NEC Table 9 Alternating-Current Resistance

[AC_DC]

Hi,

I am trying to calculate voltage drop using the formula: VD=L*R*I/1000 where R values can be found from NEC table 9. However, that table 9 is for three single conductor in conduit. What if I have more than 3 conductors in conduit, where I can find the adjustment factor or how can I calculate for more than 3 single conductors in conduit?

 

[shoaib10]

This may help....
http://forums.mikeholt.com/showthread.php?t=169786

 

[shoaib10]

I dont know if this is correct but what i may do is as per 310.15(B)(3)(a) to do the adjustment factor and then get the corresponding ampacity and look it up in table 310.15(B)(16) to get the wire size...From that you can look up in Chapter 9 table 9 for AC resistance of the cable....

 

[AC_DC]

NEC Table9

NEC Table9

My question is that after I calculate ampacity using Table310.15(B)(3)(a), then Table 310.15(B)(16), I know which wire I need to use. Since my length of wire is long, I am considering there is voltage drop. I am using VD=L*R*I/1000 where R values are from Table9 in NEC. However, that table 9 on NEC says Three-Single-Conductors in Conduit. But I have 7conductors in my conduit. Is it okay to use my R value from table9 even though I have more than 3 conductors in my conduit.

 

[K8MHZ]

The OP hit on an interesting point. The tables are only for D.C. and three phase with three wires in a conduit. Nothing for single phase.

 

[iwire]

The OP hit on an interesting point. The tables are only for D.C. and three phase with three wires in a conduit. Nothing for single phase.

The NEC is not a design manual.

I would use an online voltage drop calculator.

 

[K8MHZ]

The NEC is not a design manual.

I would use an online voltage drop calculator.

Same here, but if you look up what 'R' means in VD=L*R*I/1000, it specifies that 'R' be taken from the NEC.

I did run across this, though.

Wire resistance calculations

The n gauge wire resistance R in ohms per kilofeet (Ω/kft) is equal to 0.3048×1000000000 times the wire's resistivity ρ in ohm-meters (Ω·m) divided by 25.4² times the cross sectional area A_n in square inches (in²):
R_{n (Ω/kft)} = 0.3048 × 10⁹ × ρ_(Ω·m) / (25.4² × A_{n (in)}

)
​

The n gauge wire resistance R in ohms per kilometer (Ω/km) is equal to 1000000000 times the wire's resistivity ρ in ohm-meters (Ω·m) divided by the cross sectional area A_n in square millimeters (mm²):
R_{n (Ω/km)} = 10⁹ × ρ_(Ω·m) / A_{n (mm}

)​

For copper, ρ = 1.68×10⁻⁸ Ω per meter

 

[K8MHZ]

What is “K” in the formulas? K is the “electrical resistivity” of the type of conductor being used. The K value isa constant and can be found in most physics tables that provide resistivity of variousmaterials. This electrical resistivity is calculated by (electrical resistance x cross-sectionalarea/ longitudinal length) and is expressed as (ohms x cmil/ft) or simply (ohms-cmil/ft.) Copper has a K value of 12.9 ohms-cmil/ft and aluminum has a K value of21.2 ohms-cmil/ft. Other reference materials may show slightly different values, but for thepurposes of this discussion, these values are acceptable. These two values are derivedfrom the data found in Chapter 9, Table 8 of the Code. The K factor is found by multiplyingthe conductor’s resistance (ohm/kFT) by the conductor’s circular mil area and then dividingby 1000.

http://www.adamselectric.coop/wp-content/uploads/2015/02/Voltage-Drop.pdf

 

[iwire]

Same here, but if you look up what 'R' means in VD=L*R*I/1000, it specifies that 'R' be taken from the NEC.

Where is that specified and in realation to what task?

 

[Ingenieur]

Where is that specified and in realation to what task?

I think he means eff Z, not R
and note 2 of the NEC table states i x Z gives a good approximation of L-N V drop

 

[Smart $]

.... But I have 7conductors in my conduit. Is it okay to use my R value from table9 even though I have more than 3 conductors in my conduit.

Don't know of any online calculator offhand that compensates for the number of conductors. Anyone?

There are two scenarios in derating for the number of conductors. You either upsize the conductor to maintain a specific ampacity... or you do not upsize because the derated ampacity is sufficient. In either scenario, the derating is to ensure the conductors do not exceed a certain temperature. That temperature is 75°C. It is also the temperature at which the R value in Table 9 is established (see title)... so the crux of this paragraph is, sure, by all means, use your value from Table 9.
:angel:

 

[K8MHZ]

Where is that specified and in realation to what task?

Toward the bottom of this page:

http://www.engineersedge.com/instrumentation/voltage_drop_calculations_10192.htm

""R" is the resistance per 1,000 ft. Use NEC Table 9 for AC wiring." From this page:

http://ecmweb.com/content/basics-calculating-voltage-drop

And in this PDF:

https://www.google.com/url?sa=t&rct...t7Tvh_zztl_0Qgrwg&sig2=cHNAw3HW1pLnjJTwITk0Kg

 

[K8MHZ]

I think he means eff Z, not R
and note 2 of the NEC table states i x Z gives a good approximation of L-N V drop

No, what I am referring to is R. See above.

 

[K8MHZ]

Don't know of any online calculator offhand that compensates for the number of conductors. Anyone?

That's the crux of the biscuit. Table 9 is for a combination of resistance and reactance specifically for 3 phase, 3 conductors per conduit. The reactance will change if the number of conductors per conduit changes, but there is no adjustment factors in the table. Table 8 is for DC, resistance only, and the amount of conductors per conduit doesn't matter. Why the VD formula the OP posted doesn't take that into consideration is puzzling.

 

[Carultch]

In either scenario, the derating is to ensure the conductors do not exceed a certain temperature. That temperature is 75°C. It is also the temperature at which the R value in Table 9 is established (see title)... so the crux of this paragraph is, sure, by all means, use your value from Table 9.
:angel:

:thumbsup:

 

[shoaib10]

Don't know of any online calculator offhand that compensates for the number of conductors. Anyone?

There are two scenarios in derating for the number of conductors. You either upsize the conductor to maintain a specific ampacity... or you do not upsize because the derated ampacity is sufficient. In either scenario, the derating is to ensure the conductors do not exceed a certain temperature. That temperature is 75°C. It is also the temperature at which the R value in Table 9 is established (see title)... so the crux of this paragraph is, sure, by all means, use your value from Table 9.
:angel:

I am just trying to understand..So first he has to derate using table 310.15(B)(3)(a) to do the adjustment factor and then get the corresponding ampacity and look it up in table 310.15(B)(16) to get the wire size...From that you can look up in Chapter 9 table 9 for AC resistance of the cable....Am i right?

 

[Smart $]

I am just trying to understand..So first he has to derate using table 310.15(B)(3)(a) to do the adjustment factor and then get the corresponding ampacity and look it up in table 310.15(B)(16) to get the wire size...From that you can look up in Chapter 9 table 9 for AC resistance of the cable....Am i right?

That's certainly part of a well-designed, compliant installation.

 

[shoaib10]

That's certainly part of a well-designed, compliant installation.

Okay..Took me some time to get it..But finally..:thumbsup:

 

[Ingenieur]

That's the crux of the biscuit. Table 9 is for a combination of resistance and reactance specifically for 3 phase, 3 conductors per conduit. The reactance will change if the number of conductors per conduit changes, but there is no adjustment factors in the table. Table 8 is for DC, resistance only, and the amount of conductors per conduit doesn't matter. Why the VD formula the OP posted doesn't take that into consideration is puzzling.

how much will it change?
the magnetic field for 3 x 100 A and 6 x 50 A are the same for a closed loop (conduit) Amperes Law
in the case of balanced 3 ph most cancels anyways

 

[K8MHZ]

how much will it change?
the magnetic field for 3 x 100 A and 6 x 50 A are the same for a closed loop (conduit) Amperes Law
in the case of balanced 3 ph most cancels anyways

That's not the point. The point, at least to me, is the verbiage used. I'll use an online calculator and call it a day. The question was about the formula in post #1. Not real life. And someone thinks it matters, or it wouldn't be explicit as to 'three phase, three wires'. Why not just call it 60 Hz AC if it doesn't matter?