Conduit Type Effects on Voltage Drop and Impedance
How conduit material affects AC impedance and voltage drop. Covers steel conduit, aluminum conduit, PVC, and the importance of proper conductor grouping.
Conduit Material Matters
The type of conduit used for electrical installations affects AC voltage drop through changes in the conductor's effective impedance. This effect is particularly significant for steel conduit (EMT, IMC, rigid) where the magnetic properties of the steel create additional inductive reactance that increases the total impedance beyond the conductor's resistance alone.
Understanding these conduit effects enables more accurate voltage drop calculations for AC systems, especially for large conductors and high-current circuits where reactance can be a significant factor.
Steel Conduit Effects
Steel conduit (EMT, IMC, RMC) is magnetic, and this property affects the inductance and reactance of conductors run within it. The effect is more pronounced for larger conductors and can increase effective AC resistance by 15-50% compared to the DC resistance value.
Important Consideration
When only one conductor of a circuit is run in a steel conduit (or all conductors of one phase), severe heating can occur due to induced currents in the conduit. All conductors of a circuit must be grouped together in the same steel conduit.
Conduit Type Comparison
| Conduit Type | Material | Magnetic | Effect on Vd |
|---|---|---|---|
| EMT | Steel | Yes | Increases reactance |
| Rigid Steel | Steel | Yes | Increases reactance |
| Aluminum | Aluminum | No | Minimal effect |
| PVC | Plastic | No | No effect |
NEC Chapter 9 Tables
NEC Chapter 9, Tables 8 and 9 provide resistance and reactance values for different conduit types. Table 9 specifically shows the AC resistance and reactance for conductors in steel and aluminum conduit or PVC, allowing for more accurate voltage drop calculations.
Using NEC Table 9
- • Use effective Z values from Table 9 for more accurate AC calculations
- • PVC values apply to non-magnetic raceways
- • Steel conduit values account for the magnetic effects
- • Values given at 75°C and 60 Hz
Calculate with Conduit Effects
Our advanced voltage drop calculator can account for conduit type effects for more accurate AC calculations. Specify your raceway type for the most accurate results.
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