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How hot is too hot for PV array wire?
#1
Hey All

I have 70feet of "6 AWG, Type MTW/UL1283 Wire, 266 Strand, Temp 105C"  wire between PV array and Controller.  The wire is in EMT conduit.  Today - at 38a the wire is running 40C/105F  - e.g. 'warm to the touch' and its not a  problem...   BUT...

How hot is too hot?   

I asked because I added panels to max out my Midnite Classic 150s and as summer approaches they will reach 45+ amps and the he wire will get hotter.  Here's the specs:


The Midnite offers current limiting,  so I can control this but I'm thinking that it will become a question of how hot is too hot (unless I run 4awg wire - a big pain!!)
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#2
I am using 40ft of 10 gauge supplying 99v max 19amps.. and another 19amps 99v on another set of 10 gauge then combined inside the house the a short 3ft run of wires (4 gauge) going to inverter so that’s like 40 amps 99v,, they don’t get warm at all.. with prior configuration I had 27amps on the same gauge wire and it got slightly warm.. I’d think 6 gauge at 70ft would be thick enough to not catch fire and not have much amperage loss from heat
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#3
What voltage is your panels running at? Also, how many amps are you pulling?

Maybe you could bump the voltage up a little to reduce amp draw(?)
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#4
Maybe "Temp 105C" is maximum the cable can withstand ;-)
My conservative gut feeling would be: ~60C perfectly safe, ~75C concerning but safe

Make sure you don't have weak/hot spots in, eg connectors.

Some suggestions:
Split the array into 3p3s and 3p2s, and run another set of cables.
Replace with thicker cable.
Upgrade to Midnite Classic 250 and reconfigure array to 3p5s.
Limit charge current. <- cheapest and easiest. You won't lose much energy as the panels will rarely put out 100% anyway. Besides, on those super sunny days you'll likely get more energy than you need.
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#5
(02-19-2020, 01:28 AM)ajw22 Wrote: Make sure you don't have weak/hot spots in, eg connectors.

I agree. This could be an issue here. Perhaps there's a loose connection somewhere loosing voltage making the amps jump up higher than the installation was rated at.
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#6
Cables heat from the amps (current) flowing - which leads to a current carrying "ampacity" rating.
They are rated for a certain current in open air & need to be derated when enclosed in conduit or bundled with other wires, etc.
You also need to derate more again if cable/conduit is in the sun &/or temps are elevated. EMT conduit (metal) seems to get quite hot in the sun...
See here: https://www.copper.org/applications/elec.../a6124.pdf (there's a temp rise table in there).

This doc has cable ampacity numbers for temps to 90C rated insulation (6AWG is 75A at ambient 30C),
https://www.usawire-cable.com/pdfs/NEC AMPACITIES.pdf
This one has in conduit & 105c cable for <50V marine:
https://www.bluesea.com/resources/529/Al...zing_Chart
Looks like your 105C cable is rated for max 84A in conduit but you have to derate again for in sun/temp.
I couldn't find a clear temp derating number, & it varies with height off a roof surface & colour!
Say "EMT on roof" temp rise = 30C, you'd use the 75C numbers in the usawire table so that's 65A on a 30C ambient day.

So probably going to be OK with your predicted 45A in EMT conduit on a sunny day... but that cable's going to be running pretty hot.
Maybe run a parallel 6AWG pair in a separate conduit?
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#7
Thank you all for the comments.    I have an access box where I can check temps - here's a picture of that:


@Korishan ...  
>What voltage is your panels running at? Also, how many amps are you pulling?  
The Midnite Classic sizing tool predicts 93.9v and I observe that is mostly 97'ish volts but can occasionally go a bit lower.  So 4275w/90v = 47.5a at low end and 97v = 44a at hither voltage.    But as you know MPPT varies thru the day.      

>Maybe you could bump the voltage up a little to reduce amp draw(?)
Yes, my fall back plan is to use Midnite Classic current limiter.   You can limit current going to the batteries and then it will automatically limit incoming PV current because it has to be <= the current going out.   I'm currently running with 80a (limiter setting/circuit breaks) for the battery charging.   This would be in range of 80a*52v = 4160watts/95v = 44a from PV.    

So all settings point to 45 +/- amps on a regular basis.    

@ajw22....
>My conservative gut feeling would be: ~60C perfectly safe, ~75C concerning but safe
Yes I'm thinking that also but I appreciate the comment.   I have a temp gun and will measure/monitor the temp as summer arrives.      This temp roughly coincides with 'too hot to hold onto with fingers'  

>Make sure you don't have weak/hot spots in, eg connectors.
No hot spots - just unbroken wire directly to the Midnite Classic input lugs.

>Some suggestions:
Thank you for helpful suggestions.   I can limit current temporarily if I need to and then rewire.    Perhaps combine some of the current wiring -  use 2x6awg combined instead of 1 wire + run some new 4AWG.  Its only 70feet and is accessible and doable - just a pain to do conduit etc. 

@Redpacket...   appreciate the info.   
Its EMT conduit under the house where multiple strings combine to make the full 15panels (3s5p) power from the combiner box directly to the controller - 70feet of full 3s5p. The one conduit has 4 x 6AWG wires and the other has only 2 x 6AWG wires. If the temps stay within 50C/140F'ish range in the 4 x 6AWG wire run then I think it will be good but we'll see.


I'll monitor and report back on this.
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#8
(02-18-2020, 10:32 PM)OffGridInTheCity Wrote: I have 70feet of "6 AWG, Type MTW/UL1283 Wire, 266 Strand, Temp 105C"  wire between PV array and Controller.  The wire is in EMT conduit.  Today - at 38a the wire is running 40C/105F  - e.g. 'warm to the touch' and its not a  problem...   BUT...

(02-19-2020, 05:46 PM)OffGridInTheCity Wrote: @Korishan ...  
>What voltage is your panels running at? Also, how many amps are you pulling?  
The Midnite Classic sizing tool predicts 93.9v and I observe that is mostly 97'ish volts but can occasionally go a bit lower.  So 4275w/90v = 47.5a at low end and 97v = 44a at hither voltage.    But as you know MPPT varies thru the day.      


Ok, so 70 Feet of 6 AWG pulling around 40A. According to this chart you are exceeding the limits:


You should be using close to 4 AWG or lower your amps. 2 AWG would be better for that distance.
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#9
Offgrid..I mispoke above, I separated my array into 2 sets of wires 40’, each set is right at 20amps 100v (like 1830 w each) then combined (in parallel) inside the house. then its 40 amps 100v short run 3660w (3ft) of 4 gauge directly to outback charger.. going into that distribution block in the pic is 4 wires (2 positives and 2 negatives) 10 gauge (2 from each array) then 2 wires 4 gauge going to charge controller.. and like i said they barely get warm
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#10
(02-19-2020, 07:12 PM)Korishan Wrote: Ok, so 70 Feet of 6 AWG pulling around 40A. According to this chart you are exceeding the limits:


You should be using close to 4 AWG or lower your amps. 2 AWG would be better for that distance.
Yea, but the charts don't capture all the variables.  I see on others that I'm on the line due to distance between 6AWG and 4AWG as I go from 38amps to 48amps.   Its also a matter of power loss (as apposed to danger) in some of the charts - e.g. they lead you to believe you could lose 10% of the power. I actually haven't found power loss to be so dramatic at these distances... but I'm definitely NOT an electrian.

Its really quite startling how the distance affects things.   For example, I had a 1" 4AWG from bussbar to Shunt in my control box when I first turned on the additional 21 panels and the resulted in a few cases of nearly 200a flowing thru it!! and it barely got warm.  I upgraded to 4/0 for that as I couldn't reconcile that in my mind and in theory it can to to 240a at full bore as summer approaches.

Its 4AWG wire from Midnite Classic to Battery - about 8 feet - with 80a on each one and they are at the 101F temp.   80a!!    So distance really matters.  

The 6awg wire was installed by a certified Electrician when I did the original system as was supposed to support 50amps.    But you know, I'm OK to 'question authority' as its my house / safety Smile

-------
40amps at 105F seems OK to me...  but regardless of Electrician I don't want 'too hot to touch' (e.g. 150F'ish degrees) as it just doesn't sound like common sense.    If it stays at 120F or something like that then I think I can sleep OK Smile    IF not, I'll combine the 2 x 6AWG to get 3AWG and run a new 4AWG line.  

@Doin It - Thanks for the clarification/info!
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