Inverter size vs battery size update?

[gmanInPA]

Sunking mentioned in one of the stickies about the relationship between inverter size and battery voltage - that 2000W should be the limit of 24V. I have two MPPT 80 CCs capable of 600 V at 80 Amps. My system was sized with a 4000 W inverter (Schneider XW4024). I told the vendor I wanted to stick to 24V and this is what they pointed me to. The smallest cables I have from my power panel to my battery is a #2 (Amazon: Camco 47485 Red 3/8" Stud 36" Long 2-Gauge Marine Battery Cable and Lug Assembly Tinned) and those that came with the battery (1575 AH) which are #0 AWG and were already tipped by the manufacturer.

At present, I have a 4230W PV system, which means I could theoretically reach 176 Amps of charging. Most of the time, being a battery backup system, my inverter is doing the charging and not at very high amps, though it could do so if the SoC is low after running off batteries for some time. I also have a BTS which would allow the CCs and Inverter to adjust the charge to compensate for battery temp.

I would hope that someone selling systems such as mine was aware of maximum amps per wire gauge and that I was not sold a fire hazard. I'd also wonder why Schneider would make an inverter that supports 4000 W yet can operate at 24 V and also be UL listed.

All that aside - Is my setup problematic like this? Secondly, since this IS what I have, and I'm not eager to plunk down $6k on a 48V battery - would it be recommended to limit the amps of the CC charger based on wire gauge rather than the C/10-C/12 range?

[sensij]

I don't totally understand what question you are asking. The documentation in the manual (pdf page 44, printed page 2-12) for the inverter clearly suggests wire sizes and OCPD capable of handling 175 A. Yes, if your wire isn't sized correctly for your system, using soft limits to reduce your risk would be better than doing nothing at all. Using properly sized conductors is the better approach.

[gmanInPA]

sensij - the manual indicates the wires sizes necessary for the PDP - yes. However, based on how many CCs one has wired in, the amount of amps going through those wires could vary quite a bit, no? I have to 80 Amp CCs. Though not likely to happen, they're capable of 160 Amps of output. That is a lot of heat of course and I want to make sure that my battery wires not only match the PDP manual (which they do), but that they are also capable of withstanding the amps that could be put through them. That was one question. I am not an electrician, nor do I have the NEC committed to memory, so I'm just looking for some opinions / validation / invalidation of the battery bank wiring sizes. If I do have under-sized wires, there needs to be a come to Jesus meeting with a few people involved in making these recommendations, and some adjustments to my CC output for the sake of safety. If not, great!

Additionally, Sunking s assertion was that a 4000 W inverter with a 24 V battery bank was foolish (if I read that right). Since this particular inverter of mine is sold for 24 V configurations, I'm wondering if that assertion still holds true? Obviously Schneider doesn't agree, or only anticipates that someone will employ half of the inverter's capacity when using 24 V. Ultimately, I'm interesting in making sure nothing in my setup is riskier than it should be.

[SunEagle]

I believe the basic principal is if the inverter "wattage" can draw down the battery system faster then the chemistry allows it is too big.

For a 12v system I was told the rule of thumb was about 2 x the Ah rating of an FLA type battery is equal to 1 x the wattage of the inverter. (ie. 200Ah battery = 400 watt inverter).

It would be a different ratio for 24v and for either AGM or Lithium chemistry batteries.

[gmanInPA]

Thanks for the feedback SunEagle . My battery is 1575 AH - so twice that (3150) is between a 2000 W and 4000 W inverter. Realistically, my continuous watts when running off the inverter is < 1000 W, but it can peak close to 4000 for a few seconds. My inverter will handle peaks of 8000 W for brief periods, though I've never seen it reach 4000 and definitely not higher. At 4000 W, that would be a high discharge rate, but my 'normal' (without much attempt at conservation) load on the batteries is about 41 Amps. That doesn't seem like a very high discharge rate for the bank size - but what do I know. I am most concerned about the charging amperage with the battery cables.

[SunEagle]

Well a 41amp load on a 1575Ah system is very low. Depending on the battery chemistry if you stay within C/8 and C/12 for both charging and discharging you should be ok.

It all depends on the duration of the "brief" peak usage as well as what the Battery SOC is that can hurt you. At 4000 watts on a 24v battery is about 170 amps which is ~ C/9 and should be ok for a 1575Ah system as long as your wires are rated to handle that high amount of amps.

But if your inverter is given the chance to power 8000 watts even for a brief time it could hurt your battery system as well as overload your wires.

[sensij]

On the PV side, there are specific code requirements for wire size based on the Isc of the panels. Between the charge controller and the battery, the wiring should be sized to handle the full rating of that charge controller, generally 125% of the continuous rating. If you elect to use smaller wire (possibly not code compliant), as long as it is protected by a properly sized fuse or breaker, the risk is reduced.

If the two CC's are combined into common wiring, that section of wiring needs to be sized for the combined 160 A. If both are connected directly to the battery, then 80 A is appropriate.

Similar logic on the inverter. If you don't intend to use 4000 W continuously, and have smaller conductors, it isnt really right, but at least make sure the ocpd is sized for the conductors. Different types of fuses have different blow curves that may be tolerant to surges, but voltage drop may screw it up off you try to go too small for too long of a run.

[gmanInPA]

I have zero objections or interest in getting away with small wires. I think people doing so is often being penny wise and dollar poor. These systems are already expensive, so a few extra bucks on over-sized conductors is no big deal. Yes, they're harder to work with - I know.

My entire system was pre-wired - the CCs being wired inside of blast-proof conduit from the CCs into the PDP (something about the DC being in the same space as the AC). Since it is shielded inside, truthfully, I am unaware of what size wired was used from the CC to DC bus bars/fuse. I am going to confirm with the company that did the pre-wiring, though I'm confident they stuck to both NEC and Schneider's specs.

If I am getting north of 3000 W, it is because someone in the family is unaware that we're "off-grid" and has unwittingly used the toaster or microwave which earns them some barking from myself when that happens. Otherwise, unless everything happens to kick on at once (well, fridges, freezer) - it stays pretty low - usually between 600 W and 1200 W. I am trying to get the amp rating on one section of my battery cable that goes from a 300 A disconnect switch to the positive bus bar in my PDP. If that proves to be undersized (2 guage), I will be replacing it with 1/0 (170 Amps according to my electrician).

Anyone have any recommendations for an appropriate terminal coating/sealer? The Schneider manual mentions covering the terminal connections in such for insulation - and that is NOT currently done in mine.

[Sunking]

Additionally, Sunking s assertion was that a 4000 W inverter with a 24 V battery bank was foolish (if I read that right). Since this particular inverter of mine is sold for 24 V configurations, I'm wondering if that assertion still holds true? Obviously Schneider doesn't agree, or only anticipates that someone will employ half of the inverter's capacity when using 24 V. Ultimately, I'm interesting in making sure nothing in my setup is riskier than it should be.

Assertion is made on 100 amps of current is the technical limit of DIY capabilities. Above that really requires a professional to properly terminate cables. Even if terminated properly require frequent checks.

Second part is a battery chemical capabilities. Anything higher than C/8 charge/discharge currents the batteries do not react fast enough.

Based on those two limitation 100 amps limits 12 volts 1000 watt, 24 volt at 2000 watt, and 48 volt at 4000 watts. Sure a manufacture can make a 4000 watt 12 volt inverter, but a battery that can deliver 400 amps and cables to handle that much current is extremely dangerous at a DIY level .It is the very same premise utility's do not use low voltage. For every 100 amps requires 800 amp hour minimum battery capacity. 100 amps at any voltage is dangerous.

[gmanInPA]

Sunking lives! Was starting to wonder if a meteor hit Central America for a little bit there...

I understand your point about DIY connections. It is definitely scary to consider being close to that level of amps. Or, for that matter - even having that kind of current flowing through some spaces. In my particular case, I do have professionally terminated cables and they were installed according to the torque values supplied by Schneider. One thing that was not done that I do want to have done is to cover those terminal connections with some sort of insulating material. Schneider mentions this in their manual - but not specifics. I've asked my electrician for advice and am waiting for a response.

If I have a battery bank of 1575 AH, and I charge at C/10-12, I shouldn't be in the bad chemistry zone you mention, correct? That IS higher than 100 amps mind you, but is all on appropriately-gauged wires for the current.

Aside from the 100 Amp limit, do you see any issue with this 4000W/24V inverter setup - understanding that is is NOT a DIY install? I did some of the install myself - but not these parts.

[Sunking]

Sunking lives! Was starting to wonder if a meteor hit Central America for a little bit there...

Nope just a little vacation in the land of Fruits and Nuts, you know Calaphonie with some buddies playing golf from San Diego to Monterey

I understand your point about DIY connections. It is definitely scary to consider being close to that level of amps. Or, for that matter - even having that kind of current flowing through some spaces. In my particular case, I do have professionally terminated cables and they were installed according to the torque values supplied by Schneider. One thing that was not done that I do want to have done is to cover those terminal connections with some sort of insulating material. Schneider mentions this in their manual - but not specifics. I've asked my electrician for advice and am waiting for a response.

If I have a battery bank of 1575 AH, and I charge at C/10-12, I shouldn't be in the bad chemistry zone you mention, correct? That IS higher than 100 amps mind you, but is all on appropriately-gauged wires for the current.

Aside from the 100 Amp limit, do you see any issue with this 4000W/24V inverter setup - understanding that is is NOT a DIY install? I did some of the install myself - but not these parts.

Let's see if I can shine some light on the subject. When it comes to manufactures, do not trust any of them. You can buy 5000 watt 12 volt Inverters any day of the week. Just like weed, tobacco, and booze. Just because it is legal and legit does not mean it is safe or a good practice.

One great example is Manufacture Wire Size recommendations. Inspectors and engineers don't give a damn about what the manufacture recommends. Nor does the NEC. Two important facts most manufacture ignore.

1. NEC requirements. This pertains to the wire Insulation type (temperature), number of current carrying conductors, and raceway types. Example a 1/0 maximum current rating is as low as 125 amps for a 60 degree insulation in conduit, and up to 260 amps for a 90 degree insulation in Free Air.

2. Minimum Voltage Loss Requirements. In low voltage systems, voltage loss is a serious requirement. Total loss is to be minimum of 2 to 3% overall all. That means 1% between Panels-CC, CC-Battery, and Battery-Load. 1% of 24 volts = .24 volts. That is a tough requirement to meet. To do that means you really do not even use NEC, and forget about manufactures. A 1/0 with 100 amps is more than safe, but not worth a damn if you loose 5% of your power. Typically if cable runs or less than 10 loop feet NEC minimum works. Beyond that you have to calculate. When you do that the Manufacture and NEC may say 1/0 AWG is good, but to get down to 1% may require 4/0 AWG.

So stop and think about there questions for a moment:

What is the highest Amp Rating Controller you can buy?
How much power is that at the given battery voltage?
What is the largest conductor size Joe Home owner can terminate with auto parts store tools?
What is safe?

Well hopefully these numbers might mean something when you answer the questions, In no particular order: 6, 80, 2000, 1000, 4000.

Do any of those numbers mean anything after answering the questions?

So can you run 4000 watts at 24 volt battery? Yes if you know what you are doing, obsessive with preventative maintenance and willing to accept the risk. I am a pro, and I would not do it.

Having said all that I compliment you on asking the right questions, and buying manufactured cables. Just understand the risk and what you are asking for.

[gmanInPA]

Thanks for the answer Sunking. I have the equipment that I have, and for the present, need to work with it. I am totally fine with putting in the largest wiring I can - and tend to do that where possible anyway - because frankly, I don't want to just meet code - I want to prevent fires or equipment damage. I will probably replace one 3' cable with a larger cable just to be as safe as possible.

90% of the time, I am running nowhere near 100 AMPs from/to the battery - but at times it can and does - though the duration is probably not extensive.

Without a new battery and inverter, not sure I can do much about it other than use the inverter/CC settings to limit the charging amps - but then I'd be below C/10-C/12 charging, which would just expedite the demise of the battery.

If a 1575 AH battery bank ideally charges at ~130-150 Amps... despite the voltage, that is what I need to put into it, right?. I was under the impression that the constant current was the key - not constant voltage - at least for bulk charging.

Despite the inverter size and voltage, the charge current needed is based on the battery, right?

So let's say I did have a 48V system, I'd be only able to charge in the 90 Amp range if I had 4500 W inverter. I'd get slightly higher - around 100 Amps if I had the Schneider's 6000 W inverter. The only way to get the amps needed to charge the battery at C/10-12 with a 48V system is with an 8000 W inverter, but even there, I'd still be at the same charging amps as I am at 4000W/24V, correct? Same amount of heat on the wires as half the voltage and watts.

Since my continuous draw is ~800-1200W, having an a larger inverter seems like quite a waste - especially because to charge the battery still requires the same charging current and therefore heat.

Where I do see it making sense is the inverting from batteries... If I normally pull ~1000 W at 24 volts (~40 amps), halving that with higher voltage would definitely be nicer.

What I am I missing here...? Sorry if it is obvious and I'm just dense/

[sensij]

If you switched to a 48 V system and kept the energy capacity the same, your Ah capacity would drop from 1575 to 787 Ah. Your new C/10 rate would be 79 A, within the 90 A available from the 4500 W inverter.

[Sunking]

If you switched to a 48 V system and kept the energy capacity the same, your Ah capacity would drop from 1575 to 787 Ah. Your new C/10 rate would be 79 A, within the 90 A available from the 4500 W inverter.

Very good.

[Sunking]

What I am I missing here...? Sorry if it is obvious and I'm just dense/

I was trying to lead you into the math and connect the dots which Sensi hinted at.

For example using an 80-Amp controller, the largest you can buy, requires a 6 AWG cable between CC-Battery, and Battery-Inverter (Assuming 5-foot one-way runs) when apply the logic behind what the manufactures came up with. So if you use an 80 amp controller, the maximum Panel and Inverter wattage for Battery is:

12 volts @ 1000 watts, Battery AH range = 640 AH to 1000 AH
24 volts @ 2000 watts, Battery AH range = 640 AH to 1000 AH
48 volts @ 4000 watts, Battery AH range = 640 AH to 1000 AH.

Now connect the dots with the batteries. You have a 24 volt 1575 AH battery right. That also equals a 48 volt 787.5 AH. Examples:

Battery Watt Hour capacity = Battery Voltage x Amp Hours. So:

12 volts @ 2000 AH = 24 volts @ 1000 AH = 48 volts @ 500 AH. All = 24,000 watt hours.

What does all this tell you?

Answer is you should be operating at 48 volts. That means one single 80 amp CC, Max panel/inverter wattage is 4000 to 5000 watts, all using less expensive and easy to work with 6 AWG copper wire. You would have slept a lot better at night knowing it is safer and cost you a lot less money.

Does that turn the light in your head?