Monday, January 13, 2014

Episode 5 pt 2: 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 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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