NEC Chapter 9 Tables 8 and 9

[NEC User]

I'm trying to determine when to use NEC Chapter 9 Table 8 vs. NEC Chapter 9 Table 9.

My understanding is to use NEC Chapter 9 Table 8 for:
DC or AC 3 wire single phase applications
Does power factor matter? What about 1 phase applications with a 0.85 power factor?

My understanding is to use NEC Chapter 9 Table 9 for:
AC 3 phase applications (600V, 3-phase, 60 Hz, 75 degrees Celsius.

Is this correct?

 

[GoldDigger]

I'm trying to determine when to use NEC Chapter 9 Table 8 vs. NEC Chapter 9 Table 9.

My understanding is to use NEC Chapter 9 Table 8 for:
DC or AC 3 wire single phase applications
Does power factor matter? What about 1 phase applications with a 0.85 power factor?

My understanding is to use NEC Chapter 9 Table 9 for:
AC 3 phase applications (600V, 3-phase, 60 Hz, 75 degrees Celsius.

Is this correct?

No, it is not correct.

The values in Table 8 are mechanical properties of the wire, including resistance.
But that resistance does not include any inductance associated with the wires and the impedance of that inductance, which will be frequency dependent.
It also does not include the effect on the resistive component of skin effect. Skin effect refers to the action of magnetic fields to force more of the current to flow near the outside of the conductor, effectively increasing the resistance of the wire. This becomes significant at high frequencies and in large wire diameters.

Table 9 refers directly to the impedance of the wire after taking into account inductance and skin effect calculated for 60Hz.

I think you are confusing this with the voltage drop calculations in which you include a voltage drop in both conductors for DC, line to ground unbalanced AC and line to line 120/240 AC but do not include a second wire length in three phase balanced AC.

 

[NEC User]

Yes, I am asking this question to assist in voltage drop calculations.

I'm still confused though.
Under what conditions should I use the resistance values in Chapter 9 Table 8 to calculate voltage drop?
Similarly, under what conditions should I use the resistance values in Chapter 9 Table 9 to calculate voltage drop?

It now seems like Chapter 9 Table 8 is only for DC applications and Table 9 is for 3-phase AC applications. Then what do I use for single phase applications?

Thank you

 

[GoldDigger]

Yes, I am asking this question to assist in voltage drop calculations.

I'm still confused though.
Under what conditions should I use the resistance values in Chapter 9 Table 8 to calculate voltage drop?
Similarly, under what conditions should I use the resistance values in Chapter 9 Table 9 to calculate voltage drop?

It now seems like Chapter 9 Table 8 is only for DC applications and Table 9 is for 3-phase AC applications. Then what do I use for single phase applications?

Thank you

FWIW, the three phase in same conduit limitation in Table 9 is that the inductance will be higher if the wires are separated by any significant distance.

The resistive portion of the impedance will be the same for any 60Hz application regardless of configuration. If you are in a situation where the inductive component of the voltage drop is significant, you will need to use the information in the footnote to the table to figure out what the actual effective Z for your power factor is.

Executive summary:

You will use Table 8 to calculate voltage drop for DC circuits only, or for small wire high PF AC.

You will use the Effective Z columns from Table 9 for three phase balanced AC loads only.
For single phase AC or for different power factor loads you will use the X_L and R values from Table 9 and do some math instead of using the Effective Z values in the table.

 

[GoldDigger]

In case you would like a little more direct answer:

For two phase use the correct Effective Z applied to the double wire length (count both conductors)
For three phase, use the correct Effective Z applied to the one-way length (current cancellation in a balanced three phase circuit works for you.)

To calculate the Effective Z, see Note 2 to the table:

2. Effective Z is defined as R cos(Theta) + X sin(Theta), where ? is the power factor angle of the circuit. Multiplying current by effective impedance gives a good approximation for line-to-neutral voltage drop. Effective impedance values shown in this table are valid only at 0.85 power factor. For another circuit power factor (PF), effective impedance (Ze) can be calculated from R and XL values given in this table as follows: Ze = R ? PF + XL sin[arccos(PF)].

Had to fix up the quote since the symbol for Greek letter Theta did not come through.

 

[JDBrown]

Some jurisdictions have a set policy regarding which table you should use for voltage drop calculations. For example, the City of LA wants you to use Chapter 9, Table 8 for #2 AWG or smaller, and Chapter 9, Table 9 for larger than #2 AWG. So it might be a good idea to check with your Building Department to see if they have a standard way they want it done.