Pre-wiring for new construction
NBDrancher
Posts: 13Registered Users ✭✭
I'm curious what size conduit I should run and what wire size I need for a 40 foot run from a 3000 watt PV array/combiner box to a Four Star Solar Mini Magnum 4448PAE w/Classic 150 charge controller/48 volt battery bank. I'm preparing to pour concrete and want to do this right.
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The second thing to check for is voltage drop of the run... You may need heavier cable (and larger conduit). Again need array wiring details.
-Bill
With a Classic 150 on a 48 volt system, your string Vmp should be at least 75 volts, and your string Voc should be less than 120 volts. I guess you have 60 cell panels configured three to a string, with a Vmp of 90 volts and a combined Imp of 33.3 amps.
That voltage and current through a 40 ft run (80 ft of cable for the round trip) of 8 gauge copper, will give a voltage drop of 1.9%.
--vtMaps
The voltage drop app will ask you questions and then give you wire size. For instance, you'll need to tell the app how many volts and amps will run through this wire(s), acceptable voltage drop (use 2 or 3%), and length of wire run.
Once you have wire size, you then determine how many wires need to be run in the conduit. When you know wire size and number of wires, use the conduit fill app to size the conduit.
Do this, then consult with the experts here to confirm your results.
18 solar panels X 265 watts = 4771 watt array.
By the way, I did my earlier calculations based on a 3000 watt array because you wrote in the original post:
You still haven't answered the question about how the panels are configured. I will assume that you have chosen the most sensible configuration for 18 panels, which is six strings with three panels per string (string voltage = 93).
In that case, a 40 ft run of #6 copper will have a voltage drop of less than 2%.
--vtMaps
Thank you so much for taking the time to provide this information.
-Bill
For most areas, the maximum Vmp-array is around 100 VDC.
If you had a "cheap" PWM charge controller, your optimum Vmp-array would be ~72 VDC... This allows for Vmp to fall with high panel temperatures in the summer, and some voltage drop in the array wiring.
In the end, it is the battery bank that sets the operating voltage (input voltage range) for your inverter.
HOWEVER--there is always a however in engineering land--The operating voltage range of a 48 VDC flooded cell lead acid battery is around 42 volts to 59 volts for a room temperature battery bank under normal discharging/charging conditions.
Depending on battery construction and battery bank temperatures, charging (and equalization) voltages can reach 60-62 VDC (cold battery banks need higher charging voltages, equalization requires higher charging voltages).
And, many AC inverter mfg. seem to set their "cutoff voltages" to ~15/30/60 Volts (12/24/48 volt banks). And, especially for our friends in the great white north, they have problems with inverters shutting down in very cold weather (and during equalization).
AGM batteries do not need (and should not be subjected) to elevated voltages for equalization. Some industrial batteries (forklift, traction) seem to need elevated charging voltages as a matter of course.
So--The summary:
Your MPPT charge controller will make efficient use of the elevated Vmp-array charging voltage.
Your actual battery bank voltage will be set by Load, state of charge, Absorb, and Equalization voltage set points will be affected by temperature, charging phase, and battery construction. And 60 VDC, especially for cold climates and industrial FLA batteries, will be a real problem for folks with 60 VDC cutoff inverters (i.e., may have to shut down inverter during some charging phases).
-Bill
For starters let's pretend the controller is 100% efficient... then power in = power out.
In your case, Power in = 93 volts X 8.55 amps X 6 strings = 4771 watts
Power out depends on the battery voltage at the moment.
If the battery is at 58 volts, then 4771 watts ÷ 58 volts = 82.3 amps out of the controller. Of course the controller is not 100% efficient, so there will be less than 82.3 amps. The lower the battery voltage, the higher the current (amps) will be. That's why Bill referred to the MPPT as like a transformer.
If that works for you, I can't argue with success, but many folks have ruined batteries with a setup like that. Many 24 volt batteries require 29.5 volts for absorb and 31 volts for equalization. When panels get hot their voltage drops. If the panel Vmp is 31 volts at standard temperature, then it is several volts lower when hot. If your battery needs 29.5 volts and the panel's Vmp is 28 volts... there is a problem. And that does not take into consideration that the controller loses some of the voltage also.
--vtMaps
Bill-
I have to apologize for missing the obvious when it comes to the rate of charge to/thru the charge controller, not the inverter as I inadvertently communicated.
We do reach single-digits in the winter here and I have realized poor charging rates when the existing battery bank has been depleted overnight. My new installation will be inside a well insulated building, batteries in a plywood box vented to the outside.
another Bill-
As both you and Bill have alluded to-a balanced system must be achieved or as close to it as possible.