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How do wire ratings work? Found hot wires

CaliSunHarvester

Solar Enthusiast
Joined
Dec 25, 2022
Messages
533
Location
Santa Cruz, California, USA
Environment: 24V system
12 panels total 3.12kW pv
80A SCC

I was looking at my wiring with my new thermal imaging camera. I noticed that with peak sun (and we are just in March, mind you), the wires from the charge controller towards battery are very warm, both near the SCC, and at the DC breaker switch on both sides of the switch.
I also noticed that this switch is installed "backwards", which I will correct ASAP.

I measured twice.

At around 60A charge, the temperature was 97F (36C)
At around 75A charge, it shows a reading 133F (56C)

This wire from SCC via DC breaker to battery is:

Cerrowire-A Slipwire THHN/ THWN-2/MTW 4AWG (21.2mm2) GR11 SR VH-1 600V (UL) OR AWM 07/09/2023

I don't understand what this text means except 4AWG, the sqmm equivalent, 600V and - hopefully - the manufacturing date.

When I look up "awg 4 ampacity" various tables will qualify the result for a temperature.

e.g.
THHN/THWN-2 at 75 °C is 85 Amps.
THHN/THWN-2 at 90 °C is 95 Amps

since my wire does not have anything like 75C or 90C stamped on it, what does this refer to? Temperature of the environment? That would be very bad news, as this room is more like 20 Celsius.

Also, the wire temperature drops off within a few inches from my connections as visible in these 2 photos. Does this mean that *all* my connections are bad??? Every single one???

20240326_204251.jpg
20240326_204615.jpg

I think now that with ~80A charging, maybe I am too close to the wire limit (is it 85? or 95?) Especially if in summer I will get 80A for an extended number of hours of 80A. Right now it's probably just 1 hour. The SCC reports it as 82A actually. It could be 4 hours in August?

IIRC, this SCC cannot accept larger than AWG 4 wire.
 
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Environment: 24V system
12 panels total 3.12kW pv
80A SCC

I was looking at my wiring with my new thermal imaging camera. I noticed that with peak sun (and we are just in March, mind you), the wires from the charge controller towards battery are very warm, both near the SCC, and at the DC breaker switch on both sides of the switch.
I also noticed that this switch is installed "backwards", which I will correct ASAP.

I measured twice.

At around 60A charge, the temperature was 97F (36C)
At around 75A charge, it shows a reading 133F (56C)

This wire from SCC via DC breaker to battery is:
Cerrowire-A Slipwire THHN/ THWN-2/MTW 4AWG (21.2mm2) GR11 SR VH-1 600V (UL) OR AWM 07/09/2023

I don't understand what this text means except 4AWG, the sqmm equivalent, 600V and - hopefully - the manufacturing date.

When I look up "awg 4 ampacity" various tables will qualify the result for a temperature.

e.g.
THHN/THWN-2 at 75 °C is 85 Amps.
THHN/THWN-2 at 90 °C is 95 Amps

since my wire does not have anything like 75C or 90C stamped on it, what does this refer to? Temperature of the environment? That would be very bad news, as this room is more like 20 Celsius.

Also, the wire temperature drops off within a few inches from my connections as visible in these 2 photos. Does this mean that *all* my connections are bad??? Every single one???

View attachment 205021
View attachment 205023

I think now that with ~80A charging, maybe I am too close to the wire limit (is it 85? or 95?) Especially if in summer I will get 80A for an extended number of hours of 80A. Right now it's probably just 1 hour. The SCC reports it as 82A actually. It could be 4 hours in August?

IIRC, this SCC cannot accept larger than AWG 4 wire.

The temp ratings are part of the THWN-2 etc definition, I don’t believe they need to be printed on there unless it exceeds the standards. It should be 90C for that type.

I believe 75C and 90C refer to the wire temperature at which the device/insulation will survive to its rated lifespan (decades). Usually the things (SCC and battery terminals) you connect wire to are good to 75C, so you don’t want things heating up past that point. Double check what the SCC and battery are rated to. Note quite a few people on the forums here are willing to push the wiring in the 48V part of the system beyond NEC limits.

You want to cap to .8* 75C column ampacity , to address the continuous load derate. So you are by some design philosophies way too close to the limit for summer situation. If you believe that things like free air flow table apply (higher current allowed) then you have headroom.
 
Usually the things (SCC and battery terminals) you connect wire to are good to 75C

yeah, the wires are "hot" at the SCC as well but "cold" at the battery. "Hot" meaning 56C, so, below the 75C that you mentioned for "many things" - the SCC manual mentions a 'high temperature protection' and 90C next to it (??)


Is it normal that the wire is hot at the connections but cools off a few inches past that?

The connection at the SCC and at the DC breaker is of the same type (I don't know what it's called). The battery is a post on which I have a lug ring with a nut holding it down. The connection at the battery is probably a much better one.

I'm thinking I should redo the other connections?

Again, these hot wires are in my 24V system.

Here is a photo of my 48V system's charge-verter - clearly no big current there:

20240326_213242.jpg
 
The connections to the SCC and DC breaker are just bare wire inserted and a screw tightened down on it. I will admit, I don’t have a torque driver.

Would a good ferrule avoid that heat?

Worst case, I found that I can configure the SCC to limit charging to 72A. (85A * 0.85)
 
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This info is online, with a simple search. THHN = Thermoplastic High Heat resistant Nylon. The temperature rating is the rating of the conductor's maximum temperature. THHN-2 is 90C, THHN is 75C. If you see a 'W' THWN they add water resistance to the spec THHN wire means your conductors should never exceed 75C. The current capacity is determined by wire size. Charts all over the internet. The wire you are looking at are 'Free to Air' (not in a jacket or conduit). Since this allows better heat dissipation you can get away with smaller gauge wire for a given current.

The smaller the wire the higher the resistance per foot and the more heat you get for the same number of amps. 4AWG is good for about 70A in conduit with two other conductors, THHN-2 is close to 90A at 90C, but you would need to make sure everything it connects to is rated for 90C. Free to air you can go much higher but the resistance of the cable will cause voltage drops under high load conditions, which is why it gets hot which is generally sub-optimal. It's all about current making heat, not voltage, but as the resistance of the load gets closer to the resistance of the cable the voltage presented at the load drops (See also: Ohms law). Under very high load conditions this drop can be significant.

The temperature drops off at the breaker because it is acting as a heat sink. You seem to be well within the spec, and the temps are about what I would expect for that gauge wire with that much juice.
 
The connections to the SCC and DC breaker are just bare wire inserted and a screw tightened down on it. I will admit, I don’t have a torque driver.
The worst connection has higher resistance, it will produce more heat.
I'd say it's worth it trying to make better connections since the heat seems to only be where you're connecting the wire
Torque wrenches are pretty cheap if you want one https://www.amazon.com/dp/B000I7ZDN4

Your temps aren't that hot though, keep in mind what is hot to our hands is barely warm for most electronics.
 
Yea it can be and overall to see the wires themselves just curious to look at it
I don't have a thermal camera and my wire is oversized so can't really test temperature anything like you can there.

From the pics though and what you said if the wires are getting colder as they move away from those points then there's definitely some form of higher resistance there.
The breakers & fuses produce more heat than the wire as it's internally designed to heat up when they pop / break
Your camera won't see inside the breaker though since the house is so thick

Some fuse holders have heatsinks on them to keep the wire cooler while the fuse is heating up I guess
 
You have a pic of your stuff without the thermal image?

I went down there this morning and here are 2 pictures.

I have this big class T fuse; based on the above comments I want to do a thermal of that as well- when under load. The most I ever push there is 120A: car charging plus water heater = 2800W on AC side. 2800W / 25V + loss adjustment = 120A

My actual measurements showed a 119A pe go tell me about those connections. They were last tightened in October and again this morning.
 

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A non breaker solution here might be an IMO Disconnect for the PV, and a Class T Fuse + Battery Disconnect for the battery.


Not that I'm prescribing my convictions to others, but I do like to avoid DC breakers everywhere in my system.

But 133f/56c isn't necessarily a problem either. That's not even breaking the first barrier at 60c, and THHN is rated for 90c.

And damn this rain, I want our sunny spring already. It's gonna turbocharge all this grass growth too.
 
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I went down there this morning and here are 2 pictures.

I have this big class T fuse; based on the above comments I want to do a thermal of that as well- when under load. The most I ever push there is 120A: car charging plus water heater = 2800W on AC side. 2800W / 25V + loss adjustment = 120A

My actual measurements showed a 119A pe go tell me about those connections. They were last tightened in October and again this morning.
yea looks fine. For actual issues all I can say for yours is if the wire isn't all the way in the terminal
but your heat seems fine for the wire size and amps.

In the end if you are like me and don't like to see any hot temps, you need to just oversize your wire like I did.. but as long as your temps are below the rating it should be fine.
It's like a computer CPU, they idle at 20-60C in a computer (desktop vs laptop much hotter) all of them have maxes of 75-105C and they do fine lighting everything on fire at 95C in laptops and still last years... usually everything else melts except the cpu LOL

Just fun to think about:
all the marketing / bragging of "smaller wire" used in strands. I understand it.. there's less air pockets thus more overall wire.
I have no idea if that actually makes a difference though, but I see it on solar wires sold. Smaller strands obviously make wire easier to bend but are also better for heat/transfer. The best of course being 1 big solid core but good luck moving that around lol

maybe since your wire has large strands it's slightly hotter? a few degrees? that'd be a fun test
 
maybe since your wire has large strands it's slightly hotter? a few degrees? that'd be a fun test

Yes, the wire between the batteries and from the batteries to the inverter is 2/0 with very fine strands. I think it is called welding wire.
At 60A there is nothing going on there temperature wise.

The class T fuse shows 68F just like the 2/0 wire.

wire isn't all the way in the terminal

Nah, that would be too easy. I just removed way too much of the insulation. I am pretty sure it's in as far as possible.

When I have time I got to try the ferrules. Maybe this Sunday.
 
Yea for the terminal ends I've seen some come with some crimps that are square & metal so they perfect fit into the socket.. idk where to get those
Is that basically what the ferrules are?

What I've done for decades in small electronics & PCB is just melt solder into the tip of the wire and then clamp that into the socket. No idea about doing that with solar though, I assume it'd be fine
 
square & metal so they perfect fit into the socket.. idk where to get those
Is that basically what the ferrules are?

Yes, I think those are ferrules.
This 24V system was my first solar project and I didn't know about ferrules then (13 months ago).
With my second system, I did try to use ferrules where possible. I got the necessary tools as well.
 
So my 80A solar charger worked a bit harder yesterday. I have 12 used 260W panels on there which theoretically could deliver 3.1kW
But I thought the SCC would only pull as much as it can handle (push up to 80A into the battery bank)?

20240404_124103.jpg
20240404_124113.jpg
 
The worst connection has higher resistance, it will produce more heat.
I'd say it's worth it trying to make better connections since the heat seems to only be where you're connecting the wire
Torque wrenches are pretty cheap if you want one https://www.amazon.com/dp/B000I7ZDN4

Your temps aren't that hot though, keep in mind what is hot to our hands is barely warm for most electronics.
Your temps aren't that hot though, keep in mind what is hot to our hands is barely warm for most electronics.
I don't have a thermal camera so I use the "touch it method" with my hands. I am not sure how most people without the extra equipment would test for temp, but if you use the proper size wires, make sure the wires are tight and it doesn't feel hot to your hand it should be good.
If there is something else I should be doing please let me know, I'm old but new at this.
 
The first step is torquing properly including hopefully reading up on the nuances of torquing (using a torquing driver is a good start and probably enough, even better to read up on it beyond that. Unfortunately i don't have bibliography ready to go)

Most people use thermal cameras, there are also thermistors on multimeter (dunno how I feel about telling people to stuck those into a live system).

If you have OCPD like breakers in there hopefully the combination of overheat and high current will cause the OCPD to trip. Since the current rating is derated with higher heat. However the heating has to be close to the breaker to get this effect.
 
NOTE - when retorquing something the proper way to do it is to loosen it off then torque it to the correct value once. You cannot retorque or check by putting the screwdriver or socket on then turning until it hits value. That will over tighten it. This means to do it right you need to shut everything down first or there will be sparks and nasty stuff.


Spent a few years in a calibration lab repairing torque wrenches and pressure gauges.
 
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Your temps aren't that hot though, keep in mind what is hot to our hands is barely warm for most electronics.
I don't have a thermal camera so I use the "touch it method" with my hands. I am not sure how most people without the extra equipment would test for temp, but if you use the proper size wires, make sure the wires are tight and it doesn't feel hot to your hand it should be good.
If there is something else I should be doing please let me know, I'm old but new at this.
Why are you quoting me and saying that? I posted identically the same text a week before you lmao:
Your temps aren't that hot though, keep in mind what is hot to our hands is barely warm for most electronics.
are you replying to it or something? idgi
 
Why are you quoting me and saying that? I posted identically the same text a week before you lmao:

are you replying to it or something? idgi
Yes, I was referring to your quote, I am new to posting and didn't know how to show your quote at the time. I now know how to reply, but not yet sure how to post in general showing someone else's quote. I apologize for any confusion.
 
Has anyone suggested checking voltage drop (Vd) across the total system and then each component? That's always my first step. At the max amps of course. Vd x Amps = Watts lost across that connection = Heat
 
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