Voltage Drop Calculator |
The calculator below is intended to help estimate cable voltage drop so that you can correctly size you power wiring. You may, for example, need to get power (240 VAC, 50 Hz) from your main fuse box to your barn, that is 100 feet away from your house. And, you may need up to 30 Amps. So you want to make sure that the voltage drop, at full load, is not more than 5%.
The actual voltage drop may differ a little from the calculator's prediction, but will be very close, in most cases.
As an example, say I have one of those weed/grass trimmers that uses thin plastic line. The trimmer is corded and is rated at 240 VAC at 6.5 Amps. Obviously, to get to any distant areas, you might need a fairly long extension cord. For this example, say you need 100 feet. The question is, which extension is best? The #16, #14, or #12? The goal should be that you should not lose more than 5 percent of your voltage, measured at the trimmer. If you enter the appropriate information in the calculator, it will show that using a 100 foot #16 gauge extension, there will be a 5.4 percent voltage drop. That may be a little to much of a drop. But a #14 gauge extension will reduce that drop to 3.3 percent, keeping it withing the manufacturer's operating range. That extension should be sufficient up to about 150 feet. But for any distance greater than 150 feet, it would be prudent to use a #12 gauge extension. Or, get a battery operated trimmer.
Use this calculator to estimate cable voltage drop for sizing conductors. The calculation assumes uncoated copper or aluminum conductors operating at the temperature selected and is based on the ac/dc resistance or impedance from NEC 2005 Tables 8 and 9 for stranded conductors operating on a DC or AC 60Hz system. The ampacity of each conductor size in the dropdown menu below is based on NEC 2005 Table 310.16 for 60C insulated conductors rated 0 through 2000 volts with not more than three current carrying conductors in raceway, cable or earth with an ambient of 30C (86F).
Note that the actual ampacity and voltage drop for your application may differ from these results but in most cases will be very close to those shown here.
System Voltage: This should be the line-to-line voltage for multi-voltage and 3 phase systems. For a 120/240V single phase system select 240V. For other single phase systems select the line-to-neutral voltage.
Cable Size - These are uncoated copper or aluminum conductor sizes with CU/AL 60C Ampacity listed at 30C ambient for reference.
Conductor Temperature - NEC tables assume 75C operating temperature. If conductor is oversized a lower temperature can be used but this temperature should always be higher than ambient. This value only used for adjusting conductor resistance.
Parallel Runs - Examples for parallel runs: 120/240V single phase system with single black-red-white conductors (installed in single conduit) select "single set of conductors", 120/208V 3phase system with 2 conductors per phase and neutral (installed in 2 parallel conduits) select "2 conductors per phase in parallel", dc system with 3 positive and 3 negative conductors select "3 conductors per phase in parallel".
Cable Length - Length of cable in feet (one-way distance)
Load Current - Load current in amperes