Bussmann Series FC2 Available Fault Current Calculator

Reply from: Thomas Domitrovich   April 29 2019, 1:02 pm EDT
JW, Thanks for the note and I have read through your post and have a few comments to help answer some of your questions. I believe you touched on three key areas with regard to this fault current value at the service equipment that include the following: 1. 110.24 marking requirement 2. Equipment rating evaluations 3. Incident energy calculations It’s important that you treat each of these separately so as not to confuse the issue. Each are important and related but we can’t mistakenly let each topic impact some of the requirements. I’ll touch on each here.

1. Available fault current / 110.24 requirement: The Bussmann application was specifically designed to help those in the field address the requirements of 110.24 which first went into the NEC as part of NEC 2011. The requirement says that “Service equipment at other than dwelling units shall be legibly marked in the field with the maximum available fault current.” That sentence has not changed since it was introduced. The Bussmann application, as well as other tools in the market, can be used to calculate the maximum available fault current at that equipment. This requirement goes on to provide some other requirements that must be included in the label including the following: a. Field marking. This is not something that can be done at the manufacturers’ facility as it is dependent upon length of conductors and fault current values from the utility which will change for each installation. b. Date: This is important for liability reasons for the contractor, inspector and facility owner. Due to changes in the system fault currents can change. Adding the date ensures that everyone understands that the design considered the level of fault current as calculated at that time. c. Durability: the label must be durable for the installation environment. The goal here is obvious as labels must be readable in the future even after being exposed to the elements. d. Documentation: A detail added as part of NEC 2017 is such that the AHJ must be given the documentation as to how the calculation was made. This was added because there were cases in the field where calculation values were provided but didn’t make sense. The AHJ needs to see the data. The FC2 app already provides this information as you can print the one-line diagram out and present that to the AHJ. Note it also provides the 110.24 label that inspectors can accept as meeting the requirements of this section. e. Modifications: 110.24(B) existed from the beginning of this requirement as part of NEC 2011. It emphasizes the fact that fault currents can change when facility changes are made and must be updated.

2. Equipment evaluations: You noted that someone could conclude you can’t use this app to meet 110.24. I do agree with you that some could conclude that you can’t use this app to meet 110.24 as anything is possible. That conclusion would be incorrect. You mentioned 240.86 in your note and drew the conclusion because of language in there that the Bussmann app cannot be used to meet 110.24 marking requirement of available fault current. Keep in mind that marking the fault current is the first step of equipment evaluation. We don’t influence the marking of available fault current based on equipment being used. Fault current is the fault current based on impedances and fault contribution sources including the utility and motors. Equipment must then be selected that has appropriate ratings. You have two options: a. Fully rated equipment: This is equipment that has SCCR and interrupting ratings that exceed the available fault current where it is applied. The marking of 110.24 can provide that information to properly select equipment. SCCR and Interrupting ratings must be greater than that on the label applied to meet 110.24. The Bussmann app is certainly adequate for this process. b. Series rated equipment: This is related to your 240.86 reference. It doesn’t have anything to do with calculating fault current and everything to do with properly selecting the equipment to be adequate for the fault current at that location. The NEC requires that if you are going to select devices that will have an interrupting rating less than the available fault current, the person doing this design must be a licensed professional engineer. The reasons for that are included in all of the details necessary to ensure a proper application. This is a good example of spending money to save money. The engineer will understand the details around motor contribution that must be met. This effort goes above and beyond what 110.24 is providing and it doesn’t impact that requirement at all. If an engineer is not involved, you should not be using series rated product as mistakes there could cause equipment to be over dutied. The Bussmann app can help with this evaluation as well but the engineer will need to understand motor contribution and manufacturer information to ensure proper devices and equipment are selected. c. A leap from 240.86 saying that 110.24 marking is impacted would be incorrect. 240.86 has its own additional markings to help ensure everyone at the site understands that the installation is an “engineered” solution and not a straight forward straight rated application.

3. Incident energy: NEC 2014 added an informational note to this requirement that says “The available fault-current marking(s) addressed in 110.24 is related to required short-circuit current ratings of equipment. NFPA 70E -2015, Standard for Electrical Safety in the Workplace, provides assistance in determining the severity of potential exposure, planning safe work practices, and selecting personal protective equipment.” as a side note to this, NEC 2020 will be adding interrupting ratings in addition to SCCR as that value can be used to evaluate both. The informational note points to NFPA 70E for assistance in determining severity of exposure. There are two different ways in 70E to help understand exposure and select PPE. They are as follows: a. Calculation method: This method includes calculating fault currents and arcing currents and using that information to determine incident energy which then is used to determine PPE. What the code panel wanted to ensure is that this value of fault current of 110.24 is not used to “calculate” incident energy. It doesn’t specifically say that but knowledge of IEEE 1584 and 70E will help the qualified individual understand that. If you are not familiar with calculating incident energy I understand it would be hard to understand that detail. b. Table method: This method provides a way to determine a level of PPE for the electrical worker when a calculated method label is not available. The table method has some restrictions and available fault current is one of those restrictions. The 110.24 marking CAN be used as the parameter data for the use of Table 130.7(C)(15)(a).