ampacity-which wire type and temp rating?

[Bluedog225]

Good morning,

I need to put up some solar panels (finally).

I’ll be running a few arrays at different locations. My question relates to finding the correct ampacity chart for the wire from the panels, through underground conduit, to the charge controller.

I’m assuming I use the 10 awg (?) wire on the panels to combine the various panels at a combiner box.

From the combiner box, I plan to run to the MPPT without regard to voltage drop. It is what it is. Right?

I’d appreciate input on what type of wire and a temperature rating I should select so I can use one of the various ampacity charts. And any help on picking one of the ampacity charts would be helpful.

What wire do you guys use for this run?

Thanks

 

[DougfromdaUP]

THHN/THWN is the most commonly used wire. It’s affordable too.

Don’t forget to de-rate the wire for the number of conductors in your conduit.

I think you should at least calculate the voltage drops. So you know what you’re getting into.

If you go to a good online calculator, like Southwire’s, it will help you determine code compliance as well as give you voltage drop.

 

[12VoltInstalls]

plan to run to the MPPT without regard to voltage drop. It is what it is. Right?

Sortof. But how far is the run from panels to combiner box to the SCC unit?
There’s things and reasons to factor what the losses are and design to mitigate those losses.

 

[Bluedog225]

There are a couple of runs planned. And some more ambitious plans for later.

The first run is about 30’ for a small 6 panel array. And another 14 panel array at about 100’.

Later there may be a 200’ run for a 14 panel array.

Panels are in the 300 watt 9 amp neighborhood.

I’ll be at about 120 volts initially for a Victron 150/35 but may step up to a Victron 450/200 later.

I’ll check out Southwire using THHN/THWN.

Thanks

 

[12VoltInstalls]

What wire do you guys use for this run?

I use 10ga Solar Cable from panels to combiner. UV resistance and very tough. Temco stuff costs about the same as the solar wire from the local electrical supplier, and a bit more than other stuff on amazonian. But the temco is multi-layer and iirc tinned copper- a lot of the more “affordable” solar wire is not as thick and/or one layer, and not tinned. Not good enough in my mind for thirty years in the sun.

From combiner box to SCC I use thhn, 8ga or 10ga depending on the length of the run, string voltages, drop over distance. For other people. For my own setup I “cheated” and used some 12AWG thhn in the underground conduit because I have like 10,000 feet of it from an auction lot and my voltage was/is high enough that the loss was acceptable and I’m overpanelled a bit.
I do have a ‘new’ run not connected yet that is mostly 10ga but it’s only going to gain me a few percent.

I buy new 8AWG or 10AWG for other people.

At 100-200’ 10AWG will work if your tolerance for the voltage drop is there.
There are ‘tools’ online to show what the losses and results will be to help you plan your installation.

 

[Bluedog225]

Thanks. I guess I’m confused about ampacity.

I thought ampacity was fixed for a given gauge of wire. Without regard to distance. While voltage will drop over distance.

And if I stay within a de-rated ampacity limit in my conduit, I’d always be ok.

But I look at this chart that indicates I can use 12 awg THHN for 30 amps but footnotes that my overcurrent protection cannot exceed 20 amps.

I don’t get it.

 

[12VoltInstalls]

guess I’m confused about ampacity.

There is a ‘fixed’ or a ‘constant’ for voltage loss over distance for X-square-inches of copper wire, so many V per foot. While thinner wire can handle “Y” amps/volts and 12ga has less capacity than 10ga for example, you either need to use the formulas or a calculator to calculate the voltage loss over distance for a given wire gage. The loss is an expression of percentage per foot.
The goal is to not have so much voltage loss that the effective efficiency is reduced to an intolerable level. Plus, there is the heat potential- thinner wire has more loss/resistance over distance, and you do not want to a) create a fire hazard and b) realize higher losses as resistance increases with temperature rise.

In general, 14AWG is 15A, 12AWG is good to 20A, 10AWG/10ga is good for 30A, 8ga is good for 40A…

The charts posted are factors for calculation with formulae but as you noted, the over current protection ** is in line with what I stated above. Note the temperatures it displays as well- that reflects the temperature rating of the wire sheathing.

This is pretty basic but is a start. Don’t forget that a 100’ run is 200’ of wire. Some calculators do not factor for the complete circuit. 10ga performs way better than one might expect sometimes. Even 12ga losses with higher string volts can be acceptable, but you should know what the losses are before saying, “good’nuff” because that way you are not guessing but rather, deciding.

 

[Bluedog225]

Got it. I’m glad you pointed out the round trip aspect. I was wondering.

I’ll need to size the wire for the amps taking into account the recommended over-current protection limits and make sure voltage drop is acceptable based on round trip distance.

Thanks!

 

[wheelman55]

Ok. I’m not an expert. Here are things to consider.

Ampacity gets derated with more than 3 wires in a conduit.

Ampacity gets derated above certain temperatures.

For continuos loads, you need to uprate by 125%.

Don’t put romex in conduit as that would keep heat in.

The above exists to keep heat down. Too much heat makes fire. Fire burns your house down or worse.

There’s an invaluable book called “Ugly’s Electrical References”. It’ll help with your homework.

https://www.amazon.com/gp/aw/d/1284194531?psc=1&ref=ppx_pop_mob_b_asin_title

Use a calculator like Southwire’s.

Use the numbers from your solar electronic gear and your solar modules to design your array.

More modules in series will add voltage without increasing amps…meaning smaller wire.

It doesn’t cost too much more to upsize to the next larger AWG wire.

Best of luck with your journey and be safe.

 

[wheelman55]

Another useful link.

Derating Current-Carrying Conductors for Conditions of Use | JADE Learning

If you ask a seasoned electrician how many No. 12 AWG conductors will fit into a 3/4" EMT conduit system, you may get a response such as “one more!” While this is amusing (and typically true), there are precautions that must be taken according to the NEC whenever conductors are bundled together...

www.jadelearning.com

 

[RV8R]

Got it. I’m glad you pointed out the round trip aspect. I was wondering.

I’ll need to size the wire for the amps taking into account the recommended over-current protection limits and make sure voltage drop is acceptable based on round trip distance.

Thanks!

For 12vdc stuff, I like this Wirebarn Calculator;

Wire Size Calculator

www.wirebarn.com

Caveat ,,, Not All wire/insulation is created equally, so you have to understand the “disclaimers & notes”



Also ,,, V*A=W ,,, There is a lot to learn from that when considering “voltage drop” & being close to amperage capacity ( a little different scenario than Solar Panel “Generation” as you will just loose on panel potential I guess).