NEC Table 9

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primo.beltran

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Hi sirs, I have a question. NEC Table 9 list all Ohms to Neutral Impedances. However, the table is entitled as "Alternating Current Resistance and Reactance for 600V cables, 3Phase, 60Hz, 75 Deg C, Three Single Conductors in Conduit"

Are these installation and temperature conditions to be statisfied in order to use the Ohms to neutral impedances in our V drop calculations?

If I have more than three current carrying conductors in the raceway, or say I chose a 60 Deg C or 90 Deg C conductor temperature, will these impedances still going to be valid?

Or they can be used to all low voltage calculations.
 
primo.beltran said:
Hi sirs, I have a question. NEC Table 9 list all Ohms to Neutral Impedances. However, the table is entitled as "Alternating Current Resistance and Reactance for 600V cables, 3Phase, 60Hz, 75 Deg C, Three Single Conductors in Conduit"
Are these installation and temperature conditions to be statisfied in order to use the Ohms to neutral impedances in our V drop calculations?
Click to expand...
Welcome to the Forum.

You would use the ohm/kFT column from Chapter 9, Table 8.

The following basic formula can be used to determine the voltage drop in a 2-wire dc circuit, a 2-wire ac circuit, or a 3-wire ac single-phase circuit, all with a balanced load at 100 percent power factor and where reactance can be neglected:

VD=2?L?R?I?1000

where:
VD= voltage drop (based on conductor temperature of 75?C)
L= one-way length of circuit (ft)
R= conductor resistance in ohms () per 1000 ft (from Chapter 9, Table 8)
I= load current (amperes)

For 3-phase circuits (at 100 percent power factor), the voltage drop between any two phase conductors is 0.866 times the voltage drop calculated by the preceding formula.

Calculation Example


Determine the voltage drop in a 240-volt, 2-wire heating circuit with a load of 50 amperes. The circuit size is 6 AWG, Type THHN copper, and the one-way circuit length is 100 ft.
Solution
STEP 1.
Find the conductor resistance in Chapter 9, Table 8.
STEP 2.
Substitute values into the voltage-drop formula:

VD=2?L?R?I?1000

VD=2?100?0.491?50?1000=4.91 volts

STEP 3.
Determine the percentage of the voltage drop:

4.91V?240V=0.02 or 2%
Click to expand...
 
Thanks Chris. Folllow up question: table 8 indicates Direct Current Resistance at 75 Deg. C. Is this going to applicable to AC calculations. In some manufacturer cable Catalogs, the DC and AC resistances are in separate columns (as far as I remember). How about the AC Reactance?
 
primo.beltran said:
Thanks Chris. Folllow up question: table 8 indicates Direct Current Resistance at 75 Deg. C. Is this going to applicable to AC calculations. In some manufacturer cable Catalogs, the DC and AC resistances are in separate columns (as far as I remember). How about the AC Reactance?
Click to expand...
What is your application. As posted the above ignores reactance. Is this lighting, motor...?
 
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