Wire sizing calculator NEC

See Also

• Cable Sizing Calculator AS/NZS 3008
• AC and DC Voltage Drop Calculator with Formulas and Examples AS/NZS 3008

Parameters:

• Voltage (V): Specify the voltage and select the phase arrangement: 1 Phase AC or 3 phase AC. Currently supports AC only.
• Load (kW, kVA, A, hp): Specify the the load in kW, kVA, A, or hp. Specify cosF (load power factor) when load is specified in kW or hp.
• Maximum volt drop (%): The maximum allowable voltage drop.
• Distance (m, ft): The estimated cable or wire length in meters of feet
• Short circuit current and time (kA, ms): The short circuit current and clearing time for the protection device.
  • On low voltage circuits the let-through current is typically used, i.e. downstream of the protection device (fuse, MCB, or MCCB).
  • On high voltage circuits the prospective fault current is typically used i.e. the fault current on the primary side of the circuit breaker, contactor or fuse. This is so that the cable can also handle a fault if the primary protection fails. Furthermore, high voltage circuit breakers typically do not limit the fault current.
• Number of conductors: The number of current carrying conductors in a raceway, cable or buried directly. No more than three is allowed without applying a derating factor. Ignore the neutral and earth conductor in three phase cables.
• Insulation type: The type of insulation. Typically thermoplastic (PVC, 75°C) or thermoset (XLPE, 90°C). The important part is to select the correct temperature rating.
• Number of parallel conductors: Typically only one cable. More than one cable may be selected for high load scenarios. This will affect the derating of the cables. This is not considered in this version of the calculator.
• Cable installation: How the conductors are in installed.  The options are as specified in NEC tables 310.15(B)(16) and 310.15(B)(17).

Current rating:

• The current rating is selected from Table 310.15(B)(16) and Table 310.15(B)(17) in NEC 2017.
• The cable ratings in are based on an ambient temperature of 30°C.
• The current rating is based on the insulation type. Only PVC and XLPE cables are considered.
• The current rating is also based on installation method. Only raceways, cables, buried conductors, and free air is considered as specified in tables 310.15(B)(16) and 310.15(B)(17).
• The wire sizing calculator considers copper conductors only.

Derating:

• No derating is currently applied to the current ratings tables 310.15(B)(16) and 310.15(B)(17)..
• It is assumed that the maximum ambient temperature is  30°C and the maximum  ground temperature id 20°C. For higher temperatures, a derating will have to be applied according to NEC.
• It is assumed that there is only one three current carrying conductors in a raceway, cable or buried. And that the raceways, cables and buried conductors are spaced according to NEC to prenet derating. 
• To apply a manual derating, divide the load by the derating factor from NEC, and enter the new load value in the calculator.

Voltage drop calculation:

• The single phase AC voltage drop is calculated as:

\(V_{d1\phi}=\dfrac{I L (2 Z_c)}{1000}\)

where I is the load current, L is the distance, and \(Z_c\) is the cable impedance in O/km.

• The three phase AC voltage drop is calculated as:

\(V_{d3\phi}=\dfrac{I L (\sqrt{3}Z_c)}{1000}\)

• The impedance is calculated as:

\(Z_c =\sqrt{R_c^2 + X_c^2}\)

where \(R_c\) is the resistance and \(X_c\) is the reactance. This method calculates the impedance for the worse case power factor, i.e. when the cable and load power factor is the same.

• The cable sizing calculator uses the resistance \(R_c\) and reactance \(X_c\) values  from Table 9 in chapter 9 of the NEC.
• The values in Table 9 is based om three single conductors in a conduit. The wire sizing calculator uses the resistance and reactance values from the PVC conduit column in chapter 9.

Short circuit calculation:

• The short circuit capacity of the cables is calculated according to "P-32-382-2007, Short Circuit Characteristics of Insulated Cables,  Insulated Cable Engineers Association (ICEA), 2007".
• The short circuit capacity is calculated as:

\(\bigg(\dfrac{I}{A}\bigg)^2 t = 0.0297 \log\bigg(\dfrac{T_2+234}{T_1+234}\bigg)\)

where:

• I is the short circuit current in amperes.
• A is the conductor area in circular mils.
• t is the short circuit time in seconds.
• \(T_1\) is the maximum operating Temperature. The wire size calculator uses 75°C for PVC and 90° for XLPE.
• \(T_2\) is the maximum short circuit temperature. The wire size calculator uses 150°C for PVC and 250°C for XLPE.