Episode 5 pt 1: Voltage Drop and Wire Sizing for Solar

Voltage drop and wire size are important considerations when designing and installing solar PV systems.  Voltage drop occurs in any electrical circuit carrying power. A percentage of the power running through the conductors will be lost as heat.  This amount is dependent on the size of the wire along with the voltage and current of the load, and some environmental factors as well (circuit length, temperature, conductor material). Today I’m going to look at designing for voltage drop and how to design for it so your solar array functions properly.

The maximum voltage drop you should design for in a system is not set in stone.  For commercial systems, it can be specified in the bid documentation, but for smaller systems you are usually left trying to figure out the best value that will deliver the most energy without breaking the bank on wire.  While the National Electric Code (NEC 2011) does not explicitly provide a maximum permissible voltage drop for solar PV systems, you can find a reference point in Articles 210 and 215 in fine print notes.  They state that overall voltage drop should not exceed 3% for branch or feeder circuits and 5% overall.  Based on this, most solar systems are designed to never exceed 3% overall voltage drop as a feeder circuit, with no more than 1.5% voltage drop on the AC side.

The reasons you want to keep voltage drop low in your solar system are multiple. 

First, the obvious reason is wasted energy and wasted dollars. A solar array is a major investment, and if an upgrade from #12 to #10 wire will provide a significant decrease in voltage drop it will pay for itself throughout the life of the system.  A difference between 1% and 2% voltage drop between your inverter and the interconnection is a difference of 50W for a 5000W system, and it quickly adds up the larger it gets.

On the DC side of the inverter, you need to hit a defined voltage window.  If you size your solar strings to the lower end of that range, a high voltage drop could put you outside of the maximum power point tracking window of the inverter. This is at its worst in hot weather, when voltages are already low and the inverter is hot.  This is also some of the best production time for solar PV systems all year, optimum performance is critical and your DC wiring needs to be sized with that in mind.

AC voltage drop is possibly more important than DC for solar PV systems.  The issue is one with the electric grid and your inverter.  Your service voltage can vary depending on several conditions including weather and time, and the inverter is constantly working to match up with that voltage.  This is why on inverter data sheets you see an AC voltage range the inverter is capable of producing, typically -12% / +10%.  For a 240V inverter, this gives an upper voltage limit of 264V.  Grid voltages as high as 250V or higher have been seen for 240V services. iIf your AC wire was sized with a 5% voltage drop your inverter would see 262.5V, almost its limit and at risk of going into fault.  This is why low AC voltage drop is extremely important, and most commercial bids and lease agreements require 1% or less.

Now that you have a guideline for how much voltage drop you want to design for in your solar system, how do you do it?  Next week I’ll be digging into the NEC 2011 codebook for wire sizing methods and solar current calculations.  I'll also share some of my favorite online/mobile tools and rules of thumb to help make this as smooth and painless as possible.  Happy New Year!