Just like with skydiving, the techniques used when sizing conductors for electrical projects are imperative to the process. You wouldn’t jump out of an airplane with the wrong parachute, so why design electrical systems with incorrectly sized conductors?

Throughout my experience working in the oil, gas, and petrochemical processing industry, I have seen numerous instances of errors committed while sizing conductors and performing voltage drop calculations per the National Electrical Code® (NFPA 70 – NEC). These errors can have a wide variety of effects which range from causing equipment to operate incorrectly or inefficiently to potentially causing serious injury or death to personnel. There are many factors and considerations which must be taken into account when performing these calculations.

For the purposes of this article, assume that we are sizing a conductor which meets the requirements of Table 310.15(B)(16) (formerly Table 310.16) of the 2014 National Electrical Code. Furthermore, assume that a THHN copper conductor will be used. The first consideration one must make when determining the appropriate conductor size is the temperature rating of the conductor. In this application, a 90°C rated conductor is being used. Does this mean one should use the ampacity listed in the 90°C column of this table? Perhaps surprisingly, no, it does not. Remember that the temperature rating in this table also applies to termination points of the conductor as these will be operating at the same temperature as the conductor itself, see 310.15(A)(B) and 110.14(C). In my experience, the most common rating for a low voltage breaker terminal is 75°C. According to an Eaton® (who also manufactures breakers for Rockwell Automation®) Application Paper, the terminals on molded-case circuit breakers are rated for a maximum temperature of 75°C. With this information, the correct column to use for conductor ampacity rating is the 75°C column.

Table 310.15(B)(16) provides a starting point for sizing a conductor. This table makes a number of assumptions, the most notable of which is that the conductor is installed in an environment with an ambient temperature of 30°C (86°F). Most applications in my experience have exceeded this ambient temperature requirement (especially when installed in Texas) and have therefore required an adjustment factor to be used. Table 310.15(B)(2) contains these adjustment factors.

Voltage drop is referenced in a number of places in the NEC as an Informational Note. Section 210.19(A) IN No. 4 is the first reference to voltage drop in the code. The significance of this is that Informational Notes as defined in the NEC are NOT code requirements. These are intended to provide recommendations for best engineering practices and not enforceable as a requirement of NEC, see 90.5(C). This being said, the NEC is intended to be a minimum requirement for electrical installations. It is published by the National Fire Protection Association as a means to mitigate harm to personnel or property. In most cases, good engineering practices, industry standards, and client specifications have requirements which are above and beyond NEC; voltage drop is by far the most common example of this. Consider a 3-phase 480V MCC lineup which is fed from any number of upstream transformers and switchgear. According to NEC, the total voltage drop of all feeder and load conductors to a connected load should be no more than 5% voltage drop for reasonable efficiency of operation. If this MCC were connected to a load which contained protective relaying, even this level of voltage drop may cause nuisance tripping. Additionally, if the voltage drop is too great at the load terminals, the increase in the current passing through the load conductors may cause upstream overcurrent protective devices to trip.

Whether it is skydiving or conductor sizing, it is important not to jump into (or out of) something without understanding all necessary factors. These factors are vital to safe and reliable operational decisions. The potential issues that stem from improperly sizing conductors vary in severity but are all important nonetheless. Always make sure to double check codes, applicable specifications, and industry recommendations to help decrease the likelihood of issues that could arise.

 

References: 2014 National Electrical Code® published by the National Fire Protection Association®, Eaton® Application Paper AP01200004E

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Justin Grubbs H+M Industrial EPC

Justin Grubbs, P.E. – I&E Department Manager at H+M Industrial EPC

B.S. in Electrical Engineering

Justin has more than 7 years of industrial engineering, construction and commissioning experience. He has experience with designing, engineering, leading I&E construction, developing plant control documents, directing commissioning efforts, overseeing instrumentation specifications, validating engineering data, specifying I&E material and training operations personnel.

One thought on “Prevent Nuisance Tripping – Proper Conductor Sizing Techniques per 2014 National Electrical Code

  1. There are some common installations where the electrical load to be protected will be located in an area that is subject to a different range of environmental conditions, particularly ambient temperature. An extreme example of this type would be where a fan motor is located in a minus 40 degree ice cream freezer and its protective circuit breaker is located in a poorly ventilated motor control center room where the air temperature routinely exceeds 100 degrees F. during hot summer months. This could result in the breaker s thermal element trip point being reduced due to its hotter ambient. The breaker could experience nuisance tripping.

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