Wire Sizing

This tool will help you determine the proper wire size for various parts of a solar system.

From PV Combiner Box to the Battery Bank
PV Description:
PARAM 1.0: PV Module Voltage.
PARAM 1.1: Number of PV Modules.
PARAM 1.2: PV Module short circuit current in Amps. (Isc)
From Battery Bank to DC load run
Battery Bank Description:
PARAM 1.2: DC load from Battery Bank in Watts. (System Voltage x Amps)
Cable and System Description:
PARAM 3.0: System Voltage.
PARAM 3.1: Distance in feets of cable
PARAM 3.2:
    Compute Voltage Drop using Voltage Drop Index
    Compute Voltage Drop using Drop Factor


When selecting a wire you must consider its ampacity and voltage drop. The Ampacity is the amount of current a wire can carry. A wire will have a voltage loss due to the resistance of the wire at a given length.

To meet NEC requirements we must size the cable to the load it will carry (ampacity). NEC rates cable sizes and provides the maximum aperage that they can carry safetly. See Tables 310.16 and 310.17 in NEC 2008.

Besides meeting NEC requirements you want to avoid voltage loss caused by the cable resistance and distance. To avoid this we compute a cable size that meets the ampacity requirement and reduces the voltage drop to an ideal 2%.

By NEC a cable can not carry more than 80% of the rated capacity. To achieve this the load computed for the cable is multiplied by 125%.

When a cable is used from the PV modules NEC (690.8) requires an additional safety factor to be included to account for any extra current produced by the panels during an exceptionally sunny day.

This tool provides two methods to account for voltage drop when selecting a wire size. The first method is to compute a Voltage Drop Index (VDI) and lookup the cable size from a lookup table. The VDI is computed as (Amps * feet)/(drop% * voltage). The second method is to compute a 2% drop for the load and find a cable with a computed drop value less or equal to the 2%. The drop is computed using factors pre-calculated from data that appears in the book "Photovoltaics, Design and Installation Manual" (2007) published by "Solar Energy International".

DISCLAIMER: Always consult with a professional or licensed electrician before selecting a cable. In must locations solar installations are required to meet NEC and must be completed and certified by licensed profesionals. CARES or the authors of this tool assume no responsability over the use of this tool or its accuracy.