Does anyone know how to figure when to go to the next wire size when running wire over a long distance? Thanks.

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I need to know total feet of wire 1 way and what you are powering...Total amp draw on circuit.

You need to run UL cable. This cable requires no conduit. There is no formula, only a process to which I need to know this information. I will check back

You need to run UL cable. This cable requires no conduit. There is no formula, only a process to which I need to know this information. I will check back

There is a formula for Voltage drop. 2KIL/cm Where K is a constant for the type of wire being used (copper is 10.4) I is the amperage draw of the device at full voltage rating. L is the length of the wire one way. and cm is the circular mil area for the wire. If you rearrange the formula you get cm=2KIL/Vd where Vd is voltage drop. The N.E.C. allows for a 3% Vd. So if you have a 120v circuit with a 3% Vd the allowable Vd. is 3.6. If your device pulls 16 amps and is 300 ft away from your panel then your equation will look like this, cm=(2x10.4x16x300)/3.6volts which =27733.33cm If you check the N.e.c. chapter 9 table 8 the closest wire size to this circular mil area is #4 (always go to the next larger wire size). This is only an example. If you give me some idea of what you need to do i'll try to give you a better idea of what size wire to use.

The "Ugly's Electrical reference Book" shows a Voltage drop Equation with a K value of 12.9@75*C. The value I was using was for a temperature of 20*C from "Delmars Standard textbook of Electricity". It also states that for each degree rise in Celcius the K value will increase by .0039. Taking into consideration an increase of 55 degrees the value would increase by .2145 for a new value of 10.61. Why would these numbers very by so much from two different references? Hollywood please provide some insight if you know. I am very curious as to which is more accurate. Will check back later.

Thanks for responding. What I am putting in is a condensing unit and evap for a walk-in box. It's a 220 volt system 1 hp compressor, other than that that is all I know at the present time. The location is a 2 1/2 hour drive from me.

The condensing unit will probably have a amperage rating on it. It may also have information on it regarding overload protection. Be sure to inspect the evaporator fans to see what amperage they pull. You will need a circuit large enough for both. I think you should calculate the total overload protection by taking 125% of the full load current of the highest rated motor and add it to 100% of the other load. Then choose the closest overload protection device available, going to the next highest one. Size your wire according to the overload protection using the N.E.C. codebook. I doubt it will be too far away from the Service to really need to worry about Voltage drop, but if it is then refer to the previous replies. Use the overcurrent device rating as the full load draw when making those calculations and you will be alright. Another thing necessary is a service disconnect located near the unit you are installing. If it is subject to the elements then it(the service disconnect) will need to be a NEMA 3R enclosure(raintight). If you have any doubts about what you are doing then please call a local electrician to do the work.