Table of Contents
Cable Sizing Calculations to NEC (NFPA 70)
This guide shows how to size electrical cables using NEC (NFPA 70), with step-by-step calculation examples. It is intended for electrical engineers, contractors, and designers who need to accurately determine compliant conductor sizes.
Cable sizing under the NEC requires verifying:
- Conductor ampacity (NEC 310.16)
- Temperature and adjustment factors (NEC 310.15)
- Voltage drop (recommended limits)
- Overcurrent protection sizing (NEC 210.20, 240.4)
- Equipment grounding conductor sizing (NEC 250.122)
- Available fault current at the load
The complete calculation process is explained using worked examples, including 480 V feeder and EV charger cable sizing, demonstrating how each criterion influences the final cable selection.
ELEK also provides a Free NEC Cable Sizing Calculator that automatically applies NEC ampacity tables, temperature corrections, voltage drop checks, and OCPD selection.
For calculations based on Australian standards, see our AS/NZS 3008 cable sizing guide.
Example 1 – Three‑phase 480 V feeder: Continuous load with long run
Problem
Design a three-phase 480 V feeder to a distribution panel supplying a 100 A continuous load over a 1000 ft run. Conductors are single-conductor copper RHW (75°C) in PVC conduit, 35°C ambient, with three current‑carrying conductors; limit feeder voltage drop to 3%. The available fault current at the source is 3 kA, and the equipment terminals are rated for 60°C. Determine, per NEC, the minimum sizes for the phase and neutral conductors, the OCPD rating, and the equipment grounding conductor (EGC).
Step 1: Select the minimum phase conductor size for ampacity
To determine cable ampacity ratings, a continuous load is rated at 125% of its base load. Thus, the design current is calculated as:
[1]
\(I_\text{design} = 100\text{ A} \times 1.25 = 125 \text{ A}\)
The correct current rating table must be selected from NFPA-70 (NEC) based on the insulation type, cable type, and installation. Once the correct reference table has been identified, the correct column based on the insulation and terminal limitations must be selected. The appropriate table and column for this problem is Table 310.16, Column 3 for Insulation and Column 2 for Terminal limit.
Step 2: Ampacity correction based on Ambient
Ampacities for ambient temperatures other than those in the ampacity table 310.16 are corrected in accordance with Table 310.15(B)(1)(1). For raceways or cables exposed to direct sunlight on or above rooftops, where the distance from the roof to the bottom of the raceway or cable is less than 19 mm (3∕4 in.), add 33°C (60°F) to the outdoor temperature to determine the ambient temperature for applying correction factors in Table 310.15(B)(1)(1). In this problem, cables are inside a raceway but not on a rooftop; hence, the correction factor from the 35° row would give us 0.94, which would be applied to the ampacity column for the insulation in Table 310.16.
Step 3: Multiple conductor adjustment
The ampacity of each conductor shall be reduced as shown in Table 310.15(C) (1) where the number of current-carrying conductors in a raceway or cable exceeds three. For this problem, we have 3 conductors, hence no correction is applied:
Step 4: Ampacity ratings
To meet the design current of 125 A, the derated insulation ampacity results in 1 circuit of 1/0 AWG, with a tabulated ampacity of 150 A and a derated ampacity of 141 A. In the terminal column for the same size, we get tabulated ampacity of 125 A. Both the insulation and the terminal columns meet the design current.
Step 5: Minimum size to meet voltage drop requirements
For the voltage drop calculation, we will use 1 set of 1/0 AWG cable.
Determine cable operating temperature.
To determine the Voltage Drop for a cable selected, the operating temperature (θ°) must first be calculated using the equation:
\(\begin{align*}
& \theta_0 = \left( \frac{I_B}{I_Z} \right)^2 \times (\theta_Z – \theta_A) + \theta_A \\[1mm]
\end{align*}\)
Where:
\(I_B=\) Design current (A)
\(I_Z=\) Rated ampacity (A)
\(\theta_Z=\) Operating temperature of the cable when carrying \(I_Z\) in degrees Celsius
\(\theta_A=\) Ambient temperature
For a Design current of 125 A and a rated ampacity of 141 A, the operating temperature at the rated current for this cable type is 75 °C.
\(\theta_0=\) 66.4 °C
Determine cable resistance (Rc) and reactance (Xc)
The AC resistance of uncoated copper wires in PVC conduit and the reactance of all wires in PVC conduit at a starting size of 1/0 AWG will be used to determine the AC resistance Rc and reactance Xc of the chosen cable. The Rc and Xc values will be used to calculate the voltage drop across the cables, based on the load specified in the problem.
Calculate voltage drop based on power factor and cable operating temperature
The following equation is used to calculate the voltage drop on the cables:
\(V_{d3\phi}=IL\left[\sqrt{3}\left(R_c\cos\theta+X_c\sin\theta\right)\right]\)
\(\theta_0=\) 48.37 °C
The resistance and reactance values decrease for larger cables, with Rc = 0.062 ohm/1000 ft and Xc = 0.041 ohm/1000 ft. This reduces the voltage drop to 13.41 V (2.79%). The calculated percentage voltage drop approaches the limit. Any smaller cable would exceed the specified voltage drop limit. Therefore, voltage drop dictates the smallest cable size for this problem.
Step 6: Select the neutral cable size
The neutral carries zero current in a balanced three-phase system but is typically sized to equal the phase conductors to prevent unexpected imbalances and harmonics. A neutral conductor may be sized smaller than the phase conductor based on the load’s maximum unbalance, as per article 220.61(A), without violating standards or compliance.
Step 7: Overcurrent protective device rating
For a 100 A continuous load, the overcurrent protective device cannot be rated at 100 A because continuous operation at full rating violates the continuous-load sizing principles in Article 210.20(A). As per article 210.20(A), continuous loads must be protected by an OCPD rated at least 125% of the load, which means the minimum required device rating is 100 A × 1.25 = 125 A. The next step is confirming that the chosen rating is a standard ampere rating, since breakers and fuses must be selected from the standard sizes listed in Table 240.6(A). While 100 A is a standard size, it does not satisfy the continuous-load requirement; 125 A is the smallest standard size that does. This selection remains valid only if the conductors are sized so that their allowable ampacity (after any correction/adjustment factors and termination limits) is compatible with a 125 A OCPD, ensuring the conductors remain protected from thermal damage. Because 125 A is below 800 A, it remains within the range where standard selection practices apply, thereby avoiding the higher-threshold restrictions imposed on very large overcurrent devices as per article 240.4(B) and 240.4(C). When a conductor’s ampacity doesn’t exactly match a standard OCPD rating, NEC 240.4(B) permits using the next higher standard rating.
Step 8: Equipment grounding conductor (EGC)
In low-voltage installations, the equipment grounding conductor (EGC) is a core safety conductor because it provides a permanent, low-impedance path for fault current back to the source. Because its job is to clear faults, EGC sizing is based on the rating/setting of the breaker or fuse protecting the circuit, not the phase conductor size. The minimum EGC is selected from Table 250.122, which lists minimum sizes for copper and aluminum (or copper-clad aluminum) EGCs according to the OCPD rating. For example, where the circuit OCPD is 125 A, Table 250.122 indicates a minimum EGC of 6 AWG copper or 4 AWG aluminum/copper-clad aluminum. The EGC is not required to be larger than the circuit conductors as per Article 250.122(A).
When phase conductors are increased in size for reasons such as voltage drop, ambient temperature correction, or conductor bundling/adjustment, the EGC cannot remain at the table minimum if that minimum was based on the original conductor size. In these cases, the Base EGC (6 AWG) in this problem must be increased proportionally by circular-mil area so that the grounding path remains robust relative to the increased fault-current capability of the circuit conductors.
Original phase conductor: 1/0 AWG Cu (≈105,600 CM).
Upsized to 4/0 AWG Cu (≈211,600 CM) for voltage drop compliance.
Base EGC from Table 250.122 for 200 A: 6 AWG Cu (≈26,240 CM)
Scaling factor (CM) to account for percentage increase in size (CM) = 211600/105600 = 2.003
Base EGC rescaled: 26,240 × 2.003 = 52,559 CM
The scaled EGC requires 3 AWG copper.
Step 9: Available fault current (AFC)
Available fault current (AFC) represents the largest amount of current capable of being delivered at that point at the end of the run, as per definitions in Article 100 and 110.24. In this problem, the supply input provides a fault current of 3 kA at the source, and the calculation determines how much remains at the end of the run by modelling the effective impedance and applying a reduction multiplier. The method uses conductor resistance and reactance to form an effective impedance. C factor reflects “ease of current flow” for the wiring method, and applies a phase-dependent constant K (3-phase =1.73). The calculation then forms an F factor and an M factor, explicitly described as a reduction multiplier. If F approaches 0 (negligible conductor impedance), M approaches 1, and the load-end AFC approaches the source AFC; as F increases with length/impedance, M decreases, and the available fault current at the load drops accordingly. The AFC at the end of the run is then obtained by applying this reduction multiplier to the source value.
\(F= \frac {K\times L \times AFC_{Source}}{(PC\times C\times V)}\)
\(M=\frac{1}{1+F}\)
Calculated values of cable impedance in this problem yield a reduction multiplier M ≈ 0.554, meaning only about 55% of the source fault current is available at the end of the run.
\(AFC_{Load} =AFC_{Source} \times M\)
Using a source fault current of 3 kA, the reduced value becomes approximately 3 kA × 0.554 ≈ 1.66 kA.
Example 1: Selection summary
| Parameter | Initial selection | Final selection |
|---|---|---|
| Final cable size (phase) | 2/0 AWG Cu | 4/0 AWG Cu |
| Final number of sets | 1 set | 1 set |
| Initial ampacities |
195 A (Tabulated insulation), 142 A (Derated insulation), 145 A (Tabulated terminal) |
- |
| Operating temperature | 66.4 °C | 48.37 °C |
| Cable resistance Rc | 0.39 Ω/km | 0.203 Ω/km |
| Cable reactance Xc | 0.14 Ω/km | 0.135 Ω/km |
| Voltage drop | 25.96 V (5.41%) – exceeds 3% limit | 13.41 V (2.79%) – within 3% limit |
| Final ampacities | - |
230 A (Tabulated insulation), 216 A (Derated insulation), 195 A (Tabulated terminal) |
| OCPD rating | 125 A | 125 A |
| EGC size | 6 AWG | 3 AWG |
| Available fault current (AFC) | ≈1.66 kA (using impedance for selected size) | |
Example 2 – EV Charger Feeder: 125 A Load (DC Fast Charger, Hot Garage)
A single DC fast charger (DCFC) power cabinet is installed in a commercial parking garage with a 3-phase supply at 400 V (L-L) and a power factor of 0.95. The copper conductors (THHW, 90°C) are run overhead in a raceway near the ceiling with a terminal limitation of 60°C. It is installed 100 ft from the charger, where air temperatures can reach 45°C (113°F) during summer afternoons. The source fault level at the supply is 3 kA. The equipment grounding conductor is aluminum. The charger emits 3% triplen harmonics (3rd order). The resistance and reactance values for the cable are 0.13 Ω/1000 ft and 0.058 Ω/1000 ft, respectively.
Step 1: Calculate design current and minimum phase conductor size for ampacity
\(I_{\text{RMS}}=\sqrt{I_1^2+I_3^2}
=\sqrt{(125\,\mathrm{A})^2+(3.75\,\mathrm{A})^2}
\approx 125.06\,\mathrm{A}\)
\(I_{\text{design}} = 125.06 \times 1.25 = 156.33~\mathrm{A}\)
The correct current rating table must be selected from NFPA-70 (NEC) based on the insulation type, cable type, and installation. Once the correct reference table has been identified, the correct column based on the insulation and terminal limitations must be selected. The appropriate table and column for this problem are Table 310.16, Column 4 for Insulation, and Column 2 for Terminal limit.
Step 2: Apply ambient temperature correction (hot garage)
Ampacities for ambient temperatures other than those in the ampacity table 310.16 are corrected in accordance with article 310.15. In this problem, cables are inside a raceway but not on a rooftop; hence, the correction factor from the 41-45° row would be 0.87, which would be applied to the ampacity column for the insulation in Table 310.16.
Step 3: Multiple conductor adjustment
The raceway has three current-carrying conductors. The EGC is excluded from the count per NEC 310.15(F), and adjustment factors only apply when the count exceeds three per 310.15(C)(1). Since the neutral current is insignificant, it is not considered a current-carrying conductor and hence no correction applies.
Step 4: Ampacity ratings
The harmonics injected by the EV charger increase the effective current through the cable. The design current of 156.33 A must be satisfied by the derated insulation ampacities and the terminal ampacities. This results in a conductor size of 3/0 AWG.
Step 5: Minimum size to meet voltage drop requirements
\(\begin{align*}
& \theta_0 = \left( \frac{I_B}{I_Z} \right)^2 \times (\theta_Z – \theta_A) + \theta_A \\[1mm]
\end{align*}\)
The Design current, IB, is 156.33 A, with a rated ampacity, IZ, of 196 A. At the maximum operating temperature of 90°C, the conductor temperature is calculated to be 73.7 °C.
Cable resistance (Rc) and reactance (Xc)
The resistance and reactance of the XHHW conductor are not specified in the NEC. Hence, values must be taken from the manufacturer’s datasheet or calculated from the conductor temperature. For this example, we will assume the values of 0.13 ohm/1000 ft and 0.058 ohm/1000 ft.
Since 90° XHHW conductor impedance values are not listed in NEC Chapter 9, Table 9, use manufacturer datasheet values or calculate based on conductor temperature.
Calculate voltage drop based on power factor and cable operating temperature
The following equation is used to calculate the voltage drop on the cables:
\(V_{d3\phi}=IL\left[\sqrt{3}\left(R_c\cos\theta+X_c\sin\theta\right)\right]\)
Step 6: Select the neutral cable size
In an ideal three-phase system, the neutral current is zero; however, triplen harmonics return on the neutral. Since the neutral current is significantly lower than the phase currents, it is sized to match the phase.
Step 7: Overcurrent protective device rating
Per NEC 625.41, EVSE loads are continuous, so the OCPD must be rated at no less than 125% of the load current. With a true RMS current of 125.06 A (including harmonics), the minimum required rating is 125.06 × 1.25 = 156.33 A. Consulting NEC Table 240.6(A), the two nearest standard sizes — 125 A and 150 A — both fall short of 156.33 A. The next standard size, 175 A, is the smallest that satisfies the requirement.
Step 8: Equipment grounding conductor (EGC)
In low-voltage installations, the equipment grounding conductor (EGC) is a core safety conductor because it provides a permanent, low-impedance path for fault current back to the source. EGC sizing is based on the rating/setting of the breaker or fuse protecting the circuit, not the phase conductor size. Table 250.122 lists the minimum sizes for copper and aluminum (or copper-clad aluminum) EGCs, depending on the OCPD rating. In this example aluminum EGC was selected with a minimum size of 4 AWG.
If the phase conductors are upsized, the EGC must also be proportionally upsized, as discussed in Example 1 above.
Step 9: Available fault current (AFC)
\(F= \frac {K\times L \times AFC_{Source}}{(PC\times C\times V)}\)
\(M=\frac{1}{1+F}\)
Calculated values and cable impedance in this problem yield a reduction multiplier of M ≈ 0.84, indicating that about 84% of the source fault current is available at the end of the run. This is significant because the run was shorter than in example 1.
\(AFC_{Load} =AFC_{Source} \times M\)
Using a source fault current of 3 kA, the reduced value becomes approximately 3 kA × 0.84 ≈ 2.53 kA.
| Parameter | Selection |
|---|---|
| Final cable size phase and neutral | 3/0 AWG Cu |
| Final number of sets | 1 set |
| Ampacities for phase and neutral conductor |
225 A (Tabulated insulation), 196 A (Derated insulation), 165 A (Tabulated terminal) |
| Operating temperature | 73.7 °C |
| Voltage drop | 3.99 V (≈1%) |
| OCPD rating | 175 A |
| EGC size | 4 AWG Aluminum EGC |
| Available fault current (AFC) | 2.53 kA |
Key Engineering Takeaways
- Ampacity is only one constraint
Meeting Table 310.16 does not guarantee compliance. - Voltage drop can determine the final size
Conductor size may need to be increased beyond ampacity requirements to meet design limits. - Continuous loads increase design current
Applying 125% affects both conductor sizing and OCPD selection. - OCPD must be coordinated with conductor ampacity
The selected device must protect the conductor after all correction and adjustment factors. - EGC sizing must follow conductor upsizing
Increasing the phase conductor size requires a proportional adjustment of the grounding conductor. - Fault current reduces with impedance
The available fault current at the load depends on the conductor impedance and run length. - Installation conditions affect results
Ambient temperature, harmonics, and installation method all influence conductor size.
References
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NFPA (National Fire Protection Association). NFPA 70 – National Electrical Code (NEC), 2023 Edition. Quincy, MA: National Fire Protection Association.
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NFPA (National Fire Protection Association). NFPA 70 Handbook – National Electrical Code Handbook. Quincy, MA: National Fire Protection Association.
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IEEE (Institute of Electrical and Electronics Engineers). IEEE Std 141 (Red Book) – Electric Power Distribution for Industrial Plants. New York: IEEE.
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IEEE (Institute of Electrical and Electronics Engineers). IEEE Std 242 (Buff Book) – Protection and Coordination of Industrial and Commercial Power Systems. New York: IEEE.
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ELEK. Electrical Engineering Calculators. Available at: https://elek.com/calculators/
