calculating cma

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bloodnsand

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Ok the elecrtical wiring residential 16th edition based on the 2008 nec by ray c mullin on page 104 says to use 12 for ohms on copper and 20 for aluminum the mike holt nec bundle and the uglys book says to use 12.9 for copper and 21.2 for aluminum so my question is which do I use and how are these books based on the nec with 2 different formulas to the same calculation
 
bloodnsand said:
Ok the elecrtical wiring residential 16th edition based on the 2008 nec by ray c mullin on page 104 says to use 12 for ohms on copper and 20 for aluminum the mike holt nec bundle and the uglys book says to use 12.9 for copper and 21.2 for aluminum so my question is which do I use and how are these books based on the nec with 2 different formulas to the same calculation
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Voltage drop is actually a complex calculation for AC circuits. You didn't post the formula from the book, but most likely it is actually the DC formula. The values you question are values of K. It is not a constant. It's a variable assigned to material resistivity, and varies with temperature.

http://www.dolphins-software.com/IEEE_ExactFormulae.htm
 
Smart $ said:
Voltage drop is actually a complex calculation for AC circuits. You didn't post the formula from the book, but most likely it is actually the DC formula. The values you question are values of K. It is not a constant. It's a variable assigned to material resistivity, and varies with temperature.

http://www.dolphins-software.com/IEEE_ExactFormulae.htm
Click to expand...

The IEEE formula adjustment for temperature doesn't yield a value of 12 for copper for 20, 25, or 60 degrees celsius. Reverse calculating it give about 53.4 C. This doesn't seem to be a usual temperature we see in the electrical trade. The question then remains; what's the source of the different values for K?
 
gadfly56 said:
The IEEE formula adjustment for temperature doesn't yield a value of 12 for copper for 20, 25, or 60 degrees celsius. Reverse calculating it give about 53.4 C. This doesn't seem to be a usual temperature we see in the electrical trade. The question then remains; what's the source of the different values for K?
Click to expand...
The value of K is in ohm-cmil/foot units.

The resistivity of copper is 1.68E?10-8 ohm-meter @ 20?C.

1.68?10-8 ohm-meter = 10.10571228912 ohm-cmil/foot

Voltage drop is typically calculated at 75?C conductor temperature.

Copper resistivity adjusted for 75?C...

2.04036?10-8 ohm-meter = 12.27338757514 ohm-cmil/foot


I used these two sites for the calculation:
http://www.endmemo.com/physics/resistt.php
http://www.translatorscafe.com/cafe...istivity/1-8/ohm_meter-circular_mil_ohm/foot/
 
gadfly56 said:
The IEEE formula adjustment for temperature doesn't yield a value of 12 for copper for 20, 25, or 60 degrees celsius. Reverse calculating it give about 53.4 C. This doesn't seem to be a usual temperature we see in the electrical trade. The question then remains; what's the source of the different values for K?
Click to expand...

Those constants are made to simplify calculations, especially for non-electrical people. The key is understanding what 12 stands for.

Voltage drop is basically amps X ohms. To derive the multiplier K, try calculating the CM-ohm per foot of copper wire and you'll get a figure that will be very near to the figures being used in voltage drop calculations.

Ex. 4/0, Copper:
CM = 211600 circular mils; ohms/1000 feet = 0.049

feet per ohms = 20408.16;

Therefore K = 211,600/20408.16 = 10.4. But since we will oversize wires at around 125%, the K value chosen became 12. To check, do it with a sample wire size of aluminum.
 
topgone said:
Those constants are made to simplify calculations, especially for non-electrical people. The key is understanding what 12 stands for.

Voltage drop is basically amps X ohms. To derive the multiplier K, try calculating the CM-ohm per foot of copper wire and you'll get a figure that will be very near to the figures being used in voltage drop calculations.

Ex. 4/0, Copper:
CM = 211600 circular mils; ohms/1000 feet = 0.049

feet per ohms = 20408.16;

Therefore K = 211,600/20408.16 = 10.4. But since we will oversize wires at around 125%, the K value chosen became 12. To check, do it with a sample wire size of aluminum.
Click to expand...

That makes no sense. If you have a continuous load, you're going to go with 125% anyway, you're not going to do it twice.
 
topgone said:
Those constants are made to simplify calculations, especially for non-electrical people. The key is understanding what 12 stands for.

Voltage drop is basically amps X ohms. To derive the multiplier K, try calculating the CM-ohm per foot of copper wire and you'll get a figure that will be very near to the figures being used in voltage drop calculations.

Ex. 4/0, Copper:
CM = 211600 circular mils; ohms/1000 feet = 0.049

feet per ohms = 20408.16;

Therefore K = 211,600/20408.16 = 10.4. But since we will oversize wires at around 125%, the K value chosen became 12. To check, do it with a sample wire size of aluminum.
Click to expand...

gadfly56 said:
That makes no sense. If you have a continuous load, you're going to go with 125% anyway, you're not going to do it twice.
Click to expand...
His calculation method is correct. His resistance value is incorrect. DC resistance of 4/0 copper is 0.0608 ohm/1000ft.

211,600 ? 0.0608 ohms/kft ? 1000 = 12.86528 ohms-cmil/ft

Also note the resistance value is for stranded. The resistance value for solid would be ~2.3% less.
 
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I think it is referred to Neher, McGrath-?Temperature and Load Capability of Cable Systems?-October 1957. Ch. CALCULATION OF D-C RESISTANCE
Rdc=12.9/CI for 100% IACS copper conductor 75 oC (10 A)
Rdc=21.2 for 61% IACS aluminum 75 oC (10 B).
 
Julius Right said:
I think it is referred to Neher, McGrath-?Temperature and Load Capability of Cable Systems?-October 1957. Ch. CALCULATION OF D-C RESISTANCE
Rdc=12.9/CI for 100% IACS copper conductor 75 oC (10 A)
Rdc=21.2 for 61% IACS aluminum 75 oC (10 B).
Click to expand...

Admit it, your secret dream growing up was to become a reference librarian :D. Good work sniffing those out.
 
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