When electrical wires or cables must run through areas where they could be damaged or create a safety hazard, they are run inside protective metal or PVC pipes or tubes called conduit. Simply pulling wires and cables through a conduit does not make it meet the minimum standards of the National Electrical Code. Care must be taken to properly calculate the correct size of conduit for each job.
Contact the local office or government branch responsible for enforcing your city or county's building and electrical code standards. Local regulations may meet or exceed the standards of the National Electrical Code (NEC). Installing improperly sized conduits can be expensive if local inspectors require replacement with different conduits or wires to meet standards.
Determine what wire sizes and types will run through the conduit. Different rules determine which conduit will meet code standards, depending on how much the conduit is filled by wiring, and the number of wires used. Special restrictions apply when you mix data and electrical wires in one conduit. Conduits and wires may also be graded differently depending on manufacturer or location. International standards rate wiring sizes and gauges using the metric system of measurement while North American countries such as the United States, Canada and Mexico adhere commonly to the imperial measured Standard Wire Gauge, abbreviated as SWG. Ensure conduit diameters are indicated in inches and electrical wiring are sized according to the SWG. Use conversion tables found within the NEC to convert materials using different sizing standards or units of measurement to prevent inaccurate conduit calculations.
Refer to NEC for tables containing the approximate area data which expresses, in square inches, estimated values of the total area of wires run at varying lengths. Add the value of every wire required to run within the conduit together to find the final answer. This can be used to determine the required minimum area of the electrical conduit. Divide the answer by 0.53 for one wire, 0.31 for two wires and 0.40 where more than two conductors are used. Refer to NEC total area charts for conduits using the final total to determine an appropriate size and length required to satisfy minimum code standards.
Compare your answers with those of a licensed electrician or building inspector to ensure the conduit does not exceed its maximum fill percentage before you purchase electrical supplies or apply for permits.
Check before purchasing any electrical supplies that the same standards are used between materials. Unit conversions can cause miscalculations when nominal sizes are different. Many permits require plan details that outline which wire and conduit size, type and length will be installed. Failure to provide this information can lead to your permit being rejected.