It might have been some time since you learned about voltage drop, so what are the requirements and the calculations? Chris Halliday explains.
The voltage in a circuit will drop due to the resistance and reactance of cables.
Designers and installers must consider voltage drop to ensure safety and proper operation of the equipment, ie: adequate voltage must be supplied.
If there is too much voltage drop the equipment won’t work, or safety will be compromised – such as a motor overheating and catching fire.
We therefore need to consider voltage drop before running cables and connecting loads. A larger cable than intended may be needed to ensure compliance with voltage drop requirements. Considering the voltage drop issue at the design stage of an installation will help avoid problems.
The Wiring Rules – AS/NZS30000, Section 2 at Clause 3.6 – provides the relevant detail for low-voltage installations. Clause 7.3 covers stand-alone systems and Clause 7.5.7 covers extra-low-voltage installations.
All sparkies should be familiar with these requirements.
Low voltage
Clause 3.6.1 discusses having the voltage for a piece of equipment higher than the lower limit of the Standard for that equipment.
The clause does allow for equipment not covered by a Standard to function safely at lower voltage, but it does not say this negates other sub-clauses in Section 3.6.
Conductors shall be sized to ensure that the voltage at any point in an installation does not drop more than 5% from the nominal (refer Clause 3.6.2). This equates to 11.5V for a 230V system and 20V for a 400V system.
The clause does not consider transient currents such as motor starting, solenoid closing and other similar applications.
The permissible voltage drop can be 7% if the supply comes from the low-voltage terminals of a substation on the premises and is dedicated to the installation (refer exceptions for Clause 3.6.2).
The voltage drop limits just mentioned are the total voltage drops for an installation. Voltage drops in the consumer mains, sub-mains and final sub-circuits must be tallied to ensure compliance with the limits.
Most of the voltage drop could be allocated to the final sub-circuit if there are no sub-mains and very short consumer mains. Likewise, long consumer mains and sub-mains will severely limit the available voltage drop in the final sub-circuit.
The old Australian Standards handbook HB301, for designing to the Wiring Rules, states that 2% to 3% is considered reasonable for the final sub-circuits, but it also discusses rearranging percentages for a satisfactory outcome.
Prudent placement of main switchboards and sub-boards will help to ensure that voltage drop is not an issue. This is a design matter and cannot be left to chance.
Stand-alone systems
Stand-alone systems, in accordance with Clause 7.3, are required to have a voltage drop of no less than 11% from the nominal.
This must take account of the output voltage of the source and the drop within the installation under normal operating conditions (refer exceptions for Clause 3.6.2).
It is really the same voltage-drop limit as for a grid-connected system, as the network company can have a drop of 6% and the installation a further 5%.
Extra-low voltage
A maximum voltage drop of 10% for extra-low-voltage installations, when conductors are carrying the circuit-operating current, is applied by Clause 7.5.7.
That is, unless the extra-low-voltage equipment (not exceeding 50V AC or 120V ripple-free DC) is specially designed to operate over a wider voltage range.
The 10% limit excludes transient currents caused by such things as motor starting, solenoid closing and other similar events.
Calculating voltage drop
Two methods are generally used for calculating voltage drop, as provided by AS/NZS3008.1.1, and both use a value of current in the calculation.
Clause 3.6.2 of AS/NZS3000 provides guidance on what value of current should be used in calculations and should not exceed:
- total circuit current;
- circuit maximum demand; or
- circuit protective device rating.
For circuits such as lighting and socket outlets, where the load is distributed over the circuit length, half the current rating of the protective device may be used for calculations (refer exceptions for Clause 3.6.2).
The first method uses conductor impedance, length and current for the calculation. This is slightly complex, as the resistance and reactance of the cable must be known to allow calculation of the cable impedance (refer Clause 4.3 of AS/NZS3008.1.1).
The resistance and reactance values for various cable types are provided by Tables 30-39 of AS/NZ3008.1.1. For smaller cables the reactance will have little effect on the total impedance and so can often be ignored.
Ohm’s law is used for this method as follows:
Vd = I x Zc
Where
Vd = voltage drop in volts
I = circuit current in amps
Zc = impedance of the circuit
Most sparkies will be more familiar with the second method:
Vd = (L x I x Vc)/1000
Where
Vd = voltage drop in volts
L = route length of the circuit in metres
I = circuit current in amps
Vc = cable voltage drop per ampere-metre length of circuit in millivolts per ampere-metre (mV/Am)
The value of Vc is provided by Tables 40-51 of AS/NZS3008.1.1.
The formula can also be rearranged to calculate Vc then look up a cable with a mV/Am value less than the value specified (not equal to, as there is no built-in margin for this simplified method).
Tables 40-51 are for three-phase circuits. Single-phase values of Vc require the three-phase values to be multiplied by 1.155. Don’t get caught by this little trap for young players.
The value of Vc is based on the maximum conductor temperature permitted by the cable insulation material. Allowance can be made if the cable operating temperature is considerably less than the maximum (refer Clause 4.4 of AS/NZS3008.1.1) and for differences in power factor.
The values of voltage drop calculated can be converted to a percentage by multiplying by 100 then dividing by 230 (for a 230V system).
The Wiring Rules has a simplified version of the latter method at Table C7. You simply need to multiply the length in metres by the current in amps and divide by the allowable voltage drop percentage, then find the size of cable with a value more than that provided by the Table C7.
Examples
A sparkie wants to connect a normal 16A single-phase power circuit using standard power cable – 2.5mm² twin and earth, PVC and PVC thermoplastic sheathed.
Voltage drop in the consumer mains and sub-mains has been calculated, and 3% is left of the allowable 5% for the circuit. The final socket outlet has a route length of 40m.
A 3% voltage drop = 0.03 x 230 = 6.9V (this value must not be exceeded).
The current for the calculations will be 8A, as the socket outlets will be distributed along the circuit (refer Clause 3.6.2 exception 1 of the Wiring Rules).
The impedance method:
The resistance of the cable is found by using Table 35 of AS/NZS3008.1.1 and the 75°C value, ie: 9.01Ω/km.
Vd = I Zc
= 8 x 9.01/1000 x 40 x 2 (don’t forget there are two conductors, the active and the neutral return).
= 5.8V. This value passes the 3%/6.9V voltage drop requirement set by the installation requirements.
The mV/Am method
The Vc value is found for the 2.5mm² cable using Table 42 of AS/NZS3008.1.1 and the 75°C value, ie: 15.6mV/Am
Vd = (L x I x Vc)/1000
= (40 x 8 x 15.6 x 1.155)/1000 (don’t forget the 1.155, as the mV/Am value in the table is for a three-phase circuit)
= 5.8V
Both methods align – we must know what we are doing! Again this passes the 3% requirement.
The simplified mV/Am method
Am/%V = (L x I)/%Vd
= (40 x 8)/3
= 106.7 Am/%V
Looking up Table C7 and selecting the cable with a value not less than 106.7 Am/%V, we find that 2.5mm² is the minimum size standard cable we can use for this project. So, we come up with the same result – it’s 2.5mm² cable.
Other considerations
The Wiring Rules provides Table B1 for maximum route lengths for circuits based on loop impedance.
This does not consider voltage drop, so I have created Table 1 to compare maximum route lengths for loop impedance with voltage drop in the same circuits.
I have also created Table 2, which converts the values of loop impedance to voltage drop.
A comparison of both tables shows that, in most cases, voltage drop requirements are often more stringent than loop impedance requirements. Using loop impedance route lengths or impedance values in isolation is likely to result in non-compliance with the Wiring Rules for voltage drop.
Conclusion
Voltage drop must be considered when designing and installing electrical wiring.
Compliance with voltage drop requirements will be achieved mainly by installing a large enough cable.
Total voltage drop in a low-voltage installation is usually 5%. However, this can be 7% if the supply comes from the low-voltage terminals of a substation on the premises and is dedicated to the installation.
The allowable voltage drop must be split between the consumer mains, sub-mains and final sub-circuits to achieve an overall compliant outcome.
Maximum route lengths and fault loop impedance requirements do not generally ensure compliance with voltage drop rules.
FAQs
How much voltage drop is acceptable? ›
How Much Voltage Drop is Acceptable? The National Electrical Code says that a voltage drop of 5% at the furthest receptacle in a branch wiring circuit is passable for normal efficiency.
How much voltage drop is acceptable NEC? ›What is the minimum NEC-recommended operating voltage for a 115V load connected to a 120V source (Fig. 2)? The maximum conductor voltage drop recommended for both the feeder and branch circuit is 5% of the voltage source (120V). The total conductor voltage drop (feeder and branch circuit) shouldn't exceed 120V×0.05=6V.
What is the maximum allowable voltage drop in a cable? ›The maximum combined voltage drop on both installed feeder conductors and branch circuit conductors to the farthest connected load or outlet must not exceed five percent. This is the steady-state voltage drop under normal load conditions.
What is permissible standard voltage drop between power supply terminal and installation? ›The voltage drop between the origin of the installation (usually the supply terminal) and the fixed current-using equipment should not exceed 4 percent of the normal voltage of the supply.
What is acceptable voltage drop for 240v? ›240 Volt, Conductor Size (AWG or kcmil) Single Phase, Max 3% Voltage Drop*
How much voltage drop is acceptable through switches and wiring? ›The recommended working limit is 0.10V-drop across low-current wires and switches.
What is the voltage drop specification for wires? ›The voltage drop between the power and ground side of a particular circuit will generally be 0.1 volts or less. Expect a voltage drop of 0.2 volts or less from one end of a particular copper wire or cable to the other. A switch will usually create a voltage drop of 0.3 volts or less.
What is excessive voltage drop? ›Excessive voltage drop is caused by things like loose connections, corrosion, or sometimes by wire used in a circuit that is too small a wire gauge to handle the amount of amperage the appliance in the circuit needs to function properly.
What happens if there is too much voltage drop? ›Excessive voltage drop in a circuit can cause lights to flicker or burn dimly, heaters to heat poorly, and motors to run hotter than normal and burn out. This condition causes the load to work harder with less voltage pushing the current.
What is an acceptable voltage drop motor starting? ›As the motors are connected to different buses of the electrical system, they can be properly run up to 15% voltage drop (that is less than the standard value of 20%), the voltage drop on different voltage levels buses of the electrical system caused by starting motor and the other loads must be less than 15%.
What is an acceptable voltage drop for 24v? ›
A general guideline is that a 3 to 4% voltage drop is acceptable and will still allow the LED to perform normally.
What is the DC voltage drop rule? ›What is the 2% voltage drop rule? In the solar industry lexicon, 2% voltage drop has been known to system integrators as a hard rule that, when sizing conductors, the DC voltage drop should be limited to no higher than 2%.
What is the voltage drop for 120 240 panel? ›3. Measure the voltage at the service panel board with the problem circuit. It should be 227 volts or more between hot conductors and 113.5 volts or more between hot and neutral of a 120/240-volt, single phase system (maximum of 3% voltage drop on feeders, 2% maximum recommended).
What is the maximum permissible voltage drop in volts on a 277 volt circuit? ›On a 277 volt line to neutral circuit, the allowed branch circuit voltage drop is 3%, or (277*. 03) = 8.31 volts.
What is allowable voltage variation? ›At the electrical service, the electric utility should attempt to keep the voltage ±5% of nominal. The electrician needs to be aware of the acceptable electric utility fluctuation and plan to keep the voltage at the point of utilization between +5/-10% of nominal.
How far can wire go before voltage drop? ›...
For 120-volt circuits:
| 14 AWG | 50 feet |
|---|---|
| 12 AWG | 60 feet |
| 10 AWG | 64 feet |
| 8 AWG | 76 feet |
| 6 AWG | 94 feet |
Conductors in electrical systems should not be sized with voltage drops exceeding 3%. For a 12V system the maximum voltage drop should be less than (12 V) x 3% = 0.36 V.
How do you calculate voltage drop wire length? ›By dividing the paired wire length by 100, we get the factor by which we need to multiply voltage drop per 100 feet to determine total voltage drop. Therefore, 350 feet divided by 100 equals 3.5. Multiply 3.5 by 1.27 volts drop per 100 feet to get your total voltage drop.
Do I need to worry about voltage drop? ›Excessive voltage drop in a circuit can cause lights to flicker or burn dimly, heaters to heat poorly, and motors to run hotter than normal and burn out. It is recommended that the voltage drop should be less than 5% under a fully loaded condition.
Does voltage drop really matter? ›“If too much voltage is lost, the equipment may not function correctly or may not work at all. For some equipment, such as devices that employ compressors, large motors, or pumps, even a small loss of voltage may cause a problem.”
Does voltage drop affect amps? ›
Motors require a certain output in watts in order to function, and this is the result of a voltage multiplied by amps. So when voltage falls, resistance scales up in order to supply the difference, and the motor will also draw more current.
What is voltage drop for dummies? ›Wires carrying current always have inherent resistance, or impedance, to current flow. Voltage drop is defined as the amount of voltage loss that occurs through all or part of a circuit due to impedance. A common analogy used to explain voltage, current and voltage drop is a garden hose.
Why does voltage drop when load is high? ›As load current increases, the voltage drop in the wiring increases and the voltage delivered to the system drops. The traditional approach to solving this problem, remote sensing, regulates the voltage at the load, increasing the power supply voltage to compensate for voltage drops in the wiring.
What is a good rating for a voltage drop test on a starter? ›With a good connection, there should be no drop, or at least very little (under 0.4 volts usually, and ideally under 0.1 volts). If the drop is more than a few tenths, then there's too much resistance, the connection will have to be cleaned or repaired.
What is an acceptable voltage drop 230v? ›Clause 1.6. 2 in AS/NZS 3000:2018 refers to a nominal voltage of 230 volts plus 10% or minus 6%, which ranges between 216.2 and 253 volts. On standard installations the maximum permissible volt drop between the point of supply and any point of the installation is 5%, as specified in clause 3.6.
At what percent of voltage unbalance Should the motor not be run? ›Voltage Unbalance Standards
The American National Standard for Electric Power Systems and Equipment ANSI C84. 1 recommends that “electric supply systems should be designed and operated to limit the maximum voltage unbalance to 3 percent when measured at the electric-utility revenue meter under no-load conditions.”
With 12 guage wire, that works out to just under 57 feet with a full load of 20 amps. If you allow for 16 amps (the maximum CONTINUOUS loading permitted on a 20 amp circuit), you can go just shy of 71 feet.
What is the maximum voltage drop permitted by the CEC? ›The maximum total voltage drop for a combination of both branch circuit and feeder should not exceed 5%.
What size wire do I need to run 50 amps 100 feet? ›In most cases, 6 AWG is an almost perfectly-sized wire for a 50 amp breaker. In limited cases, you will probably have to use a larger 4 AWG wire. That's when you have a long circuit and are sending electrical current at some distance (100 feet or more).
What does the NEC say about voltage drop? ›This NEC article limits the voltage drop on any branch circuit serving sensitive electronic equipment to 1.5% of the applied voltage. Alternatively, the maximum combined voltage drops on the feeder and branch circuits going to sensitive electronic equipment should be limited to 2.5%.
Is voltage drop worse with AC or DC? ›
So--the AC wiring will have more voltage drop because there is more current being passed (by a factor of 1/0.6=1.67 times in this example).
What is considered very low voltage? ›What is low voltage (LV) electricity? Low voltage (LV) electricity means electrical energy at voltages exceeding 32 Volt AC or 115 Volt DC, but not exceeding 1000 Volt AC or 1500 Volt DC for mines as per MSIR 1995.
What is the formula for voltage drop? ›Voltage drop of the circuit conductors can be determined by multiplying the current of the circuit by the total resistance of the circuit conductors: VD = I x R.
What is the formula for voltage drop finding wire size? ›Equation 2: Calculating the Wire Size in circular mils [CM = 2 x K x L x Amps/Acceptable Voltage Drop]. Alternatively, you can algebraically manipulate Equation 1 to: Acceptable Voltage Drop ÷ 1.732 x L x Amps and then look up the wire size according to its AC resistance.
What size wire for 50 amps at 200 feet? ›For a maximum of 50 amps, you'll need a wire gauge of 6. Fifty amp breakers are most often used to power many different appliances.
What is considered low voltage according to the NEC? ›“3.1 Low Voltage (LV): A class of nominal system voltages 1,000V or less. 3.2 Medium Voltage (MV): A class of nominal system voltages greater than 1,000V and less than 100kV. 3.3 High Voltage (HV): A class of nominal system voltages equal to or greater than 100kV and equal to or less than 230kV.
What is the lowest voltage that can be fatal? ›It is sometimes suggested that human lethality is most common with alternating current at 100–250 volts; however, death has occurred below this range, with supplies as low as 42 volts.
What is the standard voltage tolerance? ›The tolerance level defined by IEC60947-2 is 0.85% for minimum nominal voltage (Un) and 1.1% for maximum nominal voltage (Un). Example: For Un = 230/240V, Minimum tolerance Level is (0.85% of 230V AC) = 195.5V, Maximum tolerance Level is (1.1% of 240V AC) = 264 VAC.
How far can you run 12 gauge wire without a voltage drop? ›You can run a 12-gauge (12 AWG) wire about 100 feet without experiencing any voltage drop as long as the total wattage of the fixtures connected to it is 100 Watts or less.
How much voltage drop is too much 12v? ›The acceptable maximum voltage drop for DC loads is 5% of nominal battery voltage. The chart and the formula on this page are provided to help you in selecting the best cost / power loss compromise. NOTE: The Cable Length in the above tables are route length which is half the total conductor length.