If your voltage-drop calculation fails

What do you do if, after sizing your cable and calculating voltage drop, your voltage drop exceeds what’s allowable?

Simple: it’s back to the drawing board. Cable sizing can be an iterative process. If voltage drop fails, then the cable’s size needs to be increased and its voltage drop recalculated.

Alternatively, you may be able to justify the excess voltage drop. If you can’t, then this change in cable size can have ripple effects throughout your design. This is why you want to wait until after you get voltage drop and cable size nailed down before sizing conduit and pull boxes (bend radius). You might find out at an inconvenient time that you have to increase your conduit or box size to handle the larger cables.



When calculating voltage drop, go ahead and calculate it for your lowest expected voltage as well as your nominal system voltage. The low-voltage condition will probably be the result of allowable voltage dips built into the system. This is typically something like 10%.

For example, a system with a nominal operating voltage of 120 volts may be allowed to dip down to 108 volts during normal operation. Make sure your voltage drop calcs consider this. Find a plant document that tells you what that maximum allowable dip is, then reference it in your design evaluation.

When sizing cables for motors, don’t forget about the voltage drop caused by the motor’s inrush current. It’s common for a motor’s inrush current on startup to be approximately six times its full-load current. This means voltage drop will be about six times greater on startup. Review the motor’s manual and make sure that your voltage drop won’t exceed its lowest rated startup voltage.