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Confused about cable sizing for PV array - help!

fafrd

Solar Wizard
Joined
Aug 11, 2020
Messages
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I’ve got a very basic question that I’ve been unable to find a clear answer on and hope someone here can help with.

I’m trying to size cables for a 3P1S PV array and am not overly concerned about cabling losses but want to be sure the cable sizes I’m using are safe.

The 3 380W panels I’ll have connected in parallel have an Imppt of 9.5A so 28.5A peak is what will be running through the wires most of the time.

Isc of the panels is 10.01A and once I correct for worst-case temps of 40C, can reach 10.08A.

So Isc for the 3P1S array needs to be rated for at least 30.24A which translates to 47.25A using the 156% rule (125% x 125%).

For any exposed PV wiring I can use the ‘chassis wiring’ ratings so 10AWG will be enough for 47.25A (though I will probably over spec with 8AWG for the short run to my through-roof junction box).

My confusion comes to the home run through the house where I plan 4-wire metal-clad conduit.

I assume that when in conduit, I can no longer use the ‘Chasis Wiring’ ratings,
but does this mean I must use the ‘Power Transmission’ ratings?

47.25A for Power Transmission means I’ll need at least 4AWG which is much larger than many of the ‘Solar Wiring Calculators’ recommend.

For example, Windy Nation indicates that 6AWG can handle up to 55A: http://www.windynation.com/cm/articles_choosing-right-wire-size.pdf

Which does not correspond to either the 6AWG limits for Chasis Wiring of 101A or the 6AWG limits for Power Transmission of 37A (and hence my confusion).

I have seen some reference to certain of these standards being limited to 3 conductors in conduit, so I’d like to understand what minimum AWG I need for 47.25A in 2-wire conduit (plus ground wire) or 4-wire conduit (plus ground wire).

I wanted to use 4-wire conduit to get 2 array’s-worth of wiring into a single conduit (and a single set of holes) but if that means I’ll need 4-4 versus 2 runs of 6-2 or even 8-2, I may just run two seperate 2-wire conduits instead.

So my question boils down to:

-what minimum AWG is needed to accommodate worst-case Isc of 47.25A in 2-wire metal-clad conduit?

-what minimum AWG is needed to accommodate worst-case Isc of 47.25A in 4-wire metal clad conduit?
 
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OK, I’ve done a bit more research and found this NEC table: https://www.usawire-cable.com/pdf/nec-ampacities.pdf

So now I understand that the NEC rates 90-degC 6AWG wire for 75A at 30C ambient as long as there are no more than 3 current-carrying conductors in the conduit. 30C is not the worst-case temperature in my attic so if I rate for worst-case ambient temps of 50C (122F) 6AWG 90degC wire is derated by 18% to 61.5A (while 8AWG will barely not cut it, being derated from 55A to 45.1A, just shy of my 45.25A requirement).

So I believe I’ve I’ve answered my first question - if I go with 2-conductor MC, I need at least 6-2 (of 90degC wire).

And I also see that the derating for 4 current-carrying conductors is a further 20%, meaning 6-4 MC is rated for 49.2A, sufficient for my worst-case Isc of 45.1A.

So I can go with either 2 runs of 6-2 or a single run of 6-4 (which makes for a pretty easy decision).

Apologies for the wasted thread, though putting the details down in writing is what led me to understand that the number of conductors in the conduit is what caused confusion with all of the solar sizing calculators.

I’ll leave my ramblings for posterity in case the process of figuring out the answer can help anyone else suffering from similar confusion.

And I would still appreciate confirmation from any pros out there that I have finally got this figured out correctly..,
 
4-4 MC is going to cost me on $6 more than 6-4 MC ($256 versus $250 for 50’), so after all of that, I probably will be laying out an additional 2.4% for heavier wire to recover 37% of my losses to wiring resistance.

With 6AWG, 100’ (50’+50’) would cause a voltage drop of 1.126V @ Imppt of 28.5a or 2.81% of Vmppt of 40.02.

With 4AWG, that same voltage drop decreases to 708mV or 1.77% of Vmppt.

I’m not normally hung up on wiring efficiency since addition PV power has become so inexpensive, but paying an addition $6 or 2.4% to get back 1% of lost efficiency (or ~15kWh / year in my case) is a no-brainer (and even safer/cooler to boot).
 
Are you in the position to consider series wiring the panels?
No, that’s been the big constraint I’m struggling with. There are several reasons I want a 1S array, chief among them that I’m dealing with some pretty severe shading issues that shift around over the course of the day.

So a 1S array should give me the best efficiency I can get and in addition, I’m getting three of these new 144-half-cut-cell panels which are essentially 2 72-cell panels in parallel and mean shade on only one half of a panel allows it to deliver 50% of Imppt @ Vmppt.

In addition, the 30A charge controller I have is also limited to 55V input, so I’d need to purchase a new one to handle 2S with ~100V Voc.

If I had no shade to deal with, series configuration, lower currents, and smaller.cheaper wires would be the way to go, but now that I’ve (finally!) figured out the safe wiring requirements for the currents I’m dealing with and see thss as t the actual costs are not prohibitive, I’m confident this will be the best solution for my situation.

I’m putting in the second pair of wires for a planned second array once we have an EV in a year or two.

Another bonus of paying a slight bit more for 4AWG is that I’ll be able to size that future array for as much as 95A derated to 76A for the 4-conductor MC and further derated to 62.3A for the worst-case temps of 50C.

This means that that future array can easily be 5P1S and possibly as high as 6P1S...
 
I can/could get 500' of 10ga at Home Depot for a reasonable cost. Several pair of 10ga might be worth considering. It may or may not be better than a larger single pair.
 
I can/could get 500' of 10ga at Home Depot for a reasonable cost. Several pair of 10ga might be worth considering. It may or may not be better than a larger single pair.
Yeah, thinner wire is cheaper for sure, but I’m trying to minimize the number of holes I need to drill and conduits I need to run from attic to basement...

I can get 50’ of 4-4 MC for $256 and the simplicity of having just that one piece of conduit to run outweighs whatever savings I might get from switching to 4 or more runs of 10-2 MC...

But thanks for the offer...
 
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