Considerations for Single-Family Residential Electrical Services Based on the 2020 NEC
For those who regularly install single-family residential electrical services, the work can often seem repetitive. Resulting in a step-by-step process that leaves many with the impression that they can do the work in their sleep. And many likely could. Then there are those who do not have much experience with residential electrical services and are trying to navigate the basics of what goes into a typical installation.Whatever side of the coin you fall on, this blog has something for you.While some of the items mentioned here are long standing NFPA 70, National Electrical Code® (NEC®) codes and processes, there are a couple changes within the 2020 NEC that will modify how residential electrical services will be installed moving forward. Pre-Installation Considerations Many residential electrical services need questions answered prior to being able to do the installation. Some residential electrical services are pre-designed and have the information already available for you. For example, in custom home scenarios, it is common to have to gather information up front from the builder and/or homeowner. The equipment being installed within the home may be minimal or it may be extravagant, but the electrical needs of both will need to be met accordingly. The size of the home and the equipment loads will have a direct impact on the overall size of the electrical service. As another example, if you are working in a tract home development, the builder has likely already pre-determined many items you would normally need to determine yourself before installation, like what size electrical service you need to install and where the electrical panel will be located. They may have already spelled out that each home within the entire development will receive a 150-amp electrical service, with the electrical panel installed in the garage, and utilizing aluminum service-entrance conductors. While there is not enough time to elaborate on a full electrical service calculation here, subscribers of NFPA LiNK™ can access a full single-family residential service calculation within NFPA DiRECT™ that will walk you through the step-by-step process. Once the legwork has been completed to come up with the electrical service size, and the desired location for some of the service equipment such as the electrical panel and meter, our next conversation needs to take place with the utility or power company (POCO). While many POCO’s operate differently, it is fairly common for them to give you information like whether the home will have power fed overhead or underground, or if you are given the option to choose. They will also confirm that the desired meter location will work for what they need. In a tract home scenario, the builder and POCO have likely already discussed these items and have a plan in place. However, in a scenario where there is a single custom home being built on a 10-acre parcel, having this conversation with the POCO becomes even more crucial to the process and helps to avoid any hiccups. For example, based on rights-of-way that the utility has available to them, they may only be able to bring the power into one specific corner of the home. If the meter gets mounted on the opposite corner of the home, the POCO may not provide power until it is relocated. Having the right conversations to gather the correct information, well before any screwdriver gets turned to install any equipment, is key to any successful project. Service Equipment Considerations The initial conversations before installation should provide some critical information, such as: where the meter and electrical panel need to be installed, if the electrical service will be fed overhead, or underground, etc. In some installation areas, the meter and electrical panel will be installed on the outside of the home in the exact same location. Within this scenario, it will also have your main service disconnect for the power into the home located at this spot. If the panel is going to be located within the home, that can have an impact on how things need to be done per the NEC. Where the electrical panel is installed within the home has a direct correlation to the NEC requirements for a main service disconnect. Per 2020 NEC section 230.70, a service disconnect is required to be installed for a building on the exterior of the building or inside nearest the point of entrance of the service conductors. In cases where the service entrance conductors leave the meter, penetrate the home, and go directly into the electrical panel, the main breaker within the electrical panel often serves as the main service disconnect required by 230.70. Where the service entrance conductors penetrate the home and do not terminate directly into the electrical panel, a main service disconnect mounted on the exterior of the home, prior to the service conductors entering the home, is necessary. Based on a change that was incorporated into the 2020 NEC, an emergency disconnect is now required per section 230.85. This disconnect is intended to give first responders the ability to shut down power to the entire home before entering to address the emergency. The NEC requires that the disconnect be installed outdoors in a readily accessible location and that it be identified as the emergency disconnect. In the previously mentioned scenarios where the main disconnect was installed outdoors within the electrical panel or in a service disconnect installed due to the distance the service entrance conductors run into the building, it will just be a matter of changing how the service disconnect is marked. It would need to be marked as an “EMERGENCY DISCONNECT, SERVICE DISCONNECT” or, if more applicable, one of the other two marking options listed in section 230.85. For an installation where the service conductors leave the meter, penetrate the home, and go directly into the electrical panel, an exterior emergency disconnect would now be required to be installed for the home. Section 230.85 requires an emergency disconnect to be installed for all new electrical services as well as when an electrical service is modified or upgraded. The 2020 NEC cycle had an additional change that will impact residential electrical service installations. New section 230.67 requires all dwelling unit electrical services to have a surge protective device (SPD) installed. The SPD must be integral with the service equipment, or mounted directly adjacent to the service equipment, and must be a Type 1 or Type 2 SPD. Many manufacturers are now offering residential electrical panels with built-in SPD to help meet this new code requirement. Similar to the emergency disconnect requirement, this SPD requirement applies to both new services as well as services that are modified or upgraded. Wiring Considerations While some items discussed in this blog are new to residential electrical services, there are many that stay tried and true. For example, sizing of service entrance conductors and conduit as well as sizing grounding and bonding conductors stay consistent with the way they have been done in the NEC for some time now. To review those requirements, the scenario below will be used to elaborate on each area. Scenario: 200-amp, 120/240-volt, single-phase, 3-wire electrical service with Type THWN copper service entrance conductors installed in Schedule 40 PVC conduit and a copper grounding electrode conductor and copper bonding wires. In order to size our service entrance conductors properly, we turn to Table 310.12 which addresses wire size for single-phase dwelling services and feeders. Using the scenario information that we have a 200-amp electrical service and are utilizing copper conductors; we determine that our service entrance conductors for this electrical service will need to be 2/0 AWG copper THWN conductors. Service Entrance Conductors Size: 2/0 AWG copper THWN conductors Knowing that we have a 3-wire electrical service consisting of (2) ungrounded (hot) conductors and (1) grounded (neutral) conductor, we look to Annex C – Table C.11 which covers installations using Schedule 40 PVC conduit. Using Table C.11, we determine that in order to fit our (3) 2/0 AWG copper THWN conductors into one conduit, we will need a minimum size of 1 ½” Schedule 40 PVC conduit. Conduit Size: 1 ½” Schedule 40 PVC conduit Grounding Electrode System Considerations The grounding electrode system in electrical services creates a common connection between electrical equipment, grounding electrodes that are present and/or required, and the earth so as they are at the same relative potential. This helps to stabilize voltage on the system and limit the voltage that can be imposed on the system by lightning strikes and other potential surges. NEC section 250.50 states that all grounding electrodes listed in 250.52(A)(1) through 250.52(A)(7) that are present in the building or structure must be bonded together to form the grounding electrode system. The following are grounding electrodes, some with specific requirements, that are permitted by the NEC for grounding: Metal underground water pipes Structural steel Concrete-encased electrodes (often referenced as “UFER ground” in the field) Ground rings Rod and pipe electrodes Plate electrodes Other listed grounding electrodes In most residential applications, the more common grounding electrodes that are incorporated into the grounding electrode system are the concrete-encased electrode (UFER), rod electrodes (ground rods), and metal water pipe, although this is becoming less common due to more use of plastic piping in lieu of metal for water supplies. NEC Table 250.66 is utilized for determining what size wire we need to use for our grounding electrode conductor, based upon the largest ungrounded (hot) service entrance conductor. With us determining previously that our service entrance conductor will be size 2/0 AWG copper THWN conductors and that the scenario also stated that the grounding electrode conductor should be copper, we can now use Table 250.66 to determine that we will use a #4 AWG copper grounding electrode conductor for this electrical service. The one caveat being the grounding electrode conductor that is ran to the ground rods is not required to be larger than #6 AWG copper. All other grounding electrode conductor connections, such as to the metal water pipe and to the concrete-encased electrode, will need to be sized as the #4 AWG copper required in Table 250.66. Grounding Electrode Conductor Size: #4 AWG copper conductor* *Except to ground rods which can be a #6 AWG copper Bonding Considerations By definition, bonding means connected to establish electrical continuity and conductivity. Unlike grounding, which intends to establish a ground connection, such as to earth, bonding connects all metallic parts on the system which have the potential to become energized. Building systems, such as metal water and gas piping, are required to be bonded per the NEC. Other systems, such as community antenna television (CATV), are required by the NEC to be bonded through an intersystem bonding termination (IBT) device. Bonding all the systems together helps to minimize the potential difference between systems. The metal water piping connection that may have been made as part of the grounding electrode system is not the same requirement as listed in section 250.104(A), which requires the metal water piping system to be bonded. Again, one is dealing with grounding and one is dealing with bonding. Based on section 250.104(A), the conductor for bonding must be sized in accordance with Table 250.102(C)(1) although it is never required to be larger than 3/0 AWG copper. Based on our 2/0 AWG copper service entrance conductors and our scenario stating that our bond wires would also be copper, we can utilize Table 250.102(C)(1) and determine we need a #4 AWG copper bonding wire for the metal water piping. Also of note, section 250.24(B) requires a main bonding jumper, for grounded systems, that connects the equipment grounding conductor to the grounded (neutral) conductor at the service disconnecting means. Typically, this main bonding jumper is provided by the manufacturer along with residential panels in the form of a screw or strap that bonds the grounded (neutral) bar to the enclosure when inserted properly. If that main bonding jumper were not to be provided, and the NEC is needed to find the correct size, section 250.28(D)(1) tells us that Table 250.102(C)(1) would also be used to size the main bonding jumper. Metal Water Piping and Main Bonding Jumper* Size: #4 AWG copper conductor *If the main bonding jumper is not provided with the panel. NEC section 250.104(B) states that “other” metal piping, such as gas piping, must also be bonded. That bonding is to be done per Table 250.122. What is different about table 250.122 versus the other tables we have been using is that it is based on the rating of the overcurrent device, often a circuit breaker of fuse, that is ahead of the circuit that could cause the system to become energized. The problem with scenarios like gas piping bonding is that we may not necessarily know what circuit could potentially energize it as the gas piping could be routed throughout the home and could be energized by any size feeder or branch circuit. However, in our scenario, the largest overcurrent device we would ever have to assume could energize the gas piping would be the 200-amp main breaker, since the is the largest circuit within the home. So, if we utilize Table 250.122 to determine the copper bond wire size we need for the gas piping bond, based on the 200-amp overcurrent device, we would select #6 AWG copper wire. Keep in mind that this # 6 AWG copper wire is the minimum size wire we need. For example, if there is already a roll of #4 AWG copper wire on the truck to take care of the water bond, there is no reason that that same #4 AWG copper wire could be utilized for the gas piping bond, since #4 AWG is larger than #6 AWG wire. Metal Gas Piping Bonding Size: #6 AWG copper conductor The last area of bonding residential services to cover is the bonding of communication systems as listed in section 250.94. Other systems, such as community antenna television (CATV), are required by the NEC to be bonded through an intersystem bonding termination (IBT) device. Again, bonding all of the systems together helps to minimize the potential difference between systems. Based on section 250.94(A), any IBT that is used must meet the following criteria: Be accessible for connection and inspection. Consist of a set of terminals with the capacity for connection of not less than three intersystem bonding conductors. Not interfere with opening the enclosure for a service, building or structure disconnecting means, or metering equipment. At the service equipment, be securely mounted and electrically connected to an enclosure for the service equipment, to the meter enclosure, or to an exposed nonflexible metallic service raceway, or be mounted at one of these enclosures and be connected to the enclosure or to the grounding electrode conductor with a minimum 6 AWG copper conductor. At the disconnecting means for a building or structure, be securely mounted and electrically connected to the metallic enclosure for the building or structure disconnecting means, or be mounted at the disconnecting means and be connected to the metallic enclosure or to the grounding electrode conductor with a minimum 6 AWG copper conductor. The terminals shall be listed as grounding and bonding equipment. In conclusion, there are many consistent ways in which residential electrical services continue to be installed. Based on the ever-changing need to continually make the world a safer place, there are also changes that happen within the NEC to incorporate those needs. This blog intends to give you the most up-to-date information based on the 2020 NEC, however, it is not intended to serve as a consultation or installation instructions for any given scenario. Electrical work should always be done by professional electricians who know the local code requirements. Those same electricians also have established relationships with the Authority Having Jurisdiction (AHJ) in that area where they can reach out and get clarification if needed. It takes all stakeholders working together to keep this electrified world a safer place for all people and property. The visual content included in this blog is from NFPA LiNK™, your custom on-demand code knowledge tool, brought to you by NFPA. Find out more about NFPA LiNK™, and sign up for your free trial, here: www.nfpa.org/LiNK Important Notice: This correspondence is neither intended, nor should it be relied upon, to provide professional consultation or services.