n8586m
Member
While I am just sitting here waiting for spring so that I can empty out my storage building which will allow me to take delivery of the new tractor I have picked out, I have seen several threads asking about wire size to run for added circuits. It appears that there is some confusion on just how to arrive at the correct wire size. Maybe I can shed some light on how to find the right wire size.
We will use the following formula:
This method is a bit more involved that the Ohms Law method, but the big advantage is you can use it for three-phase or single-phase. Here are some additional items to observe:
* Single-phase VD = 2 x K x I x D/CM.
* Three-phase VD = 1.732 x K x I x D/CM. The difference between this and the single phase formula is you replace the 2 with 1.732.
* K = Direct-Current Constant. K represents the dc resistance for a 1,000-circular mils conductor that is 1,000 ft long, at an operating temperature of 75コC. K is 12.9 ohms for copper and 21.2 ohms for aluminum.
* I = Amperes: The load in amperes at 100% (not at 125% for motors or continuous loads).
* D = Distance: The distance the load is from the power supply. When calculating conductor distance, use the length of the conductor溶ot the distance between the equipment connected by the conductor. To arrive at this length, add distance along the raceway route to the amount of wire sticking out at each end. An approximation is good enough. Where we specify distances here, we are referring to the conductor length.
* CM = Circular-Mils: The circular mils of the circuit conductor as listed in NEC Chapter 9, Table 8. I will include some standard sizes later.
Now we all remember 8th grade algebra so we can cross multply the formula and solve for the CM or wire size by picking the voltage drop we can stand for the application, usually not more than 5%.
CM= 2 x K x I x D/VD single phase, CM= 1.732 x K x I x D/VD three phase
(continued)
We will use the following formula:
This method is a bit more involved that the Ohms Law method, but the big advantage is you can use it for three-phase or single-phase. Here are some additional items to observe:
* Single-phase VD = 2 x K x I x D/CM.
* Three-phase VD = 1.732 x K x I x D/CM. The difference between this and the single phase formula is you replace the 2 with 1.732.
* K = Direct-Current Constant. K represents the dc resistance for a 1,000-circular mils conductor that is 1,000 ft long, at an operating temperature of 75コC. K is 12.9 ohms for copper and 21.2 ohms for aluminum.
* I = Amperes: The load in amperes at 100% (not at 125% for motors or continuous loads).
* D = Distance: The distance the load is from the power supply. When calculating conductor distance, use the length of the conductor溶ot the distance between the equipment connected by the conductor. To arrive at this length, add distance along the raceway route to the amount of wire sticking out at each end. An approximation is good enough. Where we specify distances here, we are referring to the conductor length.
* CM = Circular-Mils: The circular mils of the circuit conductor as listed in NEC Chapter 9, Table 8. I will include some standard sizes later.
Now we all remember 8th grade algebra so we can cross multply the formula and solve for the CM or wire size by picking the voltage drop we can stand for the application, usually not more than 5%.
CM= 2 x K x I x D/VD single phase, CM= 1.732 x K x I x D/VD three phase
(continued)