Are equipment-supplied cables really big enough for 100A?

[SeaGal]

Have just upgraded to a larger inverter that will now handle 100A (48V system). Have upgraded fuses and DC cables as far as possible - we now have 40mm2 cables to support the new load.

But, at either end of my 40mm2 cables, I am left with...

- the supplied Solis DC cables with Amphenol Surelok plug in connectors. These cables are labelled 4AWG and about 1.5m long.

and at the battery end...

- an Overkill / JBD 100A BMS which has twin 24" cables - these are soldered to the BMS and are 8ga size - so 2 x 8ga for +ve and 2 x 8ga for negative.

I am assuming that both these cable sizes are sufficient to handle 100A, as that is what Solis and Overkill/JBD supply.

They both seem a bit under-rated to me, but am guessing that, being relatively short, it is not an issue.

I do like to over engineer my designs, but am a bit stuck to improve on those cables without a) soldering new wires to the BMS and b) crimping bigger than 4AWG cables into the Amphenols... and that's assuming the Amphenols can accept bigger than 4AWG and I had the crimping tools equipment.

Thoughts?

 

[sunshine_eggo]

As far as the Overkill goes:

8awg superworm wire like on the overkill is typically silicon jacketed and rated for 200°C or higher. Each has at least a 80A rating.

 

[SamDeleted]

Have just upgraded to a larger inverter that will now handle 100A

How often will you run a sustained 100amps ?

 

[SeaGal]

How often will you run a sustained 100amps ?

100A = 5000W = max inverter output. With only a 14kWh battery, obviously won't be sustained for more than about 2.5 hours max!

In summer, hopefully when charging.?

Other times when running 2 appliances - washing machine and oven would be the longest lasting, microwave and kettle, oven and airfryer etc.

 

[SamDeleted]

Other times when running 2 appliances - washing machine and oven would be the longest lasting, microwave and kettle, oven and airfryer etc.

Even then , apply diversity and you could just about run all them off a 13a 4pin gang ,

it's fine, you're over engineering it? ( I should know I am an over engineer-er)

 

[Quattrohead]

There are two parts to the wire rating.
1. The actual current carrying capability of the wire itself. It could be glowing cherry red but still be perfectly fine to carry the load.
2. The heat rating of the insulated jacket, this is generally what dictates the wire capacity. Standard cheap wire might be good for 60°, most is rated at 75 or 90°. Silicone wire supposedly rated to 200°. I may have to try that with a soldering on one day LOL

 

[SeaGal]

There are two parts to the wire rating.
1. The actual current carrying capability of the wire itself. It could be glowing cherry red but still be perfectly fine to carry the load.
2. The heat rating of the insulated jacket, this is generally what dictates the wire capacity. Standard cheap wire might be good for 60°, most is rated at 75 or 90°. Silicone wire supposedly rated to 200°. I may have to try that with a soldering on one day LOL

Thanks. Presumably a 3rd consideration, though not actually the "wire rating" is voltage drop / losses, which I prefer to minimise.

@sunshine_eggo also said that the silicone wire is rated at 200°C which, one one hand is good to know. But, on the other hand, I don't want my BMS wires getting anywhere near that temp in an enclosed box where other components may not withstand that temperature. Not sure I want a wire at 200C directly connected to my LF280K's either

Does anyone have an idea of how hot 8awg superworm wire might get, passing 50A through it - it will be within a battery box enclosure, so not a lot of free air movement?

Maybe I should just move one of the JBD's 3 battery temp sensors and stick it to the wire and monitor it?

 

[Partimewages]

I don't have a specific but #8 in typical commercial work THHN is good for about 40 amps in most cases. You will probably be warm but not hot.

 

[Pappion]

Blue Sea Wire and fuse The Blue Sea wire and fuse chart is handy to have.
The 4awg looks ok, depends on the insulation. The resistance calculates to 0.002 ohms, for 10ft round trip. 0.2v drop at 100amps.
calculator

 

[RCinFLA]

#0 about 0.10 milliohms/ft.
#2 about 0.17 milliohms/ft.
#4 about 0.26 milliohms/ft.
#6 about 0.42 milliohms/ft.
#8 about 0.64 milliohms/ft.

Voltage drop = I*R, Power loss = I^2 *R

For wire in that gauge range, stay below 3 to 4 watts of loss per foot to avoid too much heating.

You also have terminals which are about 0.05 milliohm for compression surface to surface and 0.05 to 0.15 milliohm for actual lug depending on lug metal casing gauge/quality.

 

[SeaGal]

#8 about 0.64 milliohms/ft.
...
For wire in that gauge range, stay below 3 to 4 watts of loss per foot to avoid too much heating.

Perfect, thanks. So 50A in 2ft of #8 will be 0.64 * 2 * 50 * 50 / 1000 = 3.2W over 2 feet = 1.6W per foot = OK

 

[SeaGal]

So, in summary, the wire should be OK - nothing is going to dangerously overheat. But, when drawing 100A...

a) with 2ft of 8AWG for the BMS I'm going to lose 3.2W for each cable handling 50A each, times 2 for +ve and -ve = 12.8W and
b) with the 10ft of 4AWG for the Solis connection, I'll lose another (0.2v drop at 100A) = 20W per cable = 40W.

So a loss of just over 50W for a 5000W draw = 1% loss. Not ideal, but acceptable. It's just a pity that the suppliers don't provide more beefy cables.

 

[RCinFLA]

Keep in mind a small difference in resistance in parallel wired due to terminal connections will upset the current sharing balance.,

 

[ddonq1412]

Hi popped in as i have the same issue, well mine is an issue. I recently changed from a solis 3.6 to a 5kw inverter suing a seplos mason box. when I upgraded the inverter i added a Projoy Battery Isolator 160V 125A DC MCB and using the 25mm cables that came with the solis unit to connect to batter the cable lenght is now more than 1m in leter from inverter to battery. Every time i ramp up the charge or im using my battery to change my ev at close to 80amps it will trip the circuit breaker between the battery and inverter. Im thinking the cable should be thicker.

 

[SeaGal]

Keep in mind a small difference in resistance in parallel wired due to terminal connections will upset the current sharing balance.,

Hopefully not too much difference as the Overkill BMS has the twin cables crimped together one end and soldered to the BMS the other - though obviously talking of very small resistance here anyway.

 

[SeaGal]

Im thinking the cable should be thicker.

A thicker cable won't prevent the breaker from tripping. A 125A breaker should not trip at 80A.

 

[740GLE]

Hopefully not too much difference as the Overkill BMS has the twin cables crimped together one end and soldered to the BMS the other - though obviously talking of very small resistance here anyway.

If it’s a difference it will be a % or 2 nothing radical.

 

[Hedges]

Does anyone have an idea of how hot 8awg superworm wire might get, passing 50A through it - it will be within a battery box enclosure, so not a lot of free air movement?

Ampacity table says in 30C environment, 90C 8 awg wires 3 in a bundle can carry 55A. So I'd expect about 60C rise above ambient.
Table for single conductors in free air allows more current. Your BMS may have several such wires, but not exactly bundled together.

Power dissipation goes as square of current, so temperature changes at same non-linear rate.

 

[SeaGal]

Your BMS may have several such wires, but not exactly bundled together.

Thanks. It's 2 cables, crimped together at the battery end, lying side-by-side and soldered onto the BMS at the other, next to each other.

Like this...

 

[sunshine_eggo]

I use 8awg superworm between my Victron with ~50A output to 48V - gets comfortably warm to the touch at peak output.