# How much voltage drop is OK?

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. Voltage drops greater than this will require a more efficient type of plug or a replacement system breaker. The voltage drop across any single outlet or light fixture should not exceed 15% for safety reasons.

In other words, if you have 30 volts coming in, you can only draw 20 watts (5 volts x 16 amps). If you try to pull more than this, you'll need a better power supply unit (PSU). PSUs are available with built-in overload protectors that will shut off the power if you try to pull too much current from one outlet or light fixture. Before you buy such a PSU, however, be sure to check how much current it can handle because this may affect your choice of components. For example, some cheap PSUs on the market today can only handle a few hundred milliamperes of current before they shut down; this means that if you use these units to power several heavy appliances such as heaters, air conditioners, and vacuum cleaners, you might get burned out their batteries very quickly. More expensive units usually have higher capacity ratings so they can handle several amps for longer periods of time without shutting down.

## What is the maximum recommended voltage drop for a branch circuit?

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). This means that the maximum allowable voltage on any one conductor within the cable group should be no greater than 5% of the live voltage.

In other words, if the power company supplies 120 volts across its wires, then each conductor within the cable must be able to carry at least 8 volts. If any single conductor gets down to 4 volts, then it's time to replace the cable.

The voltage drop across wiring devices such as switches, receptacles, and fuse boxes is usually small relative to the total voltage drop across the entire system. Thus, it can be safe to work with lower voltages near these devices. However, if you are installing new wiring, then you should always use the highest-quality wiring devices available. These days, most homes are wired with aluminum wiring, which has a higher resistance than steel wire of equal size. As a result, if you use regular light switches, instead of molded-case circuits, the voltage will drop faster than expected, which may cause electric shocks.

The voltage drop across appliances such as heaters, air conditioners, dryers, and dishwashers can also be high.

## How big should a voltage drop wire be?

Frequently, voltage drop calculators would push you to increase wire size at a 3.01 percent drop—the lower size would compute to a 3.35 percent drop, which is completely good. 4.15 percent is OK. 5.03 percent is allowed. The rule is concerned with the overall voltage drop from the meter to the final outlet; it should not be allowed to exceed 8% unless there is a compelling justification. Voltage drops greater than 8% can cause problems for some appliances and equipment.

In general, cable needs to be large enough to carry the current you need while keeping resistance low. The more conductors in a cable, the less resistance there is between any two points inside the cable. Therefore, using multiple conductors in a cable is helpful when high currents need to be carried across long distances. Cable is usually specified by its maximum allowable temperature when exposed to continuous direct sunlight, called "sunrise to sunset rating." This is because electricity cannot be absorbed by metal objects, it must pass through the object to do work. If it gets too hot, it could cause damage or destroy sensitive electronics housed in other materials. The maximum safe temperature for copper is 105 degrees Fahrenheit; above this temperature, it will begin to lose its ability to conduct electricity.

Copper has the highest possible electrical resistance of any known material, so even a small amount of resistance can dramatically reduce the flow of current if the load is heavy enough.

## At what length should you worry about a voltage drop?

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 minutes before they need to be recharged. A voltage drop of 15% or more indicates that there is a problem with the supply and it should be taken to a technician for repair.

## What voltage drop is acceptable in the UK?

Maximum voltage drop values are allowed in BS7671 Table 4Ab I 3% for lights (6.9V) or 5% for other purposes (11.5V). These values may need to be adjusted depending on what mode you are operating your lamp in. For example, if you are using a dimmer switch then this will affect how much voltage is dropped across it each time you switch off a light. A 6.9V lamp in a case where 3% maximum voltage drop is specified will see its power reduced by 66%, while a 11.5V lamp in a case where 5% is specified will see its power reduced by 50%. In either case, this has the effect of reducing the lamp's lifetime.

In general, if you are not sure how much voltage you are dropping with certain operations then you should try and stay below this value. For example, if you are not sure whether your lamp needs to be replaced soon then you should use lamps that drop less voltage when switched off.

The voltage drop across a resistor is equal to IR, where R is the value of the resistor. So, if you want to keep voltage drops to an absolute minimum then you should choose resistors that produce as low a resistance as possible.

## What voltage drop indicates high resistance?

If a connection is excellent, there should be minimal or no voltage loss and a voltage drop of less than 0.4 volts for most connections, ideally less than 0.1 volts. However, a voltage drop of more than a few tenths of a volt across a connection indicates high resistance and the need for cleaning or repair. A voltage drop this large can cause damage to devices connected later in the line pool.

The voltage drop across a resistor is given by E = I * R, where E is the voltage lost in the resistor, I is the current through it and R is the resistance value. So, for example, if a 100-ohm resistor has a voltage drop of 1 volt when current 10 amps are flowing through it, then the resistance value is 100 ohms.

When measuring resistance with a multimeter, make sure that you are using the correct terminals. If you are not sure which ones they are, try both sets of wires from one device to another. The difference in resistance between good connections and bad connections can be very small, so be sure that you are reading the right number. It's also important to remember that measurement error will add some randomness to your results. So even if you're using the correct terminals, if two connections appear to be about the same resistance value when measured individually with a meter, they may in fact be different when used together as part of an entire cable system.

##### Steven Bitting

Steven Bitting has been working in the automotive industry for over 20 years. He started out as a parts delivery person, but quickly progressed to become a mechanic. Steven's always looking for ways to improve himself as an individual and as a mechanic, and he takes every opportunity that comes his way to learn more.

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