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Help with Battery Cable and Class-T Fuse Sizing

Greg55618688

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Mar 16, 2021
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Hello,
Newbie here (not a trained electrician) but trying to learn and do a solar install on my retired ambulance RV. Probably my 1st mistake.
Sorry if this has been answered already. I’ve been reading but haven’t found answers yet.

Most Expected Loads: Watts Amp Approx Amp
Load Hours per Day
12 VDC
LED lights (max 10 lights) 0.5 0.417 2
IPhone/IPad 5
Cell booster 20 0.17 17

120 VAC
Single burner induction burner 1500 12.5 62.5
Micorwave 1500 12.5 41.5
Cpap (x2) 160 1.3 94
Laptops (x2) 3.4 .33 28
Mini Split AC/Heat pump 9000 btu 1000 8.5 400 - 600
Snomaster Dual Zone Fridge 66 0.55 30-40
______ ____ ___
Total 4250 36.37 890

Solar Components I have:
6-210 watt panels
Victron MPPT 150/60 Solar Charger
Victron Multi Plus Inverter 24/3000/70
Victron Lynx
Victron Orion DC-DC Converter
16-CATL 3.2v 271a prismatic cells in 8s2p 24v/560a
Electrodaucus SBMS0

Approximate length of positive cable from batteries to dual shunts (for SBMS0) will be 5.5 feet, with doubled up 1” x 5” x ¼” aluminium flat bar from shunts to disconnect switch, and 1” x 5” x ¼” aluminium flat bar tp Lynx.

Approximate length of negative cable from batteries to Victron BMV-712 shunt will be 5.5 feet. Victron shunt connected to Lynx with doubled up 1” x 3” x ¼” aluminum flat bar.

Approximate length of +/- cables from Lynx to Inverter will be 3.5 feet.

What gauge battery/inverter cable described above is required?
What size Class-T fuse should be used?

Does anyone have any concerns with this setup?

Thanks, Greg
 

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I like your clean installation and well thought out posting with requisite information.
 
I'd try to move the Multiplus to center in your compartment as it will exhaust air out the top on either side; one side of which in the picture is blocked as it's up against the edge of the compartment.

For cables from battery to Lynx, and Lynx to the Multi I would go 2/0 AWG (325A) - you could get by with 1/0 or perhaps even 1 AWG but not really a point as such a small cost factor over the other equipment in your build.
 
I’m going to put how I calculate fuses and wire size. This might not be to code, but will be more conservative meaning thicker wires and smaller fuses So less of a fire risk.

=====
I use this calculator to figure out voltage loss and try for between 1% to 3%. https://www.calculator.net/voltage-drop-calculator.html

I then look at an ampacity chart to make sure the wire can handle my highest expected constant load with 60 degree Celcius rating.


If you do that, you’re likely to come out to around 4/0 wire. Many will think that’s way to thick, but I don’t like the idea of skinnier wires that are rated for my system but are so hot you can’t touch them without getting burned.

========

For battery fusing, this has to do with your highest expected watt usage divided by the low voltage cutoff on your inverter and multiply this by this by 1.15 to account for DC to AC conversion losses, and multiply this by 1.25 to prevent nusiance trips. For fusing, I went more conservative than this and divided by 12 volts instead of 10.5 volts. This gets me a smaller rated fuse than using the low voltage cutoff. It could also lead to nuiscance fuse blowing, but I’ve never seen less 12 volts on my system when the batteries are running a heavy load and never more than 155 amps draw.

Another thing I check is that the wiring is rated for the fuse I put in. Not necessarily at the 60 degree rating, but could be higher. Fusing does need to protect the wiring.
 
Are your batteries going below the inverter? If so you might consider moving the distributor below the inverter-keep the high current cabling as short as possible. Even placing it sideways or upside down may increase the wiring "neatness" factor. Can the shunt be attached directly to the distributor-fewer connections are always better.........
 
The inverter is 3000VA continuous (less at elevated temperature), 6000VA surge. How long can it sustain 6000W?

Induction cooktop + microwave + A/C might be OK for 3 minutes, maybe not.
Best to have cooktop off while powering microwave. In hot weather, only one of the the loads at a time.


3000W/20V(low battery)/0.9(inverter efficiency) x 1.12 (DC ripple current) x 1.25 (avoid nuisance trips) = 233A
250A class T fuse is good. And as fat a cable as possible to minimize voltage drop and heat.

1200W PV. 13,000 Wh battery, max charge rate 0.09C

If PV gave high charge rate like 0.5C I would recommend setting low-temperature charge disconnect at a temperature well above 0 degrees C, because allowed charge current is reduced at temperature extremes. But your charge rate is low enough, just cut out below freezing.

Nice thing about Victron is it can use a shunt to measure battery voltage and regulate charge current, for instance if you had PV capable of 0.5C it could regulate down to 0.15C, but still deliver additional current when needed by inverter. Not necessary in your case, but good for people with smaller battery or larger PV array.
 
The inverter is 3000VA continuous (less at elevated temperature), 6000VA surge. How long can it sustain 6000W?

Induction cooktop + microwave + A/C might be OK for 3 minutes, maybe not.
Best to have cooktop off while powering microwave. In hot weather, only one of the the loads at a time.

https://www.solar-electric.com/lib/...eet_Multiplus_inverter_2kVA-and-3kVA-120V.pdf
3000W/20V(low battery)/0.9(inverter efficiency) x 1.12 (DC ripple current) x 1.25 (avoid nuisance trips) = 233A
250A class T fuse is good. And as fat a cable as possible to minimize voltage drop and heat.

Seems like the math is for a 24 volt system. Unless I’m reading wrong, OP has a 12 volt system.

I really don’t like the idea of running an induction cooker off a 12 volt system, even if it is the only thing plugged in. A microwave may be OK if you’re warming up, not cooking, food for one or two people a couple times a day. If the microwave turns into ten to twenty minutes at a time usage, then a 12 volt system is not so good. I have kids who are not as big fans as me about saving energy, especially when I’m not around.
 
Seems like the math is for a 24 volt system. Unless I’m reading wrong, OP has a 12 volt system.

Yes, I did it for 24V.

Some 12V loads listed, but ...

"Victron Multi Plus Inverter 24/3000/70
...
Victron Orion DC-DC Converter
16-CATL 3.2v 271a prismatic cells in 8s2p 24v/560a"

So 24V system, and DC-DC is probably for the 12V.
 
Elaborate post and tidy layour, nice! I remember the Multiplus manual giving specific recommendations about cable and fuse size. I don't immediately see reasons to deviate from those? FYI Victron also wrote a rather decent guide about wiring, https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf. Given that most of battery current will be caused by the inverter (?), I don't expect you'll get much advantage from a (much) larger total cross section for the battery cable.

1/4" for the flat bars sounds ok. Take care of the connections: aluminum is an excellent conductor yet aluminum oxide (which automatically forms on the outside) is an insulator, so the oxide layer has to be removed at the contact areas.

The main fuse is usually scaled such that it is the last fuse to blow (hence higher rating than all other fuses) yet it should still blow before any damage occurs. It is thus sized to the components (wiring, switches, shunt) between the battery and the fuse box (Lynx in your case).

Other comments: your main switch looks somewhat small. Might want a bit more solar Watts for running airco. Not immediately clear to me how you'll route the cable from the MPPT to the Lynx. For your battery configuration, probably 2P8S instead of 8S2P (parallel connection first)? I'm probably wrong but the washer on the Lynx positive looks like a galvanized one. Not a fan of those as they tend to corrode quickly, I prefer stainless 304 (A2) or 316 (A4).

Looks like a very nice system in the works. Please post a picture when it's finished!
 
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I would be at 150 amps max on a Class T fuse. Will handle 2x rating for over 30 seconds to cover surge performance.
 
Hello, the old newbie here again.

Thank you all that responded. I am always amazed that so many take the time and effort to help out complete strangers.

I took a few suggestions from the previous responses and I’m in the process of rearranging the Lynx and Multiplus inverter to make shorter battery cable length runs. I also reduced the length of several aluminum bars between the shunts-shut off switch-lynx.

Now the approximate length of positive cable from battery bank to dual shunts (for SBMS0) should be about 3.5 feet, with two 1” x 3.5” x ¼” thick aluminium flat bars (doubled up) from shunts to disconnect switch, and two 1” x 3.5” x ¼” aluminium flat bars (doubled up) to the Lynx.

Now the approximate length of negative cable from battery bank to Victron BMV-712 shunt should be about 3.5 feet. The Victron shunt is connected to Lynx with two 1” x 3” x ¼” aluminum flat bars (doubled up).

Now the approximate length of the positive and negative cables from Lynx to Inverter should be about 1,5 to 2 feet.

I have enough 2/0 cable for the battery bank to Lynx and Lynx to Inverter runs. I’m thinking of running double 2/0 cable runs. Does anyone have any thoughts or concerns with this?

I've tried, but I’m still not confident in determining the correct size Class-T fuse for this system. Would a 400a Class-t fuse be correct for this system?

.
I provided this system information again with this post because it lost some of its format/structure when it was pasted into the initial thread and I didn’t notice it before.

(not likely all loads would be running at same time, but just in case)

Most Expected Loads: Watts Amp
Load
12 VDC
LED lights (max 10 lights) 0.5 0.417
IPhone/IPad
Cell booster 20 0.17

120 VAC
Single burner induction burner 1500 12.5
Micorwave 1500 12.5
Cpap (x2) 160 1.3
Laptops (x2) 3.4 .33
Mini Split AC/Heat pump 9000 btu 1000 8.5
Snomaster Dual Zone Fridge 66 0.55
______ ____
Total 4249.9 36.37


Solar Components I have:
6-210 watt panels
Victron MPPT 150/60 Solar Charger
Victron Multi Plus Inverter 24/3000/70
Victron Lynx
Victron Orion DC-DC Converter
16-CATL 3.2v 271a prismatic cells in 2p8s 24v / 560a
Electrodaucus SBMS0



Thanks, Greg
 
Double runs of 2/0 is good to reduce resistance.
If maximum continuous current exceeds ampacity of one 2/0 wire, you run the risk of bad connection in one overheating the other.
Separate fuse per wire would protect it, but gets expensive.
Parallel runs of 2/0 or larger are common for commercial wiring.
You might be able to use 4/0.

Aluminum is a good conductor, but native aluminum oxide causes high resistance and overheating.
Busbars ought to be tin plated aluminum (as found in many breaker panels.)
If DIY with bare aluminum, brush off native oxide and apply oxide inhibitor. Oxide forms instantly so I would scrub a second time after applying inhibitor, wipe off to remove particles, re-apply.
Avoid bare copper on aluminum, better that at least one is tinned.
Find correct torque specs.
Check voltage drop under high load (will show up as heating after a while, but voltage shows up sooner.)

I presented math earlier that said 250A class T fuse was necessary and sufficient. Unless something has changed.
400A is OK if the conductors have sufficient ampacity. But a single 2/0 would not.
 
120 VAC
Single burner induction burner 1500 12.5
Micorwave 1500 12.5
Mini Split AC/Heat pump 9000 btu 1000 8.5

Victron Multi Plus Inverter 24/3000/70

I would fuse the battery by inverter wattage (3000), divided by low voltage cutoff (21) Multiplied by inverter inefficiency of 1.15, and a nuisance3000 tripping factor of 1.25. I get 205 amps, which to me is close enough for a 200 amp fuse.
 
I would fuse the battery by inverter wattage (3000), divided by low voltage cutoff (21) Multiplied by inverter inefficiency of 1.15, and a nuisance3000 tripping factor of 1.25. I get 205 amps, which to me is close enough for a 200 amp fuse.

After observing how high ripple current from battery is, I now recommend an additional 12%, factor of 1.12. That brings it to 225A fuse.

When same power comes from battery with ripple following the AC current delivered, (mean) average current is the same but RMS current is about 12% higher, causing more heating of the fuse.

 
[...]

I have enough 2/0 cable for the battery bank to Lynx and Lynx to Inverter runs. I’m thinking of running double 2/0 cable runs. Does anyone have any thoughts or concerns with this?

I've tried, but I’m still not confident in determining the correct size Class-T fuse for this system. Would a 400a Class-t fuse be correct for this system?

[...]
Victron MPPT 150/60 Solar Charger
The 150/60 should have a fuse between 70 and 80 A according to the manual.
Victron Multi Plus Inverter 24/3000/70
[...]
For the 24/3000/70 Victron specifies a single 50 mm^2 cable for runs up to 15 ft (at least according to the 230 V version manual, but the DC current is the same for the 110 and 230 V version). 2/0 AWG is about 70 mm^2, so a single 2/0 cable should do. Double runs are better of course, but I doubt you'll see much difference given that your runs are rso short. Concerning the fuse, the manual specifies 300 A. If you do a double run, then use two 150 A fuses, one for each run.

Regarding the battery main fuse: a 400 A battery main fuse implies that a current of slightly under 400 A can flow continuously. This would be problematic for the main switch, which appears to only have a 300 A continuous rating (https://www.bluesea.com/products/6004/Single_Circuit_ON-OFF_with_Locking_Key_-_Red). Therefore, a 300 A Class-T battery main fuse appears more appropriate.

However, this means that the main fuse may trip before the inverter fuse (as both are 300 A). If you use a heavier battery main fuse, then the main switch should be upgraded. Alternatively, you can try using a lower rated inverter fuse, for example 200 A (or a pair of 100 A ones if double runs). 200 A at 24 V should be sufficient given that the 12 volt version can do with a 400 A fuse (double voltage <=> half current).
 

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A 3000VA MP sustains 2400 watts at 25 degrees C, not 3000. Also, give it 10cm of room on all sides (per documentation); notably your left side has no breathing room.
The manual calls for 300 amp fuse on the inverter with a 50mm^2 (1 AWG) cable if you're under 5m cable length (otherwise 2 cables).

(yeah, what @kgol just posted)
 
Hello, old newbie here again.

Thank you to everyone that replied to my novice questions, much appreciated.

I feel more confident with the battery wiring and class-t fuse recommendations and suggestions.

Would it be safe/acceptable to install the T fuse connected directly to the doubled up aluminum bar before the positive connection to the Lynx instead of with a class-t fuse holder (additional space for this is possibly not available )? This would also eliminate additional cable/bar connections with resistance.

Thanks, Greg
 
only caution that the fuse is not structural and to lay the thick heavy cable carefully to minimize physical loading on the fuse to reduce strain
 
Hello, I should have explained clearer. I'm wondering if bolting the T fuse between sections of the already existing aluminum bar currently being used before or after the battery cutoff switch to the Lynx terminal would be okay to do

Thanks, Greg.
 
Hello, old newbie here again.

Thank you to everyone that replied to my novice questions, much appreciated.

I feel more confident with the battery wiring and class-t fuse recommendations and suggestions.

Would it be safe/acceptable to install the T fuse connected directly to the doubled up aluminum bar before the positive connection to the Lynx instead of with a class-t fuse holder (additional space for this is possibly not available )? This would also eliminate additional cable/bar connections with resistance.

Thanks, Greg
I think the cable between the battery and the main fuse should be as short as possible and therefore would not put the main fuse at the Lynx. The idea behind this is that if you put the fuse at the Lynx then the positive cable from the battery to the Lynx remains unprotected. Suppose that you have some chafe in the positive battery cable and that it shorts with the metal chassis (I assume that there is quite some metal around in your van and that it is connected to the battery negative), then you'll get pretty nice fireworks because this short circuit current does not go through the main fuse. Therefore, I would put the main fuse close to the battery such that the positive cable between battery and fuse is short (< 1 ft or so).

I wouldn't bother too much about resistance. If you use short 2/0 cables, as you wrote in a previous posts, and your connections are well executed (lugs well attached, contact surfaces sanded/cleaned, see what Hedges wrote), then the losses will be minimal, especially as you have a 24 V system.

I don't think a dedicated fuse holder takes up that much space, especially as the fuse holder can help support the aluminum bars (?). Thus I would try to use a dedicated class-t fuse holder. However, if you put the main fuse close to the battery, then it probably won't be connected to the aluminum bars.
 
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