Cable size question. 100A Breaker with 6 AWG? Doesn't seem right.

[woe]

So I just ordered an RUiXU RX-12K. Should be delivered next week. Starting to plan everything for the install but came across something that seemed strange in the manual.

For the connection to the Grid Panel it says to use a 100A breaker and 6awg cable.



This seems too small from everything I've read. It's only going to be a 3ft run but still seems off. Does this seem right? I would just do 4awg but I happen to have some spare 6awg THHN cable so would be nice if I can use it.

***************
EDIT:
These cable will be single in air and the reality it is highly unlikely it will get even close to 100A considering all my loads on my inverter will likely max out close to 45A.

So maybe I'd be fine with the 6awg but switching to a 60 or 75A breaker. The whole point of the breaker is to protect the cable right?

So if my theory is correct I'll install a 60 or 75A breaker which is fine got 6awg THHN, and if everything works we're good. But if the breaker trips, it tells my I should increase to a 100A breaker and increase the wire size too?

******************

For my battery I have 8x 12v 280ah I'm going to wire in a 4s2p config. I know it's not preferred to go 12V but I got them at a very good price. And I know lots have this config without issue.
My plan was to use busbars to form the 4S with a battery balancer, and 2/0 from the 2x 4S battery banks to a 500A distribution block. Then 2/0 from the block to the inverter. Does that sound right too? Or do I not need 2/0 to from the batteries?

Thanks for you advise.

Cam

 

[1201]

What is the grid pass through current?

 

[Backyard]

I'd use 4awg or larger myself.

 

[woe]

What is the grid pass through current?

Website says integrated 200A transfer relay.

 

[Berkeloid]

Looking at my go-to reference on Wikipedia, it says a 6AWG cable can carry 75 amps - if you're willing to let the cables get up to 90°C (194 °F). However this is for cables bundled together in a warm environment.

My second go-to reference says that 6AWG cable can carry 101 amps, but only if the cables are single (not bundled together) and hanging in air where convection will take care of the heat build up.

So in my uneducated opinion, putting a 100A breaker on the 6AWG cable is probably acceptable, but not great. You'll start wasting power by heating up the cables well before you get to the 100A rating as well, so your system will be more efficient at high loads if you opt for thicker cables.

 

[time2roll]

Does not seem to comply with NEC. Rating is 12k? so maybe the real current is closer to 50 amps 240v.
100a must be for surge performance less than 1 second (as if).

 

[niktak11]

You need 3 AWG copper THHN for the grid connection

 

[woe]

Looking at my go-to reference on Wikipedia, it says a 6AWG cable can carry 75 amps - if you're willing to let the cables get up to 90°C (194 °F). However this is for cables bundled together in a warm environment.

My second go-to reference says that 6AWG cable can carry 101 amps, but only if the cables are single (not bundled together) and hanging in air where convection will take care of the heat build up.

These cable will be single in air and the reality it is highly unlikely it will get even close to 100A considering all my loads on my inverter will likely max out close to 45A.

So maybe I'd be fine with the 6awg but switching to a 75A breaker. The whole point of the breaker is to protect the cable right?

So if my theory is correct I'll install a 75A breaker and if everything works we're good. But if not I should increase to a 100A breaker and increase the wire size too?

Cam

So in my uneducated opinion, putting a 100A breaker on the 6AWG cable is probably acceptable, but not great. You'll start wasting power by heating up the cables well before you get to the 100A rating as well, so your system will be more efficient at high loads if you opt for thicker cables.

 

[woe]

Does not seem to comply with NEC. Rating is 12k? so maybe the real current is closer to 50 amps 240v.
100a must be for surge performance less than 1 second (as if).

Please check my edit in the Original post.

Do you in that would be a good option?

 

[Berkeloid]

These cable will be single in air and the reality it is highly unlikely it will get even close to 100A considering all my loads on my inverter will likely max out close to 45A.

So maybe I'd be fine with the 6awg but switching to a 75A breaker. The whole point of the breaker is to protect the cable right?

So if my theory is correct I'll install a 75A breaker and if everything works we're good. But if not I should increase to a 100A breaker and increase the wire size too?

Yes this would stop your 6AWG cables from getting excessively hot. However you may still have to consider how the inverter works. If it has a ~100A surge when it switches on, that won't trip the 100A breaker but it could be enough to trip the 75A one. So if you use a 75A breaker you may have to use one that doesn't trip as quickly on overcurrent - there are various classifications based on how much overcurrent they tolerate and for how long, before tripping.

It's often a bit of an unknown when you vary from the manufacturer's guidelines like this, so while there's no problems doing so, you may have to do a bit of trial and error to make sure everything still works as expected. Ultimately, as long as you're prepared to go back to the 100A breaker and thicker cable, then there's no harm trying out the 75A breaker first and seeing how you go. Just make sure you give it a thorough test, don't just switch it on once and call it good. But if you can power it on and off multiple times even with all your high current devices plugged in, then you can be confident that it won't let you down right when you need it the most.

Otherwise, yes, if you choose to stick with the 100A breaker, going with thicker cable is probably going to be safer in the long run. I'm not sure I'd feel comfortable knowing that at any moment a partial fault in some random appliance could result in my battery cables getting almost hot enough to boil water.

(Notwithstanding the other comments here questioning whether it can ever get up to 100A, that might be worth considering too. If it's only a surge rating and it shuts off after 100A for a few seconds, then it's not going to be a problem with the 6AWG cables as they will need some time at that current to heat up.)

 

[woe]

Yes this would stop your 6AWG cables from getting excessively hot. However you may still have to consider how the inverter works. If it has a ~100A surge when it switches on, that won't trip the 100A breaker but it could be enough to trip the 75A one. So if you use a 75A breaker you may have to use one that doesn't trip as quickly on overcurrent - there are various classifications based on how much overcurrent they tolerate and for how long, before tripping.

It's often a bit of an unknown when you vary from the manufacturer's guidelines like this, so while there's no problems doing so, you may have to do a bit of trial and error to make sure everything still works as expected. Ultimately, as long as you're prepared to go back to the 100A breaker and thicker cable, then there's no harm trying out the 75A breaker first and seeing how you go. Just make sure you give it a thorough test, don't just switch it on once and call it good. But if you can power it on and off multiple times even with all your high current devices plugged in, then you can be confident that it won't let you down right when you need it the most.

Otherwise, yes, if you choose to stick with the 100A breaker, going with thicker cable is probably going to be safer in the long run. I'm not sure I'd feel comfortable knowing that at any moment a partial fault in some random appliance could result in my battery cables getting almost hot enough to boil water.

(Notwithstanding the other comments here questioning whether it can ever get up to 100A, that might be worth considering too. If it's only a surge rating and it shuts off after 100A for a few seconds, then it's not going to be a problem with the 6AWG cables as they will need some time at that current to heat up.)

Yeah that's my thought. Running new cable would be easy so changing them out of it trips would not be too much work.

The thought was just because I already had the cable. Also have a 60A breaker spare so I may just try that. If there's an issue I'll buy a 100A and 4awg cable.

Any thoughts on the 2/0 Battery cables?

 

[Brett V]

My 12k Growatt called for 6 awg on the AC in and out. I went with that but only 60 amp breakers. On the DC side, I originally had a very short < 4 foot 1/0 cable set from the batteries to the inverter. It only got slightly warm one time when I was dumping around 100 amps in to the batteries from the AC charger and a generator. Once I increased the battery plant size and relocated it to the other side of the garage, my DC cable set was 15 feet so I went with 4/0 cables. Everything has been trouble free.

 

[Backyard]

Yeah that's my thought. Running new cable would be easy so changing them out of it trips would not be too much work.

The thought was just because I already had the cable. Also have a 60A breaker spare so I may just try that. If there's an issue I'll buy a 100A and 4awg cable.

Any thoughts on the 2/0 Battery cables?

Insulation has to be rated for the voltage. And you'll need ferrules on the ends as fine strand doesn't play nice with some connectors.

 

[Berkeloid]

Any thoughts on the 2/0 Battery cables?

How many amps can the inverter pull continuously? Assuming 12 kW at 48 V that's around 250 A. 2/0 cable is roughly rated for around 200 A on the safe side, and 280 A if you don't mind a fair bit of energy burned off as heat.

Assuming the original 100 A breaker is the full 12 kW (based on 12000 W / 120 V = 100 A) then if you're using a 60 A breaker instead, you'll limit yourself to 120 * 60 = 7.2 kW max before the breaker trips, so 7200 W / 48 V = 150 A max on the battery cables. 2/0 cables are plenty for that (in fact 1/0 cable will do that fine too, as its conservative rating is 150 A). Using 2/0 cable now though means you won't need to replace it if you decide you need more than 7.2 kW one day and switch to the 75 or 100 A breaker to achieve it.

As to varying the cable size, I'd still stick with the same size to join the two 4S packs in parallel. Technically the current will be split evenly between the two packs so you only need to be able to handle half the total current on each pack, however if you do that you limit yourself because as the packs age, one may deliver more current than the other, so it's no longer a 50/50 split. In an extreme case if you lose one 4S pack entirely, the other one will take up the full load. If you've used smaller cables, suddenly the cables on the remaining pack are overloaded and the fuses you hopefully used will blow (if the inverter is drawing enough current at the time, of course). This means one failing 4S pack could knock the whole system offline, even if the other 4S pack is working fine.

So if you stick to the same cable (2/0 in your original message) it means each 4S pack can take over the whole load of the system if the other pack fails or is out of service because you're working on it, giving you more flexibility and reliability.

 

[going_commando]

These cable will be single in air and the reality it is highly unlikely it will get even close to 100A considering all my loads on my inverter will likely max out close to 45A.

So maybe I'd be fine with the 6awg but switching to a 75A breaker. The whole point of the breaker is to protect the cable right?

So if my theory is correct I'll install a 75A breaker and if everything works we're good. But if not I should increase to a 100A breaker and increase the wire size too?

Cam

It is a code violation and very bad practice to run wires singly in free air from the inverter to the panel. They need to be either a cable assembly or in conduit. Free air ratings are for overhead aerial cable suspended on a steel messenger wire, not for inside a residence.

Riuxu obviously doesn't understand the NEC to call for a 100 amp breaker protecting #6 wire. The breaker is there to protect the wire, so must be sized to not overload the wire. That is straight up dangerous and negligent.

 

[chamilun]

is ac amperage different than DC amperage?

 

[Berkeloid]

There are some things you need to take into account for exact calculations (mostly around inductive loads and power factor, and of course conversion losses), but for back-of-the-envelope calculations you can treat them the same.

 

[rogerheflin]

How many amps can the inverter pull continuously? Assuming 12 kW at 48 V that's around 250 A. 2/0 cable is roughly rated for around 200 A on the safe side, and 280 A if you don't mind a fair bit of energy burned off as heat.

Assuming the original 100 A breaker is the full 12 kW (based on 12000 W / 120 V = 100 A) then if you're using a 60 A breaker instead, you'll limit yourself to 120 * 60 = 7.2 kW max before the breaker trips, so 7200 W / 48 V = 150 A max on the battery cables. 2/0 cables are plenty for that (in fact 1/0 cable will do that fine too, as its conservative rating is 150 A). Using 2/0 cable now though means you won't need to replace it if you decide you need more than 7.2 kW one day and switch to the 75 or 100 A breaker to achieve it.

I think that inverter is a 240v inverter. So that would make it closer to 50A@240 (so a 60A breaker) for 12kw of output.

 

[1201]

is ac amperage different than DC amperage?

Yes

 

[Backyard]

is ac amperage different than DC amperage?

When measuring the AC voltage in RMS then for practical purposes, No

If the peak to peak on the AC is the same as the DC then the AC will have an average less current. But most AC voltage is measured using RMS so it's essentially the same as DC

 

[woe]

How many amps can the inverter pull continuously? Assuming 12 kW at 48 V that's around 250 A. 2/0 cable is roughly rated for around 200 A on the safe side, and 280 A if you don't mind a fair bit of energy burned off as heat.

Assuming the original 100 A breaker is the full 12 kW (based on 12000 W / 120 V = 100 A) then if you're using a 60 A breaker instead, you'll limit yourself to 120 * 60 = 7.2 kW max before the breaker trips, so 7200 W / 48 V = 150 A max on the battery cables. 2/0 cables are plenty for that (in fact 1/0 cable will do that fine too, as its conservative rating is 150 A). Using 2/0 cable now though means you won't need to replace it if you decide you need more than 7.2 kW one day and switch to the 75 or 100 A breaker to achieve it.

If I'm not mistaken it's 60A at 240V . The user manual says max surge load for 10s is 17kw.

As to varying the cable size, I'd still stick with the same size to join the two 4S packs in parallel. Technically the current will be split evenly between the two packs so you only need to be able to handle half the total current on each pack, however if you do that you limit yourself because as the packs age, one may deliver more current than the other, so it's no longer a 50/50 split. In an extreme case if you lose one 4S pack entirely, the other one will take up the full load. If you've used smaller cables, suddenly the cables on the remaining pack are overloaded and the fuses you hopefully used will blow (if the inverter is drawing enough current at the time, of course). This means one failing 4S pack could knock the whole system offline, even if the other 4S pack is working fine.

This is an excellent point. 2/0 is the answer. Thank you

So if you stick to the same cable (2/0 in your original message) it means each 4S pack can take over the whole load of the system if the other pack fails or is out of service because you're working on it, giving you more flexibility and reliability.

 

[woe]

It is a code violation and very bad practice to run wires singly in free air from the inverter to the panel.

When I say free air, it's still going to be inside a wall. Just not bundled together.

From what I understand conduit is not required is cables are inside the wall.

They need to be either a cable assembly or in conduit. Free air ratings are for overhead aerial cable suspended on a steel messenger wire, not for inside a residence.

Riuxu obviously doesn't understand the NEC to call for a 100 amp breaker protecting #6 wire. The breaker is there to protect the wire, so must be sized to not overload the wire. That is straight up dangerous and negligent.

 

[Backyard]

When I say free air, it's still going to be inside a wall. Just not bundled together.

From what I understand conduit is not required is cables are inside the wall.

Only if it's a cable rated for such installation such as romex. And, to add, inside a wall does not qualify as free air

 

[obsoletion]

Only if it's a cable rated for such installation such as romex. And, to add, inside a wall does not qualify as free air

And romex is derated for current carrying...

 

[woe]

Only if it's a cable rated for such installation such as romex. And, to add, inside a wall does not qualify as free air

Thanks!