Page last updated Mar 29, 2012 @ 01:14am

Voltage Drop Calculator - mini system

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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 resistance or impedance from NEC 2005 Table 9 for stranded conductors operating on an 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.

Units herein are American Wire Gauge (AWG) and English (feet).

Note, to run this calculator JavaScripts must be enabled on your browser.

Do not mix single phase and three phase in same calculation as results will not be accurate.

This load flow assumes 1 p.u. (100%) Voltage at supply transformer primary.

Click here for alternate calculator that only includes a single cable and fixed load.

 

 Nominal L-L for multi-voltage and 3 ph systems.  Select 240V for 120/240V single ph system.  For other single phase systems select the L-N volts.
Nameplate rating and impedance for transformer.
   
Uncoated CU or AL conductor sizes with CU/AL 60C Ampacity listed at 30C ambient for reference.
NEC assumes 75C operating temp. If oversized a lower temp can be used but it should always be higher than ambient.
ft Length of cable 1 in feet (one-way distance).  See NOTE 1 for parallel runs.
   
Amps Bus load current in amperes and PF. Assumed constant with varying voltage.
   
Uncoated CU or AL conductor sizes with CU/AL 60C Ampacity listed at 30C ambient for reference.
NEC assumes 75C operating temp. If oversized a lower temp can be used but it should always be higher than ambient.
ft Length of cable 2 in feet (one-way distance).  See NOTE 1 for parallel runs.
   
Nameplate voltage is usually slightly less than system voltage above.
LR code Highest of kVA/hp range will be used for calculation.
Motor locked rotor PF. This data is not normally on nameplate.
     
Hide  Display 137 intermediate values      
Bus voltage and current before motor start.
Bus voltage and current during locked rotor.
Bus voltage and current after motor running.
Motor terminal voltage and current during locked rotor.
Motor terminal voltage and current after running.
See NOTE 2 below when interpreting results.

 

NOTES:

  1. 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".

  2. Voltage drop for ac systems should total no more than 5% under full load conditions.  Drop may be significantly larger during surge or motor starting conditions -- sometimes in the 15% to 25% range if other devices on the system can withstand this momentary dip.

  3. For most ac systems using cables of adequate ampacity, voltage drop is not a concern unless cable lengths are well over a hundred feet.

 

UPDATES:
3/10/2010 a correction made to steel conduit formula to fix run error.  Also fixed convergence formula that affected 3-phase motor run case.
6/21/2010 corrected 500kVA 3ph transformer Sbase value.
1/6/2011 added note to not mix single and three phase in same calculation.
5/16/2011 added larger transformer kVA sizes and more impedances; deleted references to dc.

   

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