Branch Circuits, Feeders, Service Calculations, Motors, Motor Circuits, Controllers, and More

CODE CITATIONS Article 210—Branch Circuits; Article 220—Branch-Circuits, Feeder, and Service Calculations; Article 225—Outside Branch Circuits and Feeders; Article 430—Motors, Motor Circuits, and Controllers; Parts of NFPA 20-1999 Standard for the Installation of Stationary Pumps for Fire Protection are also mentioned. Small Appliance Branch Circuit Q: The National Electrical Code (NEC) requires at least two 20-ampere small appliance branch circuits for a one-family dwelling unit. Sometimes I add extra circuits to supply the counter top receptacles in the kitchen, even though two circuits can carry the load. Do I have to add these extra circuits to my load calculations? Also, do I have to recalculate the service conductor sizes and overcurrent protection if two small appliance branch circuits are added to an existing installation? A: According to the words in Sections 220-16(a) and 210-11(c)(1), the extra-small appliance branch circuit load of 1500 volt-amperes for each circuit does not have to be added to the load calculations if the dwelling unit is supplied from a single feeder. This answer may not make sense, but the language in these two sections (210-16(a) and 210-11(c)(1)) indicates that a load of 3000 volt-amperes must be used for calculating the service-entrance conductor sizes regardless of the number of small appliance branch circuits that are supplied from a single feeder. Here is the way these parts read: “210-11(c)(1) Small-Appliance Branch Circuits. In addition to the number of branch circuits required by other parts of this Section, two or more 20-ampere small-appliance branch circuits shall be provided for all receptacle outlets specified by Section 210-52(b).” Notice that only two 20-ampere small-appliance branch circuits are required. Now let’s look at the words in Section 220-16(a): “Small Appliance Circuit Load. In each dwelling unit, the load shall be computed at 1500 volt-amperes for each 2-wire small-appliance branch circuit required by Section 210-11(c)(1).” Since only two small-appliance branch circuits are required, it can be said that only 3000 volt-amperes must be added to the calculations, regardless of the number of small appliance branch circuits installed in the kitchen. However, I do not think that is the intent, because the next sentence in Section 220-16(a) requires a load of 1500 volt-amperes for each small-appliance branch circuit where the load is subdivided through two or more feeders. This is a quotation of the second sentence in Section 220-16(a): “Where the load is subdivided through two or more feeders, the computed load for each shall include not less than 1500 volt-amperes for each 2-wire small-appliance branch circuit.” If the additional small-appliance branch circuits are added to the service calculations, the added load will be 4.375 amperes for a 240-volt, three-wire service. This assumes that the demand factors in Table 220-11 are used. This table allows the two extra small-appliance branch circuits to be put into the calculations at 35 percent (3,000 x 0.35 divided by 240). Where the total volt-ampere load exceeds 120,000, the added load at 240 volts is 3.125 amperes. If the optional method for calculating the size of the service is used, the added load is 5 amperes at 240 volts. For an existing one-family dwelling unit, I would not be concerned about the added load where the service is at least 100 amperes, three-wire and the floor area computed in accordance with Section 220-3(a) is 2,400 square feet or less. We know that computing the minimum ampacity of service-entrance conductors by using Table 220-11 or Section 220-30 results in conductor ampacities that are larger than necessary. For this reason, Table 310-15(b)(6) was introduced into the NEC. This Table allows No. 4 copper conductors with Type THW insulation to be rated for 100 amperes. Many utility companies supply a 100-ampere, three-wire, single-phase service on a residence with a No. 4 triplex aluminum service drop cable. These are the reasons I would not be concerned about overloading a service to a one-family residence because of the addition of two small appliance branch circuits. Number of Feeders to a Building Q: I recently ran into a situation where the existing main service panel is attached to a pole located about 60 feet from a house which, according to my calculations, presents a load of 122 amperes. The main service panel has a placard on it, which states, “Maximum Size Breaker on Bus 100 Amps.” I fed the house underground through a two-inch PVC conduit with a set of No. 2 AWG conductors to one panel serving a large shop area (which will accommodate power tools) and a large exercise room, and a set of No. 4 AWG feeder conductors to another panel serving the residence portion of the house (living room, dining room, bedroom, kitchen, and bath). The No. 4 feeders pass through the first panel in the house to the second panel. Both panels have 100-ampere main circuit breakers and are protected at the main service panel with 100-ampere circuit breakers. The inspector did not say anything about this setup on his rough-in inspection, but upon final inspection said the No. 4s running through the first panel violated Sections 373-8 and 384-3(g). These two sections deal with space in the panel through which the No. 4s run, but when I told the inspector that there was sufficient space for the wires, he retracted his first complaint and said that it was in violation of Section 225-30 (each building shall be served by one feeder circuit unless permitted in (a) through (e)). I said that I thought he was permitting this setup as I wired it on his rough-in inspection under Section 225-30(b)(2), which says, “...a single building sufficiently large to make two or more supplies necessary.” He said that that Section pertained only to commercial or industrial installations, and red tagged the job. I consulted the NEC Handbook and could not find anything that prohibited the method that I used. Going by his ruling, I would have to replace the existing 200-ampere main service panel on the pole with one that would allow a 125-ampere circuit breaker on its bus and feed the second panel in the house from the first one. What is your reading on this issue? A: The inspector is correct in not allowing two feeders to go to a one-family residence, but it is unfortunate that this problem was not mentioned during the rough-in inspection. You should have asked for special permission to install the two feeders before beginning the job. It is possible that you would have received special permission from the electrical inspector because the service equipment is existing and circuit breaker maximum ampere ratings are limited to 100 amperes, but the calculated load is 122 amperes. I disagree with the electrical inspector that the phrase, “A single building or other structure sufficiently large to make two or more supplies necessary” only applies to commercial or industrial installations. Large buildings or structures are not defined in the NEC, but a multi-family dwelling (apartment building) could be large enough to make two or more feeders necessary. Therefore, Item (2) in Section 225-30(b) could be used to allow multiple feeders for a large apartment building. The 100-ampere overcurrent device protecting the No. 4 copper conductors may be too large. If No. 4 copper conductors with Type THWN insulation are used, the ampacity of these conductors is 100. This ampacity is taken from Table 310-15(b)(6). Assuming a 120/240-volt, single-phase service, there are four current-carrying conductors in the two-inch PVC conduit which means that the conductors must be derated to comply with Section 310-15(b)(2)(a). The ampacity of the No. 4 conductors is now reduced to (100 x 0.80) 80. The overcurrent protective device should be replaced with an 80-ampere circuit breaker unless there are motor loads, such as an air conditioning compressor, fan motors, etc. Sections 430-62 and 430-63 provide information on sizing feeder overcurrent protective devices where a feeder supplies motor loads in addition to lighting and appliance loads. Transfer Switch Used as Service Equipment Q: In the June 2000 issue of Electrical Contractor magazine, you discussed using a transfer switch as part of the service equipment. I believe you did not completely answer the question. In order for a transfer switch to be listed as “Suitable for Use as Service Equipment,” it will require a disconnect on both the normal and emergency services ahead of the transfer switch proper. This isolating switch is shown as “Arrangement B” in NFPA 20––1999 Standard for the Installation of Stationary Pumps for Fire Protection. Please refer to Figure A-7-8 Typical Fire Pump Controller and Transfer Switch Arrangements. The logical method is shown as Arrangement A. It has the transfer switch as an integral part of the Listed Electric Fire Pump Controller. Then, both of the disconnects are part of the listed controller and the assembly is marked, “Suitable for Use as Service Equipment.” This is required in order to service the transfer switch, the controller, motor, and pump, if necessary. Complete isolation from both electrical sources is required and provided. A: While it is true that a transfer switch used with a fire pump is required to provide isolation from the normal and auxiliary power supplies, these isolation switches are not required for transfer switches that are used for other than fire pumps. According to the UL Standard for Transfer Switches, there must be an “off” position on the transfer switch in order to obtain a “Suitable for Use as Service Equipment” designation. The transfer switch must include a means for isolating the load contacts from the normal and auxiliary contacts. The manufacturer determines how to do this, then the testing laboratory evaluates the arrangement. FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.

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