Question

Question: Trying to do voltage drop for MI cable. Is there a table in the NEC with MI cable effective Z at .85 PF? Table 9 has all the conductor values but I'm sure if this also applies to MI cable. Thanks

Answer 1

You're right to be careful about applying the standard conductor values to MI cable for voltage drop calculations. While Table 9 is the go-to for many conductors, it's not directly applicable to MI cable for voltage drop without adjustments. Here's why and how to approach it

Why Table 9 Needs Adjustment for MI Cable Voltage Drop:
Construction Differences

MI cable's solid copper conductors, dense magnesium oxide insulation, and copper or stainless steel sheath result in different inductive and capacitive characteristics compared to typical insulated conductors in conduit or cable assemblies. This affects the overall impedance.

Higher Operating Temperatures

MI cable is often used in high-temperature environments. The resistance values in Table 9 are typically based on lower operating temperatures (often 75°C). The resistance of copper increases with temperature, impacting voltage drop.

How to Calculate Voltage Drop for MI Cable

Get the Manufacturer's Data

The most accurate way to determine the impedance (and therefore voltage drop) of MI cable is to obtain the manufacturer's data sheet for the specific cable you're using. They will often provide: Resistance per unit length (at a specific temperature, or with a temperature correction factor) Reactance per unit length (X) Impedance per unit length (Z) - ideally at the power factor you are using. Sometimes even pre-calculated voltage drop per amp per foot figures.2.

Calculate Resistance at Operating Temperature

If the manufacturer only provides resistance at a reference temperature (e.g., 25°C), you need to adjust it to the expected operating temperature. Use the following formula:

R₂ = R₁  [1 + α  (T₂ - T₁)]

Where: R₂ = Resistance at the operating temperature (T₂) R₁ = Resistance at the reference temperature (T₁) α = Temperature coefficient of resistance for copper (approximately 0.00393 per °C) T₂ = Operating temperature (°C) - Important: Use the expected operating temperature of the MI cable, which could be significantly higher than typical conductor temperatures.

T₁ = Reference temperature (°C)3.

Calculate Reactance (If Necessary)

You may need to calculate reactance. Usually you can neglect this as it is very small for MI cable. If you choose to include the reactance in the calculation: The manufacturer is the best source of reactance per unit length (X).4.

Calculate Impedance (If Reactance Is Used)

If you calculated a new resistance and are using a reactance value:
Z = √(R² + X²)
Where: Z = Impedance per unit length R = Resistance per unit length (at operating temperature) X = Reactance per unit length Make sure units are compatible (e.g., ohms per foot or ohms per meter).5.

Calculate Voltage Drop

Use the appropriate voltage drop formula:

Single-Phase

VD = 2 I Z L cos(θ)

Three-Phase

VD = √3 I Z L cos(θ)Where: VD = Voltage Drop I = Load Current (Amps) Z = Impedance per unit length (Ohms/foot or Ohms/meter) L = Length of the circuit (feet or meters) - use the same unit of length as Z cos(θ) = Power Factor (e.g., 0.85)

Important Considerations

Operating Temperature

Don't underestimate the operating temperature of MI cable. It's often used in locations where it's exposed to high ambient temperatures or high heat from the process it's serving. This significantly affects resistance and voltage drop. Consult the manufacturer's specifications and application guidelines.

Load Type

The power factor of the load is critical. If you have a highly inductive load (e.g., motors), the power factor will be lower than 0.85, which will increase the voltage drop. A purely resistive load (heaters) will have a power factor of 1.
0.

NEC Requirements

The NEC recommends limiting voltage drop to 3% for feeders and 5% overall (feeder plus branch circuit). See NEC 210.19(A) FPN 4 and 215.2(A) FPN 2.

Safety Factors

Consider adding a small safety factor (e.g., 5-10%) to your calculated voltage drop to account for uncertainties in load current, temperature, or cable characteristics.

In Summary:*Don't directly use Table 9 for MI cable voltage drop calculations. Get the manufacturer's data, adjust resistance for operating temperature, and use the appropriate voltage drop formula. Always be conservative and consider the specific application of the MI cable. When in doubt, contact the MI cable manufacturer for assistance. Flag for review

Answer 2

You're right to be careful about applying the standard conductor values to MI cable for voltage drop calculations. While Table 9 is the go-to for many conductors, it's not directly applicable to MI cable for voltage drop without adjustments. Here's why and how to approach it

Why Table 9 Needs Adjustment for MI Cable Voltage Drop:
Construction Differences

MI cable's solid copper conductors, dense magnesium oxide insulation, and copper or stainless steel sheath result in different inductive and capacitive characteristics compared to typical insulated conductors in conduit or cable assemblies. This affects the overall impedance.

Higher Operating Temperatures

MI cable is often used in high-temperature environments. The resistance values in Table 9 are typically based on lower operating temperatures (often 75°C). The resistance of copper increases with temperature, impacting voltage drop.

How to Calculate Voltage Drop for MI Cable

Get the Manufacturer's Data

The most accurate way to determine the impedance (and therefore voltage drop) of MI cable is to obtain the manufacturer's data sheet for the specific cable you're using. They will often provide: Resistance per unit length (at a specific temperature, or with a temperature correction factor) Reactance per unit length (X) Impedance per unit length (Z) - ideally at the power factor you are using. Sometimes even pre-calculated voltage drop per amp per foot figures.2.

Calculate Resistance at Operating Temperature

If the manufacturer only provides resistance at a reference temperature (e.g., 25°C), you need to adjust it to the expected operating temperature. Use the following formula:

Topic: MI cable voltage drop