3/8 Lug on 5/16 Bolt

[dmholmes]

Thanks @chrisski. Using the low voltage shutdown for my inverter I get 183A based on that formula, so that would indicate a 200A fuse.

Fusing seems to be more an art than I science, I'm seeing several different approaches that arrive at different answers. Since fusing protects the wire, and my wire is overkill for my inverter, wouldn't it make sense to fuse for the wire's max amps * 1.25? In this case, 2 feet of 2 AWG wire rated for 175A so a 225A fuse?

 

[chrisski]

I really wish I knew that one. Another formula I was told for fusing solar panels, and I’m told this is code, is to multiply the open circuit amps X 1.56.

I wish this NEC code was not such a mystery and you’d think there‘d be something published for this.

I thought you had to be an electrical engineer to understand this, but on another forum a guys kid who was an electrical engineer could not answer questions like that.

 

[dmholmes]

Yes, I'd love AN answer instead of answers

Same for cables. Quite a difference in the cable sizes between these 2 sources:

• https://theinverterstore.com/faq-items/what-size-of-cable-should-i-use-with-my-inverter-and-battery/
• https://www.altestore.com/howto/recommended-inverter-cables-sizing-and-breakers-or-fuses-a62/

 

[John Frum]

Fusing and wire size is not that hard.

$dc_amps = $inverter_continuous_watts * 1.15 inverter efficiency / $system_voltage
$fuse_amps = $dc_amps * 1.25 fuse headroom
$fuse_amps is the minimum fuse size for that load
The max fuse size is the ampacity rating of the wire

To spec some wire to accommodate $dc_amps lets look at voltage drop

plug in $dc_amps, $round_trip_circuit_length and $system_voltage to this calculator

https://baymarinesupply.com/bosns_corner_wire_sizes *

Now that you have found the awg spec required for 3% voltage drop
Check the ampacity table below to see if that wire's maximum ampacity exceeds the awg spec we just found
Pick an awg that satisfies both requirements

You have now speced a wire that won't melt before the over-current protection trips
These are minimum values
Now do you really want your wires getting to 105 celcius?
If not, have a look at the NEC tables for more conservative ratings

https://www.usawire-cable.com/pdf/nec-ampacities.pdf

* Calculator for unbundled wire in open air, outside of engine spaces. Chart is based on USCG, ABYC, and UL standards for marine-rated (105°C dry / 75°C wet) wire.

Was that so hard?

 

[dmholmes]

@smoothJoey thanks for the calculations. The math is not hard, looking up tables is not hard. It's knowing if it's correct and who to trust with so many different answers. It's enough to make me reconsider going back to a portable power station and not deal with it.

 

[John Frum]

@smoothJoey thanks for the calculations. The math is not hard, looking up tables is not hard. It's knowing if it's correct and who to trust with so many different answers. It's enough to make me reconsider going back to a portable power station and not deal with it.

I showed you the math and the reasoning behind it.
If you won't take my word for it please let me know and I'll stop wasting my time.

 

[dmholmes]

Nothing I wrote was directed at you, my apologies if you feel offended. It's not your fault that nobody seems to agree on this. Here is what I came up with:
DC amps = 115a (1200w * 1.15 inverter efficiency / 12v)
Fuse amps = 143.75a (115a * 1.25 fuse headroom)
115a, 4 feet round trip cable, 12v system voltage
2/0 cable for 0-3% voltage drop
Max ampacity (Bay Marine, outside, 1 conductor) 330a
Max ampacity (USA Wire) 145a, 175a, 195a depending on temperature

Check the ampacity table below to see if that wire's maximum ampacity exceeds the awg spec we just found
Pick an awg that satisfies both requirements

I'm not sure what to do with this bit. The 2/0 cable shows maximum ampacity of 330a. What is the awg spec we are comparing to?

 

[dmholmes]

And really, 2/0 cable for 1200w inverter? I must have something wrong there .

 

[John Frum]

What is the awg spec we are comparing to?

The calculator gave me 6 awg
The table labeled "Ampacity Limits" right below the calculator gives me 4 awg
So that means 4 awg minimum with over current protection rated at ~150 amps
Now if you don't want your wires to get hotter than boiling water you could refer to the nec requirements which indicate
1 awg at 90 celcius
or 1/0 awg at 75 celcius
or 2/0 awg at 60 celcius

 

[dmholmes]

The calculator gave me 6 awg

Same here, I goofed and took the AWG from the middle for some reason last time.

The table labeled "Ampacity Limits" right below the calculator gives me 4 awg
So that means 4 awg minimum with over current protection rated at ~150 amps

What is relating to the "Ampacity Limits" table? I see 160 for the 4 AWG but not sure what that is based on.

 

[John Frum]

What is relating to the "Ampacity Limits" table? I see 160 for the 4 AWG but not sure what that is based on.

You question confuses me.
Pure copper of 4 awg and insulation rated for 105 Celsius not bundled and in open air can pass 160 amps continuous without the insulation melting off.
There are 2 criteria to selecting wire as I described previously.
First is voltage drop.
Second is, that it should have a higher ampacity rating then the over-current protection used to protect it.
We want the fuse to protect the wire and not the other way around.

 

[John Frum]

I guess specing wire and over-current protection is trickier than I had imagined.

 

[dmholmes]

You question confuses me.
Pure copper of 4 awg and insulation rated for 105 Celsius not bundled and in open air can pass 160 amps continuous without the insulation melting off.
There are 2 criteria to selecting wire as I described previously.
First is voltage drop.
Second is, that it should have a higher ampacity rating then the over-current protection used to protect it.
We want the fuse to protect the wire and not the other way around.

Confuses me too

In the original instruction I'm not sure what this means:
"Check the ampacity table below to see if that wire's maximum ampacity exceeds the awg spec we just found
Pick an awg that satisfies both requirements"

We have done the calculator and gotten 6 AWG cable up to here. What are we looking at in the ampacity table? The 6 AWG row? Which column?

 

[John Frum]

We have done the calculator and gotten 6 AWG cable up to here. What are we looking at in the ampacity table? The 6 AWG row? Which column?

The first column under "Outside Engine Spaces" shows the ampacity for a single wire in open air.
We want to find the first wire thickness equal to or greater than the one speced in the calulator that also exceeds fuse amps.

 

[dmholmes]

I guess specing wire and over-current protection is trickier than I had imagined.

What I currently have is from @Will Prowse's book for inverter wire and fuse. My inverter is 1200w so I had to derive from the 1000w and 2000w recommendations.

• 1000w inverter 4 AWG wire, 100a fuse
• 2000w inverter 2 AWG wire, 210a fuse

I have 2 AWG wire which is bigger than required. It's only after trying to add the fuse that I started questioning my setup.

 

[dmholmes]

The first column under "Outside Engine Spaces" shows the ampacity for a single wire in open air.
We want to find the first wire thickness equal to or greater than the one speced in the calulator that also exceeds fuse amps.

Okay, I think I got it. 160 for the 4 AWG in the table exceeds my calculated 143.75a.

 

[dmholmes]

So my "for dummies" version of
"Check the ampacity table below to see if that wire's maximum ampacity exceeds the awg spec we just found"
would read
"Check the ampacity table below to see if that wire's maximum ampacity exceeds the $fuse_amps just found"

 

[dmholmes]

4 AWG wire and 150a fuse would be the end result, without considering NEC?

 

[John Frum]

4 AWG wire and 150a fuse would be the end result, without considering NEC?

It could be.
But if you don't want your wires to ever get to 105 celcius(even though they are speced for it) you could go to the NEC tables to pick more moderate temperature specs.
I feel like I explained this all a couple of times.