BASIC NEC CODE RULES AND DESIGN PRACTICEWire Ampacity and Size1. Maximum loading for any branch circuit is 80% of rating of circuit forampacity of wire for any load. NEC 220-2, use NEC 310-16 for ampacity.This applies to not more than 3 phase conductors in 1 conduit.WarningVoltage Drop NEC 310-16 wire ampacity tables do not take into account voltage drop,only thermal limit. Overhead triplex, use 310-16 for wire ampacity.Rule of ThumbIf circuit is loaded up to ampacity and distance to load is around 100 feet, then voltagedrop will govern and will require larger wire size. If more than 3 phase wires are in oneconduit, then wires must be derated according to Note 8, after Table 310-19 as follows:TABLE 1Number of Conductors*1-3*4-67-2425-4243 and abovePercent of Values in 310-168080706050*If circuit is loaded up to 80% ampacity, then 6 phase wires can be put in one conduit.Neutral conductors supplying balanced loads do not have to be counted.Note: Minimum Circuit size is #12 with 20 Amp CB.Circuit Breaker Size2. Maximum loading of any circuit breaker is 80% of rating for non-motor loadswith exceptions as noted below. Breaker cannot be larger than ampacity of wire,except for motors and a few other loads.TABLE 2LoadResistance Loads, Heat, Stoves, Toasters, Water HeaterLightingMotors except Hermetically Sealed 00-250% NEC 430-152Motors Hermetically Sealed, Air conditionersAnd heat pumpsWelders6/14/02Chapter 3: NEC CODE Rules and Design PracticeMaximum Circuit BreakerSize % of Current125%125%175%200%1/13

MCP Breakers for Motors125% or next larger sizeStandard breaker sizes are listed Page 70-74, 240-6 NECConduit Sizing1.After circuits have been derated to get proper wire size use Table 3A if wires are allsame size. Use Page 70-57"9, or Tables 4 and 5 if wires are all different size. Groundwires should be run with all branch circuits.Example:Load Furnace 15 KW, I phase, 230V, find current, KVA, wire size, circuit breaker size,conduit size.I KW 15KW 65.2 AmpKV .23KVKW KVA, P.F. 1.0KVA 15Wire SizeWire Size Load ampacity 65.2 Amp 81.5 Amp.8Use NEC 310-16, 75 C, THW, wire#4 wire has 85 ampsUse #4 2 #4 and, 1 #4ground Circuit Breaker SizingCB Load 65.2 Amp 81.5 Amp.8.8Next higher CB size is 90 Amp.NEC 240-6 and 240-3 exception 1, conduit-size, 3 #4 in conduit Table 3A 1" conduit which allows 3 #4 maximum in a 1" conduitExample:Load: Lighting circuit 9-4 tube 4', 50VA/tube 115V fluorescent circuits. find I, circuitbreaker, wire.(9 fixtures) (4 tubes/fixture) (50V A/tube) 1800 VA 1.8 KVAI KVA 1.8 KVA 15.6 AmpKV.115 KVWire Size Load 15.6A 19.5 amps.8.86/14/02Chapter 3: NEC CODE Rules and Design Practice2/13

NEC 310-16, THW, 75 C #12 20 AmpCircuit breaker size Load 15.6 20 Amp breaker2#12 in conduit Table 3A, 1/2" conduit will take up to 4 #12 THWExample:Load: Motor 3 HP, 1 phase, 230V run ground wire with all motorsI 17A NEC 430 148Wire Size 17A 21.5A.8NEC 310-16 #10 is next size wire good for 30 Amp CB (I)(2 to2.5) NEC 430-152CB Size (17 A) (2) 34AUSE 35A circuit breaker.Conduit Size3#10, (2 power,1 ground), Table 3A, THW, 1/2" conduit will take 4 #10ExampleLoad: Unit Heater 10 KW, 230V, 3 phase, find I, wire size, circuit breaker sizeConduit SizeI 10 KW 25.1 Amp.23 KV 3Wire Size- 25.1A 31.3 Amp.8NEC 310-16 Use #8 good for 45 amp, 3 #8 1 #8 ground in 1" RGC Table 3ACircuit Breaker 25.1A 31.3 Amp so use 35A.8Conduit Size if different size wires are used6/14/02Chapter 3: NEC CODE Rules and Design Practice3/13

1. If different size wires are used in one conduit, the total area filled cannotexceed the percentage area given in Table 4, page 70-637 NEC. Use Table 5,Page 70-638 to get cross sectional area, then add all areas of all wires andpick the next largest conduit sizeExample: 3 #12, 4 #10, 2 #8, THW, what size conduit?From Table 5, Page 70-6383 # 124 # 102# 8in23(.0172) in2 .05164(.0311) in2 .012442(.0598) in2 .1196.18364 in2Page 70-582, Table 4, Over 2 Conductors, not lead covered, 40% fill.3/4" conduit will allow 21 in 2.Since .1836 in is less, use 3/4'' conduit.If, runs are over 50', increase conduit 1 size to facilitate pulling.6/14/02Chapter 3: NEC CODE Rules and Design Practice4/13

CIRCUIT LOADING Non-Motor LoadsCIRCUIT BREAKER SIZE Amp*Non-motor load1520 Normally smallest 030035040045050060070080075 C THW WIRE SIZEany load12121010888664433211/02/03/04/0250 MCM350 MCM or 2-1/0 Parallel500 MCM or 2-2/0 Parallel600 MCM or 2-3/0 Parallel700 MCM or 2-4/0 Parallel2-250 MCM Parallel3-3/0 Parallel3-4/0 Parallel3-300 MCM ParallelMAXIMUM Ampsany 240280320360400480560640* For motor Loads multiply running current by 1.75-2.50 to get circuit breaker size as shownin Table 2 above.6/14/02Chapter 3: NEC CODE Rules and Design Practice5/13

CIRCUIT LOADING GUIDE LINESMaximum Loading - for a 20 A Circuit20 Amp Circuit use for all lights and outlets @ 115V Maximum load is:I (20 A)(.8) 16 Amp maximumMaximum KVA for 20 Amp circuit is (16 Amp)(.115 KV) 1.84 KVA 1840VAOut lets (Duplex 1 outlet)Outlets 180VA per outlet. .18 KVAMax. outlets per circuit l.84 KVA max. 10.1 outlets per 20A circuit, use 10.18 KVA per outletFluorescent Fixtures 4 Foot TubeF40-T12 Tube 40 VA 10 VA Ballast Loss 50 VAVA1 tube2 tube3 tube4 tube6 tubeFixture Fixture Fixture Fixture Fixture 50VA 100VA 150VA 200VA 300VA KVA.05KVA.1 KVA.15KVA.2KVA.3KVAMaximumFixtures Per20 A Circuit36181296General Design Guidelines1) Lights and outlets should be on separate circuits to allow maximum loading oflighting circuits, and have maximum capacity available for outlets.2) Light circuits (20A) should be loaded up as close to 1.84 KVA aspractical to minimize the number of circuits.3) Outlet circuits are required to have GFI protection in the following locations:a. Residential constructionb. Any other location where portable equipment is likely to be used withgrounded surface or wet areas, shops, warehouses, comfort station, etc.,GFI outlets are about 20 , GFI breakers are about 35.6/14/02Chapter 3: NEC CODE Rules and Design Practice6/13

c. Reduce outlet loading from the 10 maximum per 20 Amp circuit to less inareas of large load, shops, repair facilities, etc.d. Each separate piece of fixed equipment must have its own circuit. Welders,grinders, drill presses etc.4. Minimum Service SizeMinimum size service that should be installed is 100 A, 20 circuit for very smallbuildings, comfort stations 200 A or 225 Amp, 40 circuit 120/240V, 1 phase forall others. This may be too small for actual load, must do load calculations.5. Future GrowthAllow for future growth. The service entrance is the most expensive part ofthe electrical system to replace. Use a minimum of 25% over the connectedload. Include ampacity and panel circuit space both. 50% is preferable.6. Voltage DropCheck service entrance to panel and branch circuits run over 75-100 feet forvoltage drop, this may increase wire size.7. Limit voltage drop to the following:Panel to LoadVoltage drop %Heating, Lights OutletsMotorsService Entrance to Transformer3%2%, 1% preferable2%8. Building Electrical Design Ordera.b.c.d.e.f.g.h.i.6/14/02Select voltage, phaseDo lighting calculation and layout lightingDo heating calculations and select equipmentLay out outletsCircuit any special equipmentFill out load calculation sheets and size panelMake up panel directoryBalance panelSize feeders to panel (just like branch circuits. 80% loadedwire same size or larger than ampacity of main breaker.Chapter 3: NEC CODE Rules and Design Practice7/13

j. Size grounding conductors NEC 250-94k. Fire alarm circuits must be tapped ahead of main breakerwith a separate panel.l. Do short circuit calculations, see Section 10.9. Underground WiringUse XLP-USE wire for underground wiring, # AWG Copper minimum.6/14/02Chapter 3: NEC CODE Rules and Design Practice8/13


1.1n Oregon, over current protection must be provided at the meter pole for each of the servicedrops to the buildings This protects the conductors from a short circuit fault should it occur betweenthe meter and the main breaker in the building.The over current device (fuse or circuit breaker)must be sized for the ampacity of the service drop, and available fault current.2.The service drop conductors must be sized from Table 310-17. If Triplex is used, Table 31016 must be used, as directed by NEC Article 321-5. The service drop ampacity must be the same asthe ampacity of the service entrance. ,THW or XLP insulated conductors should be used for overheadservice drops. XLP insulated conductor should be used for underground, and Table '3C of Chapter 9gives the conduit size required for XLP conductors.3.The service entrance conductors must be sized from Table 310-16. Table 3A of Chapter 9gives the conduit-size for THW insulated conductors used for an overhead service entrance. Table 3Cof Chapter 9 gives the conduit size forty insulated conductors used for underground service entrances.The ampacity of the service entrance conductors should be sized for the total capacity of the building'spower panels. For example, two 200 amp panels should have a 400 amp service to allow for addedcircuits in the future.4.4.The main breakers in the building provide over current protection for the power panels, andmust not be sized larger than the ampacity rating of the power panels.5.NEC allows a maximum of six hand movements at one location to shut off all the power of abuilding with a large power-distribution system. A switchboard or multiple individually mountedcircuit breakers must be at the same physical location as the service entrance to shut off panels locatedelsewhere in the building.6.All the buildings served by a common transformer must be reevaluate to insure that the.existing short circuit protection is adequate for the larger transformer required when additionalbuildings are added to the compound service system. The existing breakers must be upgraded tohandle the increased short circuit potential.7.The service entrance, service drop, and fused disconnect on the meter pole must bechanged to a higher ampacity when additional service is needed for the building.COMPOUND MATERING (USER-OWNED SERVICE6/14/02Chapter 3: NEC CODE Rules and Design PracticeDROPS)10/13

METER ON INDIVIDUAL BUILDINGS (UTILITY-OWNEDSERVICE DROPS)l.1.The utility company is responsible for everything up to theweather head-for an over head service, and up to the meter formost underground service entrances. All future maintenanceand upgrading is done by the utility.2. Items 3,4,5, and 6 in the Compound Metering section is stillthe responsibility of. the user.6/14/02Chapter 3: NEC CODE Rules and Design Practice11/13

SERVICE ENTANCES AND GROUNDINGSingle Panel Service1.The neutral is grounded to the power panelwhich is grounded to the water pipe, and to theconcrete foundation or to ground rods.2.All the branch circuits must have their groundwires attached to the -g grounded neutral.3.The main breaker in the panel serves as a single pointto shut power off for the entire building.Remotely Located Panels from a Main Panel1.2.3.panelbar.6/14/021.2., & 3 of single panel service apply to the mainpanel.The subfed conductors and the circuit breakers in themain panel are sized to the ampacity rating of thesubfed panels.The neutral in the subfed panels must not begrounded. The neutrals and grounds of the branchcircuits in the subfed panels must be kept separate.The subpanels must have a separate grounding bar forthe branch circuit ground wires. A separate groundwire must be run with the conductors from the mainpanel grounding point to the subfed panels groundingbar.Chapter 3: NEC CODE Rules and Design Practice12/13

1. The neutral in Panel A and Panel B are grounded to the panel which is grounded by aground wire to the service entrance grounding point in the wire gutter.2. Subpanels C and D are remotely located and require over current protection at theservice entrance for the conductors from the wire gutter to the panels. The individuallymounted circuit breakers also enable someone to shut off all the power at the serviceentrance. All the above must be located in the same room with a maximum 15 feetbetween the wire gutter and the panels or circuit breakers.3. All power must be shut off with a maximum of 6 hand movements. The main breakersin Panels A and B, plus the individually mounted breaker for panels C and D, can beshut off with 4 hand movements.4. Panels C and D require ungrounded neutrals, and separate grounding bars. A separategrounding wire must be run from the wire gutter to each subpanel, and connected tothe ground bar. The ground wires of the branch circuits of Panels C and D must beattached to a separate ground bar in each panel.6/14/02Chapter 3: NEC CODE Rules and Design Practice13/13