How to size electrical wire?
Here are the steps to follow when sizing electrical wire:
Determine the current load: You need to know the maximum amount of current that will flow through the wire. This information can usually be found in the specifications of the electrical device or system you are working with.
Calculate the wire gauge: Once you know the current load, you can use an electrical wire size calculator or a wire gauge chart to determine the appropriate wire size. The wire size is usually measured in American Wire Gauge (AWG) and is based on the diameter of the wire.
Consider voltage drop: Depending on the distance between the power source and the device or system you are working with, you may need to consider voltage drop. Voltage drop can cause a decrease in the voltage of the electrical supply, which can affect the performance of the device or system.
Choose the appropriate wire type: Once you know the wire gauge and voltage drop, you can choose the appropriate wire type. Common types of wire include copper and aluminum, which are both available in different grades and types.
Basic formula for sizing electrical wire:
The basic formula for sizing electrical wire is based on the amount of current that will be flowing through the wire and the maximum allowable voltage drop. Here is the formula:
Wire size (in circular mils) = (2 x Length of circuit x Current in amperes x Allowable voltage drop) / % of voltage drop / Specific resistivity of conductor
Where:
- Length of circuit: the length of the wire in feet or meters from the power source to the load.
- Current in amperes: the maximum amount of current that will flow through the wire.
- Allowable voltage drop: the maximum allowable voltage drop for the circuit. This value is usually specified by the manufacturer of the device or system you are working with.
- % of voltage drop: the percentage of the voltage drop allowed for the circuit. This value is usually 3% or less for residential circuits and 5% or less for commercial and industrial circuits.
- Specific resistivity of conductor: the resistance of the wire per unit length, which is determined by the material and size of the wire.
The result of this formula will give you the wire size in circular mils. Circular mils is a unit of measurement for the cross-sectional area of a wire. To convert the wire size from circular mils to the appropriate AWG wire gauge, you can use an electrical wire size chart.
How to calculate voltage drop?
To calculate voltage drop, following formula is used:
Voltage Drop = (Current x Resistance x Length) / 1000
Where:
- Current: the amount of current flowing through the wire, in amperes (A).
- Resistance: the resistance of the wire, in ohms per thousand feet (Ω/kft). This value can be found in a wire size chart or calculated based on the wire material and size.
- Length: the length of the wire, in feet (ft).
The result of this formula will give you the voltage drop in volts (V).
For example, let's say you have a 100-foot long wire carrying 10 amperes of current, and the wire has a resistance of 0.1 ohms per thousand feet. Using the formula, we can calculate the voltage drop as follows:
Voltage Drop = (10A x 0.1 Ω/kft x 100ft) / 1000 = 1V
This means that the voltage at the load end of the wire will be 1V less than the voltage at the power source end of the wire.
It's important to note that voltage drop should be kept within allowable limits to ensure proper operation of the electrical equipment or system. The maximum allowable voltage drop depends on the application and is typically specified by the manufacturer or the National Electric Code (NEC).