Different wire sizing charts/calculators confusion

JoshuaD · Dec 9, 2023

Why does every single wire sizing chart/calculator I look at give me a different gauge wire for the same ampacity. I've been trying to size the wires for my system for several weeks now, but every chart or calculator gives me a different AWG (sometimes drastically different) for the same ampacity -I even reached out to the solar company I bought my equipment from and got yet another completely different answer. This is by far the most confusing/frustrating part of the van build for me. Feedback? Help! lol

zanydroid · Dec 9, 2023

Probably because they could be using different temperature rating charts or deviating from NEC (which I'm not sure is applicable to van). Or they could be factoring in voltage drop to "try to help you out", but not telling you.

You need to provide system and solar panel voltage info to get help on this. Maybe also which code / construction codebook you want to use but this is less important.

Solar panels you can use the same calculations as used for residential installs, this is agnostic to your battery / inverter side voltage. Well, depends on if you decide to put all your panels in parallel I guess, which is common in van but rare in residential. In that case you would need combiners going into probably #6 or #4.

JoshuaD · Dec 9, 2023

Okay, so I have 12V system, 2x 200W panels (wired in parallel), 50amp (DC-DC) Charge Controller (10awg connecting those), 2x 100ah lithium batteries, 2000W inverter --the batteries to the inverter is where I'm getting the biggest variances in AWG sizing.

zanydroid · Dec 9, 2023

JoshuaD said:
Okay, so I have 12V system, 2x 200W panels, 50amp (DC-DC) Charge Controller (10awg connecting those), 2x 100ah lithium batteries, 2000W inverter --the batteries to the inverter is where I'm getting the biggest variances in AWG sizing.

I'm pretty confident #10 is OK for the PV conductors either in series or parallel config, unless those are bizarre solar panels.

That wide range of inconsistency makes sense because 12V is super vulnerable to voltage drops, and with a 2000W inverter you have a massive current going through it. You're going to be wasting a ton of power following NEC ampacity on 12V. The voltage drop is determined by ampacity. 1% on 240V is 2.4V which is 20% on 12V

What does a voltage drop calculator tell your heart when you punch in the numbers for your system?

JeromeS13 · Dec 9, 2023

JoshuaD said:
Okay, so I have 12V system, 2x 200W panels (wired in parallel), 50amp (DC-DC) Charge Controller (10awg connecting those), 2x 100ah lithium batteries, 2000W inverter --the batteries to the inverter is where I'm getting the biggest variances in AWG sizing.

From my limited experience, most 2000 watt inverters recommend 2/0 DC cables for 12v systems.

zanydroid · Dec 9, 2023

JeromeS13 said:
From my limited experience, most 2000 watt inverters recommend 2/0 DC cables for 12v systems.

What does the voltage drop come out to on that, for the length of wire in your system?

JoshuaD · Dec 9, 2023

zanydroid said:
I'm pretty confident #10 is OK for the PV conductors either in series or parallel config, unless those are bizarre solar panels.

That wide range of inconsistency makes sense because 12V is super vulnerable to voltage drops, and with a 2000W inverter you have a massive current going through it. You're going to be wasting a ton of power following NEC ampacity on 12V. The voltage drop is determined by ampacity. 1% on 240V is 2.4V which is 20% on 12V

What does a voltage drop calculator tell your heart when you punch in the numbers for your system?

With a 3% voltage drop, the calculator is suggesting 2 AWG wire. The inverter came with 4 AWG (and the company that sent it says that I need 1/0 AWG).

JoshuaD · Dec 9, 2023

zanydroid said:
What does the voltage drop come out to on that, for the length of wire in your system?

For 4ft of wire and 3% voltage drop, it's showing 2 awg --on this voltage drop calculator anyways?‍

DIYrich · Dec 9, 2023

Difference between chassis and conduit. Also length of wire. Some are focused on code (over heating). Others factor in voltage drop. DC battery seems to have the most variation. Maybe because the lower voltage is less watts for the same amperage.

zanydroid · Dec 9, 2023

JoshuaD said:
For 4ft of wire and 3% voltage drop, it's showing 2 awg --on this voltage drop calculator anyways?‍

Can you post a screenshot to confirm correct data entry? And what URL?

AWG #2 is very sus b/c you need >166A for 12V @ 2000W, and that is way, way past what NEC allows for #2 for regular wiring situations.

JoshuaD · Dec 9, 2023

zanydroid · Dec 9, 2023

Thanks. Does the length account for out and back (IE doubles for you)?

I am not familiar with the website, maybe some RV or boat folks are.

For a very short run you can control the conditions of installation pretty well so going above NEC limits may be OK. Like using 90C or 105C columns from a RV or boat code manual.

JoshuaD · Dec 9, 2023

zanydroid said:
Thanks. Does the length account for out and back (IE doubles for you)?

I am not familiar with the website, maybe some RV or boat folks are.

For a very short run you can control the conditions of installation pretty well so going above NEC limits may be OK. Like using 90C or 105C columns from a RV or boat code manual.

Yes, out and back. Possibly 6ft at most -but still suggests the same awg either way

zanydroid · Dec 9, 2023

OK. The vehicle space isn't something I'm super familiar with so I can't provide too much more info.

There will also be difference in current rating / safety factor based on:

how hot the terminals at the ends can get
what quality of insulation you are using

The NEC guidelines are conservative & good in that they don't require the terminals to be super robust, since they tend to stop at 75C or 60C. However they are bad in that they'll push you to use some really massive wire for a 12V system. Probably bigger than your terminals can hold without adapters.

wheelman55 · Dec 10, 2023

For such a short run the cost of wire is not going to break your budget.

If this was me, I would use the best wire I could find in 1/0 and call it a day. You’ve got lots of other things to do with your time.

Here’s a link to the wire that I use for both solar and automotive.

Awesome OFC battery cable

Q-Dog · Dec 10, 2023

In a vehicle I like to go up one size over whatever they recommend, because the temperature inside the vehicle can get quite a bit higher than ambient when parked in the sun, and the wire can't shed heat as fast if the air around it is hot. This is the same reason they derate the wire when installed in an engine compartment.

JoshuaD · Dec 10, 2023

wheelman55 said:
For such a short run the cost of wire is not going to break your budget.

If this was me, I would use the best wire I could find in 1/0 and call it a day. You’ve got lots of other things to do with your time.

Here’s a link to the wire that I use for both solar and automotive.

Awesome OFC battery cable

Yep, I like you're thinking. I might even go 2/0 since that was the biggest awg recommended. Why not?‍

Different wire sizing charts/calculators confusion

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