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!
Melville electric
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