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How do you compute voltage drop and cable size over distance?

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βˆ™ 2014-07-28 15:00:03

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The voltage drop in a cable is the current multiplied by the resistance. For a twin-wire cable the resistances of the two wires are added. Cable resistance for different sizes of cables can be looked up in wire tables.

More detailed answer for USA, Canada and countries running a 60 Hz supply service.

Voltage drop and cable size can be calculated by looking at tables that are available in the National Electrical Code. Some of these tables are difficult to understand and will require you to seek assistance. You can also determine the amount of voltage drop over a distance by finding the resistance of the cable for a specific temperature(given in ohms/1000ft)from the cable manufacturer or electrical wholesaler. If you know the largest amount of current that will flow in the cable, then use the formula:

Vdrop = Current X Distance(Ft) X 2 X Ohms per 1000Ft x 0.001

[Note: the 0.001 fixes an earlier error in the formula where the contributor didn't divide out 1000 ft by using R/1000ft!]


Voltage drop is a complicated calculation done by electricians. I don't know what you're trying to run but let me just give you the rule of thumb. If you run out over 200 feet of wire for the job in hand, you should jump up a wire size in order to have sufficient voltage at the other end. For example. Let's say you are going to wire and out building for your workshop and you need only 120 volt receptacles at the shop end to run your power tools. Power tools run at the nominal voltage of 120 volts, but they operate within a range, say 108 to 128. This allows for how close your are to your distribution companies source. I don't know what your starting voltage is so that's why the rule of thumb. To your outbuilding you only need #12 to carry 20 amps for your tools and lights etc. If you have to exceed that distance, then you would jump up to #10 in order to have sufficient voltage at the other end. If you are starting at 115 volts at your house, and are running 100 feet, I would still up the wire size. You can't go wrong by going one size bigger on your wire. When in doubt, always consult a qualified electrician.


Many electricians wire for no more than 5% voltage drop. This is usually fine since appliances and power tools are made to operate over a range of voltages. For a more expensive way to wire a house, you can try wiring for no more than 2% voltage drop. In this case, if you are running 20 amps, you need to up the wire size to #10 if you go more than 35 feet. In general, the percentage voltage drop decreases with a higher starting voltage. To see this, play around with the formula for voltage drop given in terms of resistance. The resistance of various sizes of wires can be found in the CRC handbook. Practically, this means you lose less power to voltage drop if you choose a 240 volt appliance instead of a 120 volt one. And you lose a lot with low voltage lighting running on 10 or 12 volts.


The formula for voltage drop is Vd=KIL/CSA where Vd stands for voltage drop, K stands for the resistivity factor of the material [I think copper is around 7 and aluminum 11], I is the current in Amperes, L is the round trip length of conductor, CSA is cross-sectional area of the conductor in circular mils

You can get the CSA values from chapter 9 table 8 of the NEC

CSA for 14 is 4,110; 10 is 10,380; 8 is 16,510; 6 is 26,240


As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.




2014-07-28 15:00:03
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Q: How do you compute voltage drop and cable size over distance?
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Related Questions

Will voltage drop over cable distance?

If the voltage is supplying any current through the cable, i.e. if there is any 'load' at the end, then the voltage will drop through the cable.

How do you compute voltage drop and cable size over distance in israel?

You can find it out from wire tables. It depends on the current in amps and the material used (copper/aluminium), but not on which country you are in. Try searching for wire tables and look for voltage drop per 100 metres or such.

What effect does it have on the voltage drop in an electrical cable if the resistance of the cable is decreased?

It will decrease the voltage drop.

What are formula to calculate cable size and voltage drop?

The formula to calculate cable size and voltage drop is Vdrop = current x distance times 2 times Ohms per 1000ft times 0.001.

Why drop voltage in xlpe cable is more than PVC cable?

The voltage drop in a wire has nothing to do with the insulation. Voltage drop has to do with the cross sectional area of the wire.

How do you compute the voltage drop over a 500 Meter underground Teck cable if you have a 230 115 Volt single phase system?

You have to know the connected load amperage is at the end of the cable to do the calculation.

How do you find cross sectional area of cable when you have the lent current and the voltage drop?

cross sectional area of cable * voltage drop

How do you compute the voltage drop over a 500-meter underground cable?

You need the voltage drop per meter, from data on the cable, and then multiply by 500. Alternatively find the resistance per metre of a single cable of the same wire size, from wire tables, then multiply by 2 x 500 x resistance (in ohms) x current (in amps).

What is voltage drop when utilized with cable runs?

A: ANY CABLE will have a definite impedance and if current flow there is going to be a definite voltage drop because of it

Is there a rule to distance of cable from a power source to low voltage lighting without voltage drop and do you have to put transformers or outlets in the yard?

Voltage drop depends on the size &amp; length of the wire and the wattage of the bulbs. If they make an LED replacement bulb , you would hardly notice the voltage drop.

How do you compute voltage drop?

The answer will depend on many factors such as:distance between the two points for which a voltage drop is to be calculated i.e the length of the cable runwire size and material (copper, aluminum...)type of insulation used in the cabletype of cable fixing (in the open air, along a wall, in a duct, underground, etc.)expected ambient temperature at full loadFor more information please see the Related Link and the answers to the Related Questions shown below.

How do you compute the voltage drop over a 500 meter underground cable if you have a 440 volt three phase system?

You need two more pieces of information: (i) what is the current being drawn, or the load in kVA, (ii) how thick is the cable. Then look up the wire resistance in ohms per meter for the cable, multiply by the length (500) and multiply by the current, then that is the voltage drop.

What is the voltage drop for 16 mm Cable over 100 meters?

The voltage drop depends on the current through the cable.For DC current in cable of 16 mm diameter, at 68&Acirc;&deg; F, the voltage drop is(0.00857) x (current, Amperes) volts.

What is the formula for calculating the voltage drop across the cable?

The formula for a simple DC voltage drop across a cable is:VDrop = Vmeasured at the input of cable - Vmeasured at the output of cableThis formula may seem simplistic however keep in mind that the sensitivity, accuracy and resolution of measuring instrument instrument is what is really important.If you know the DC current flowing through the cable and the impedance of the cable then you can use Ohm's law. Vdrop = I R where Vdrop is the voltage drop across the cable in volts I is the current flowing through the cable in amperes R is the resistance of the cable in ohms.ADDITIONALCalculation of the voltage drop is given by the formula belowVoltage Drop (Volt) = [(mV/Am)/1000] * I * lWhere mV/Am = millivolt drop per meter per ampere of the cable(this information is given in the table based on IEE Wiring Regulation)I = Current in the cable (in Ampere)l = Distance of cable (in Metre)See related links below

What is the formula for calculating voltage drop?

Since the voltage drop occurs across both conductors, we must take the resistance of both conductors in the cable into account. So, if the resistance of one conductor is R, then the total voltage drop along that cable will be:Voltage drop = I x I x 2R

When is a voltage drop calculation considered when determining the cable size?

The voltage drop calculation is considered when you know the size of the load (in amperes) to be supplied and the distance the cable will need to run. In Canada the volt drop for a branch circuit cannot exceed 3 percent. Always consult a professional when installing electrical equipment.

How do you compute voltage drop and cable size over distance in the UK?

If you are referring to residential wiring, then refer to the appropriate tables in BS 7671:2008 Requirements for Electrical Installations (the IEE Wiring Regulations). It's too complicated to be answered in this forum and requires prerequesite knowledge you may not have.

What is a voltage drop?

It's when there is a supply with a given voltage, and then a piece of equipment is fed through a long piece of cable so that the voltage supplied at the equpiment is somewhat less. The voltage drop is the amount of reduction caused by the resistance of the cable. Voltage drop should be less than 5% of the nominal voltage, e.g. 12 v for a 240 v system.

What size cable to run 30 meters to power 15amp motor?

To do a voltage drop and cable size calculation, need to know what the voltage is.

Is it okay to use thicker cable to avoid voltage drop?

It is mandatory to use a larger wire size to overcome voltage drop at the load.

What is the advantage of using higher voltage as opposed to lower voltage when operating heavy electrical loads?

For appliances that need a lot of power, the advantage of a higher voltage is that less current is needed, because power = voltage x current. That means that the cable doesn't need to be so thick. It also means that for a given percentage of voltage drop in the cable, the allowable volt-drop in the cable is higher when the supply voltage is higher. This also allows the use of a thinner cable especially when the cable is 100 ft long or more.

How do you calculate the cable size for a run of 30 meters?

to calculate the cable size of a run of 30 meters long you first will have to know the current of the appliance use the voltage drop formula V d = (mVxIxL)/1000 once the voltage drop is less than 2.5% of the nominal voltage, the cable should be upsize.

What will the voltage drop on a 1.5 mm cable run 1.5 kilometres be?

This is a voltage drop question. The amperage of the circuit must be given. Without the load amperage this question can not be answered.

What size cable for 37.5 kw 3phase motor?

Incomplete data: To have an accurate result - please provide the following; 1. Voltage Rating - Low Votage or Medium Voltage Applications? 2. Efficiency 3. Power Factor 4. Distance - One way distance from source 5. Location and method and arrangement of Installation 6. Allowable or acceptable voltage drop 7. Type of cable - PVC or XLPE cable with same size has different ampacity

What size of armoured cable to run 400 m to carry 5kw?

This is a voltage drop question. To answer is question the voltage must be stated.