LifePO4 Cables to 12KW Inverter

[RobertGreen]

Here's info about certification: https://ramuk.intertekconnect.com/W...e5daea9b30cb7d3c862587ec0012f22c?OpenDocument

I am permitting this project and trying to do everything to code. I am trying to set this up in a legit way and not cut any corners or do things in a non-standard way. To me, bundling 3 wires seems non-standard.

Running conductors in parallel is explicitly allowed in the NEC at least for large conductors in certain applications where a massive enough single conductor would be impractical. See section 310.
You just don't run into it very often in residential settings.

 

[John Frum]

Yes, it is in the manual. But it makes no sense! What is "3*2AWG" supposed to mean? Surely this doesn't mean 3x of the 2 AWG wires combined together? That would never pass inspection. What size wire would be the equivalent of this because I'd much rather pay for that.

2 awg is roughly 35mm2
4/0 awg is 120mm2 which is over 3 times the combined diameter but it would not dissipate heat as well as 3 separate wires.
You could go to 300mcm which is 150mm2.

 

[UncleSam]

@UncleSam does the growatt have mechanical lugs or studs?
If it has mechanical lugs what is the largest wire that will fit?

12000 ac watts / .85 conversion factor / 48 volts low cutoff = 294.12 service amps
294.12 service amps / .8 fuse headroom = 367.65 fault amps
That means 4/0 awg pure copper wire with 105C insulation and a 400 amp fuse.
If it has multiple terminals you could do 2 * 2/0 awg.

The batteries should not be daisy chained.
4 awg with a 150 amp class t fuse will be fine if the are sensibly paralleled.

Thanks very much for the details. I've attached a picture of the battery connections on the growatt.

When you said 4 AWG were you referring to the battery to busbar wires? If so, why do these batteries come with 6 AWG wire (16mm^2)? Also, they have breakers on them as well and instructions make no mention of using a fuse I think. I definitely am putting a fuse on the positive line between the inverters and battery rack, and maybe on the negative too (my understanding is these cables neeed to be exactly the same).

 

[John Frum]

Thanks very much for the details. I've attached a picture of the battery connections on the growatt.

When you said 4 AWG were you referring to the battery to busbar wires?

Yes

If so, why do these batteries come with 6 AWG wire (16mm^2)?

The batteries are 100ah batteries and the cell data sheets likely say you can pull 100 amps continuous from them.
100 amps / .8 fuse headroom = 125 amps.
I would guess the breaker on the unit is rated for 125 amps continous.
That is just a smidge over the aybc rating for 6 awg pure copper with 105c insulation.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

I specced 4 awg and 150 amp fuses to just give a little breathing room.

Also, they have breakers on them as well and instructions make no mention of using a fuse I think.

Don't trust that breaker until @RichardfromSignatureSolar says it can withstand the dead short ampacity of the battery.
You need to supply the over current protection as close as possible to the positive battery terminal.

I definitely am putting a fuse on the positive line between the inverters and battery rack, and maybe on the negative too (my understanding is these cables neeed to be exactly the same).

The positives and negatives don't have to be exactly the same but all the positives need to be the same and all the negatives need to be the same.
One more thing... The inverters positive and negative should be attached to the opposite ends of their respective busbars.
This balances the resistance across the batteries.

About those batteries.
They are made of good quality blue prismatic LFP cells that are optimized for capacity over c-rate.
The data sheets usually say you can pull 1c continuous(100amps in this case) from a cell.
Typically the cells generate significantly more heat if discharged past .5c as ion starvation starts just above .5c.

 

[OffGridICF]

Each battery came with a 6 AWG wire, which as far as I can tell seems to be correct since I'm connecting these all to the busbar in parallel. However, I can't for the life of me find out what cable size I need for the batteries to the inverter. It sounds like I need to support 300A (and, from what I understand, a little more for safety). But this chart doesn't go beyond 200A: https://www.bluesea.com/resources/1437

In fact, most charts don't seem to. Am I missing something obvious here?

Ampacity of various wire gauges:
https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/

Area of various wire gauges, useful in comparing wires to bus bars or if paralleling wires:
https://www.engineeringtoolbox.com/awg-wire-gauge-circular-mils-d_819.html

More ampacity information, including some estimations for short vs. long wire lengths:

The 1 amp/ 700 Circular mils seems to be the most conservative, other sites provide/allow for 1 amp per 200 or 300 Circular mil. For shot wire lengths use 1A/200 Circular mil, for longer wire runs use 300 Circular mil, and for very long wire runs use the table above, 1 amp / 700 Circular mil.

http://www.interfacebus.com/copper_wire_awg_size.html

Chart for copper bus bar ampacities:
https://www.copper.org/applications/electrical/busbar/bus_table1.html

Interesting discussion on paralleling wires and the effect of surface area on heating:
https://forums.mikeholt.com/threads/ampacity-of-parallel-conductors.33720/

 

[UncleSam]

The batteries are 100ah batteries and the cell data sheets likely say you can pull 100 amps continuous from them.
100 amps / .8 fuse headroom = 125 amps.
I would guess the breaker on the unit is rated for 125 amps continous.
That is just a smidge over the aybc rating for 6 awg pure copper with 105c insulation.

Ahh, I just realized why they are 6 AWG. I looked at the wire and it says "200 degrees C 750V" so it should be more than enough since it is better than 105C rated wiring.

Don't trust that breaker until @RichardfromSignatureSolar says it can withstand the dead short ampacity of the battery.
You need to supply the over current protection as close as possible to the positive battery terminal.

I'll have to think about that one. If I can't trust the breaker then I'm not sure I can trust any other part of the battery. I have to have a basic assumption that these batteries will do what they are labeled to do, and my understanding is they are of good quality.

The positives and negatives don't have to be exactly the same but all the positives need to be the same and all the negatives need to be the same.
One more thing... The inverters positive and negative should be attached to the opposite ends of their respective busbars.
This balances the resistance across the batteries.

Thanks, yes I had heard about connecting at opposite ends of the busbar.

Ampacity of various wire gauges:
https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/

Thanks for the various links, OffGridICF. Unfortunately, I specifically need DC ampacity charts. That said, I was finally able to find a wire at 4/0 that actually says the AMP rating (405A when dry). I may not buy this specific wire but I think this will cover me:

4/0 AWG Black Multi Listed Flexible Wire THHW / TEW / MTW / AWM / BC-5W2 / SAE TYPE SGT / CT

Class K Flexing Service Wire with 30 AWG 100% copper stranding for superior flexibility. Multi listed building wire, welding cable, boat cable, battery cable.

store.polarwire.com

EDIT: Signature solar also sells 4/0 cables that appear to be 105C rated.

 

[UncleSam]

Running conductors in parallel is explicitly allowed in the NEC at least for large conductors in certain applications where a massive enough single conductor would be impractical. See section 310.
You just don't run into it very often in residential settings.

Just saw this. Interesting! Problem is wouldn't you end up having to put a t class fuse on each of the 3 conductors? Ouch.

 

[John Frum]

If I can't trust the breaker then I'm not sure I can trust any other part of the battery.

 

[UncleSam]

Don't trust that breaker until @RichardfromSignatureSolar says it can withstand the dead short ampacity of the battery.

Just a followup, here's what the manual for the LifePO4 batteries says:

● Short Current Protection While discharging, if the current exceeds the maximum continuous current rating by 100% and remains for greater than 1 second, the battery module output circuit breaker will trip and disconnect the battery module terminals to shut off discharging. The BMS will require a manual reset. Resolve the fault condition to avoid further shutdowns. The settings are shown in table 3.6

So the breaker will trip if it exceeds 100% of the max charging current. That's why I feel like it is over-engineering to add fuses for each battery.

 

[Roger Harding]

I have the same Growatt 12kw inverter and EG4LL batteries, but I have not been able to get the BMS communication to work, even though it worked fine with my previous 3000LVM ES inverters; have you had any luck there?

 

[UncleSam]

I wish I had better news, but I found out that I was not sent the DMV-US-MPV inverter. The sticker actually says DVM-MPV, so basically I was sent a non-US version which is not ETL certified and which is not meant for North America. Now it makes sense why the manual did not have any information about how to wire it for split phase output. Unfortunately I have to return it, and interestingly as a result of my phone call Signature Solar took down that product from their website. I've gotten very frustrated with the errors in the Growatt manual as well as on the product listing, I'm going to be switching to another brand. That said, I think the Signature Solar folks are great and really try to help you out, but they are in desperate need of some quality control when it comes to the product listings.

 

[Roger Harding]

I wish I had better news, but I found out that I was not sent the DMV-US-MPV inverter. The sticker actually says DVM-MPV, so basically I was sent a non-US version which is not ETL certified and which is not meant for North America. Now it makes sense why the manual did not have any information about how to wire it for split phase output. Unfortunately I have to return it, and interestingly as a result of my phone call Signature Solar took down that product from their website. I've gotten very frustrated with the errors in the Growatt manual as well as on the product listing, I'm going to be switching to another brand. That said, I think the Signature Solar folks are great and really try to help you out, but they are in desperate need of some quality control when it comes to the product listings.

Mine does not say US on it either, but it puts out 120/240 very nicely. As a trial run, I ran my entire house off of it for 12 hours with my main breaker turned off, with the inverter back feeding the panel. I had less flickering and other anomalies than my pair of SPF 3000ES inverters in split phase.

 

[UncleSam]

Mine does not say US on it either, but it puts out 120/240 very nicely. As a trial run, I ran my entire house off of it for 12 hours with my main breaker turned off, with the inverter back feeding the panel. I had less flickering and other anomalies than my pair of SPF 3000ES inverters in split phase.

I'm glad it's working for you, that's great. I'm sure it's still a good unit, but I personally don't feel comfortable if I'm not sure what it is I'm actually using. The attached PDF is the spec sheet that was linked to in the product listing. The specifications are vastly different from the actual unit (e.g., max AC input is "100A" but the unit itself says "70A" and has an 80A breaker. That's a big deal when you're trying to run your whole house. There's also no ETL sticker, because "DVM-US-MPV" is ETL certified but not "DMV-MPV". If you're trying to do a permit and get things inspected (as I am) then that's a huge problem.

 

[Stiffmeister]

I have a 12kw Outback Radian inverter. I'm running 2 AWG from four batteries to a Victron Lynx Distributor. I'll be running two pair of 4 gauge from the Lynx distributor to the Radian and two pair to my 7.2kw Flexpower. I never plan on draining anything close to that from the batteries at night. It's the best I can do based on the 22 mm max diameter output of the Lynx distributor. Only during the daytime for pool pump/heaters will I be using a lot of juice and that can be powered by the three 150/100 Victron charge controllers (connected to the Radian inverter).