Double Check My Wire Size

[PaulLad]

Here's the numbers I came up with for my new 48V system:

4 - 100w panels in series: 18.5V (VMP) x 4 = 74 V @ -12 deg C = 86.4V
3 parallel strings: 5.41A (IMP) x 3 = 16.23A x 1.25 = 20.29 A
86.4V x 20.29A = 1753W
panels are 60 ft from controller
20.29 amps @ 60 ft circuit = 8 awg wire?

 

[JBertok]

I use the panel short circuit current ratings for conductor sizing, and even adding a few amps to your scenario, 6AWG is still plenty. Your current level is low enough I'd even consider combining with 3-way MC4 splitters versus a combiner box if you don't have to be inspected.
This all assumes that the maximum PV output voltage is within the MPPT operating range of your chosen charge controller, or even PWM with this configuration is possible too. What charger are you using?

 

[OzSolar]

#10 is just fine.

To meet code one must size wire to Isc x 1.56 but there is a movement to change code to Imp x 1.25 which I'm comfortable with.

Do you see that Isc rating on that panel?

Let's use 6.2 for Isc x 3 x 1.25 = 23.25 amps so that's #10 copper.

For performance I use NOCT numbers. For a good starting point those numbers are STC x .8

I use this voltage drop calculator.

.8 x 16.23 = 13 amps. Looks like #12 would be just from a performance standpoint. Keep in mind most systems only spend a few hours a year generating at rated output so there's no ROI in stepping up to a larger wire size.

A few other points,
-You may need fuse those panel since you at more then 2 strings. The label on those panels will have a "max series fuse" on it.
-Why not two strings of six modules? Perhaps you have a SCC rated at 100v max? I'd look into one that could do 150v if it's not to late.

 

[PaulLad]

I use the panel short circuit current ratings for conductor sizing, and even adding a few amps to your scenario, 6AWG is still plenty. Your current level is low enough I'd even consider combining with 3-way MC4 splitters versus a combiner box if you don't have to be inspected.
This all assumes that the maximum PV output voltage is within the MPPT operating range of your chosen charge controller, or even PWM with this configuration is possible too. What charger are you using?

Thanks. I'm still confused about the circuit length. I seem to remember seeing that whatever the distance is from the panels to the charge controller, the circuit length will be twice that (there and back). Is that accurate?

 

[PaulLad]

Do you see that Isc rating on that panel?

5.86

 

[PaulLad]

A few other points,
-You may need fuse those panel since you at more then 2 strings. The label on those panels will have a "max series fuse" on it.
-Why not two strings of six modules? Perhaps you have a SCC rated at 100v max? I'd look into one that could do 150v if it's not to late.

Yes, my SCC is rated at 100v max. Its the only SCC I could find @ 48v 20a. I could still change SCC's but how much does it save me to go two 6S2P? Max series fuse is 15 amps so 62SP would stay under that.

 

[Brucey]

Yes, my SCC is rated at 100v max. Its the only SCC I could find @ 48v 20a. I could still change SCC's but how much does it save me to go two 6S2P?

A Victron 150/35 would be a good fit and only $184.

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[OzSolar]

Yes, my SCC is rated at 100v max. Its the only SCC I could find @ 48v 20a. I could still change SCC's but how much does it save me to go two 6S2P?

1) Higher voltage means lower amps lost on that home run. It could mean smaller wire size but it may close to the line it doesn't matter. EG: It's not like you're turning #4 into #14.

2) No need for a fused combiner box. If it's still in the box I would sell it and get a 150/35. I know I'd be happier in the long run.

 

[JBertok]

Okay, for 6S2P your open circuit voltage is going to be too high for the MPPT; that has to stay under 100V. The Vmp of your panels is quoted at 18.5V, so the Voc is going to be somewhere around 21-22V. 4S is the limit with those panels and that MPPT.

And yes you are correct - "circuit" implies round trip, a combined distance of both positive and negative conductors. AC wire size charts are different and generally in one-way "distance", meaning you don't have to double the actual distance.

 

[Q-Dog]

If you have more than one string of panels, you measure wire length from where the strings combine.

 

[PaulLad]

2) No need for a fused combiner box. If it's still in the box I would sell it and get a 150/35. I know I'd be happier in the long run.

Why would you be happier in the long run? Is it because you could add more panels without buying another CC? This system is strictly for emergency backup so there's no benefit for me in that regard.

 

[scrubjaysnest]

Yes, my SCC is rated at 100v max. Its the only SCC I could find @ 48v 20a. I could still change SCC's but how much does it save me to go two 6S2P? Max series fuse is 15 amps so 62SP would stay under that.

I went with 100v/20A victrons and 12 awg 105c wire. I can hardly move 47# panels so went with 100watt. 3S2P for 1 array to 1 cc. Voc is 21.82 and Isc is 5.55.

 

[PaulLad]

2) No need for a fused combiner box.

Thanks. Looks like it would be a wash between the more expensive SCC and saving the combiner box.

 

[PaulLad]

I went with 100v/20A victrons and 12 awg 105c wire. I can hardly move 47# panels so went with 100watt. 3S2P for 1 array to 1 cc. Voc is 21.82 and Isc is 5.55.

Is that relevant to my situation? I'm not seeing how.

 

[PaulLad]

Apparently, I just need a fuse on each series string @ $10 ea. No combiner box is necessary. Esp. since this is strictly an emergency backup system, I'll just go with fuses.

 

[Zapper77]

Do you have the MPPT specifications? Will 4-100 watt panels put out sufficient voltage to activate the MPPT for a 52.1v nominal system?