What is the maximum allowable voltage drop? According to a footnote in the National Electrical Code (NEC 210-19 FPN No. 4), a voltage drop of 5% at the furthest receptacle in a branch wiring circuit is permissible for normal efficiency. It also signifies that the resistance of the circuit is less than 0.4 ohms. This would indicate that a 15-amp circuit could be used instead of the required 20 amps.
The NEC allows for some variance in practice. For example, a voltage drop of 10% or greater between the main line and the farthest outlet should be prevented to minimize risk of fire. In other words, a 20-amp circuit should not drop more than 2 volts from main to farthest outlet. However, this does not mean that a 16-amp circuit will meet the requirement. The code allows for some variance based on location and practice within an industry or organization.
In general, the closer you get to the source, the higher the voltage will be. As it passes through each device along the way, some voltage will be lost in resistors and other such items. If the voltage drop across any one device exceeds 10%, then you will need to replace it with one that can handle the load. For example, if your circuit requires 20 amps but the voltage drop is greater than 10%, then you will need to replace the receptacles on the circuit with ones that can handle at least 30 amps.
Five percent The NEC advises that the maximum total voltage drop for both the feeder and branch circuit not exceed 5%, and that the maximum voltage drop for either the feeder or branch circuit not exceed 3%. (Fig. 1). If the voltage drop on a feeder exceeds 5% but does not exceed 8%, it is considered a minor disturbance; if it exceeds 8% but does not exceed 15%, it is considered a moderate disturbance; if it exceeds 15% but does not exceed 20%, it is considered a major disturbance.
In addition to the voltage drop, there are other factors that can affect the distribution system's ability to carry current from source to destination. For example, the impedance of the load may cause problems if it amounts to more than 10% of the full-load resistance of an aluminum conductor. An inductive load such as that found in many motors will cause voltage drops throughout the network. In this case, voltage changes will occur as the motor starts and stops operating.
Voltage fluctuations caused by loads that are less than full capacity must also be taken into account when calculating the voltage drop across transmission lines. For example, if a three-phase 240-volt load averages only 120 volts during any given cycle, then the voltage drop for that line is 40%. This means that the remaining 160 volts has to make its way through two lines instead of one.
Under fully loaded conditions, it is suggested that the voltage loss be less than 5%. However, most power supplies only operate at this level for several seconds before they must switch to an alternative source of power. For example, if the battery being charged drops below 14.8 V, the power supply will automatically shut off to prevent damage to itself or your equipment.
The voltage across any load represents lost energy, so keeping the voltage across any given load as low as possible is desirable for efficiency reasons. In addition, large loads can cause voltage drops in nearby wiring or devices which may cause them to fail prematurely. For example, if a current-limiting resistor is placed in series with a motor load to protect other equipment from damaging currents, then the voltage dropped across this resistor will cause its early failure.
Finally, excessive voltage drops can signal an electrical problem requiring repair before it causes more damage. For example, if a fuse blows due to a high current draw, this indicates a need to replace the fuse or possibly the circuit breaker handling the load.
In conclusion, electrical power losses are inevitable, but minimizing them helps preserve both energy and equipment life.
In a lighting circuit, the allowed voltage drop is 2% of the supply voltage plus one volt. The maximum allowable voltage drop in a power industrial circuit shall not exceed 5% of the specified supply voltage. Any wire installation's insulation resistance should not be less than 1 M Oh.m at 120 V.
The rule applies to all wiring methods (continuous and cut back), whether inside or outside walls, covered or exposed conduits, metal or non-metal.
If the supply voltage exceeds 120 volts, then the permitted voltage drop will be twice that amount (240 volts). If the supply voltage is less than 120 volts, then the permitted voltage drop will be equal to 5% of the supply voltage (12 volts).
These are the highest limits for residential installations. For other applications such as factories, warehouses, schools, hospitals etc., higher voltage allowances apply. Consult an electrician if you need more voltage across your house wiring system.
It is important to note that the rule does not specify a maximum value for wiring insulation resistance. Unless otherwise specified by a code requirement, it is recommended that you allow 10 MOhms of resistance for house wiring. This will provide adequate safety factors for future needs when adding lights, appliances, etc. Installations with lower values may present a risk of electrical shock.
The wiring method also affects how much voltage we can take on each conductor.