Off grid greenhouse

Knotrusty

New Member
Joined
Feb 12, 2021
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15
Looks like I've got most of the components.
The plan:
24 volt system

Solar panels 37.6 Open circuit voltage 8.39 amps. 6 panels wired in series parallel = 112.8 V and 16.78 A

Growatt 3000 Max PV input 145 VDC

2 12v 200 ah batteries wired in series 24 x 200 = 4800 ah 4800 ah / 2 = 2400 usable.

I think I’m down to a DC circuit breaker from the panels to the inverter. A fuse between battery and inverter, a couple MC4 combiners and the battery cables. Not sure what size fuse I should use between batteries and inverter or DC circuit breaker. Also, should there be a battery cut off switch between batteries and inverter.

The help has been greatly appreciated and I hope to have this up and running in a few days. Thank you !!
 

Knotrusty

New Member
Joined
Feb 12, 2021
Messages
15
The plan:

24 volt system

Solar panels 37.6 Open circuit voltage 8.39 amps. 6 panels wired in series parallel = 112.8 V and 16.78 A

Growatt 3000 Max PV input 145 VDC

2 12v 200 ah batts wired in series 24 x 200 = 4800 ah 4800 ah / 2 = 2400 usable.



I think I’m down to a DC circuit breaker from the panels to the inverter. A fuse between battery and inverter, a couple MC4 combiners and the battery cables. Not sure what size fuse I should use between batteries and inverter. They recommend 1.25% of load. Just not sure what the load is for the inverter.
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,114
I think I’m down to a DC circuit breaker from the panels to the inverter. A fuse between battery and inverter, a couple MC4 combiners and the battery cables. Not sure what size fuse I should use between batteries and inverter. They recommend 1.25% of load. Just not sure what the load is for the inverter.

Continuous wattage rating / 24V / efficiency = max continuous input current
(efficiency might be 0.9, for instance.)
x 1.25 for minimum fuse size.

OK to round fuse up a bit. You never want it to blow (unless a wire gets shorted.)
Wire needs to be large enough that its ampacity exceeds fuse rating.

If wire is long, do the same calculation using peak wattage rating and determine wire gauge for 3% to 5% IR drop. But keep fuse size based on continuous wattage.

Often 2/0 is a good cable size.

What type of battery? Lead-acid can deliver 4000A or more into a short. For LiFePO4 the internal resistance specs suggest up to 20,000A although wire resistance could reduce that. Class T fuses are good (have 20kA interrupt rating) but some other types are good enough. You can also get DC breakers of suitable rating, e.g. Midnight Solar has 175A and 250A breakers.
 

Knotrusty

New Member
Joined
Feb 12, 2021
Messages
15
Thank you !!
So:
(3000/24/.90) X 1.25 = 173.6 Would a 175A fuse work or should I go larger ?

2/0 cable for the battery connections together and to the inverter.

The batteries are Renogy 12V 200AH AGM
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,114
Thank you !!
So:
(3000/24/.90) X 1.25 = 173.6 Would a 175A fuse work or should I go larger ?

2/0 cable for the battery connections together and to the inverter.

The batteries are Renogy 12V 200AH AGM


Specifications tab:
"Maximum Discharge Current: 2000A (5S)"
"Nominal Capacity: 200Ah@20hr-rate to 10.5V @25°C"

Repeat your math using 10.5 x 2 = 21V. That will bring it to about 200A fuse.
Is 90% efficiency the right value for your inverter? (Not "peak", but whatever efficiency at 100% load.)

2000A discharge at 21V would be 42kW, plenty of surge!

I expect its short-circuit current to be around 4000A or so.

MRBF is good for 5000 AIC at 32V, probably adequate. Higher priced class T is better.


2/0 with 90 degree insulation in 30 degree environment, single conductors in free air, is good for 300A


2/0 is 0.079 milliohms/foot. 20' total round-trip length 0.00016 ohms x 400A surge = 0.064V or 0.3% of 21V, quite low.
 

Knotrusty

New Member
Joined
Feb 12, 2021
Messages
15
Thanks again. I really appreciate you assistance. I'll post when I get it installed
 
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