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 go over
how to calculate your maximum current for a designed voltage drop in a solar system.

Based on last week’s article, the amount of voltage drop to
design for in a solar PV system is usually around 3% unless otherwise
specified. This should be split between
the DC and the AC conductors, so 1.5% maximum DC and AC voltage drops are the
end goals. If microinverters are being
used, there are no DC conductors that need to be sized. In that case, the AC voltage drop should
still be kept below 1.5% to help with grid synchronization. Now, it’s time to break out the NEC 2011
codebook and start figuring out how to size our wires.

Current Calculations

Before getting into voltage drop and circuit distances, the
first thing we need to figure out is how much current to design for. Looking at solar panel and inverter data
sheets, there’s a few different values for current we can use. On the DC side, our starting point will
always be the solar panel’s Isc current rating at STC. This is based on NEC article 690.8, Circuit
Sizing and Current. We are concerned
with 690.8(A)(1) and 690.8(B)(1)(a) for DC current calculations, which state:

690.8(A)(1): The maximum current
shall be the sum of parallel module rated short-circuit currents multiplied by
125 percent.

690.8(B)(1)(a): Overcurrent
devices, where required, shall be rated to carry not less than 125 percent of
the maximum current calculated in 690.8(A).

While these talk about overcurrent devices, our circuits
will have to be sized based on these same requirements. Now, looking at the solar panel’s Isc rating,
it has to be multiplied by 125% per 690.8(A)(1). Then to size wire for it, an additional 125%
multiplier needs to be added per 690.8(B)(1)(a). Taken together, the DC current multiplier for
wire sizing is 1.56 times Isc.

On the AC side, the inverter’s rated continuous current
(make sure it’s for the correct AC voltage!) needs to be used for wire sizing
calculations. Like on the DC side,
there’s a factor that needs to be applied to find our AC wire sizing
current. Looking at the same NEC
article, the inverter output circuit current can be found using 690.8(B)(1)(a)
and again referencing 690.8(B)(1)(a), which together state:

690.8(A)(3): The maximum current
shall be the inverter continuous output current rating.

690.8(B)(1)(a): Overcurrent
devices, where required, shall be rated to carry not less than 125 percent of
the maximum current calculated in 690.8(A).

Unlike on the DC side, only one 125% multiplier needs to be
applied on the AC current to find our maximum current for wire sizing. The final AC current multiplier for wire
sizing is 1.25 times rated continuous current.
Next, we’ll select wire sizes and calculate voltage drop based on our
solar current calculations. To keep post
length down, I’m breaking it up into multiple submissions.

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