Choosing a feeder cable from solar shed to off-grid cabin

Question

I have questions regarding electrical feeder sizing for an off-grid solar system. I am building a purpose-built shed that will support the solar panels and house all of the equipment such as charge controller, batteries, inverter etc. An A/C feeder will run in conduit 18" below ground for 140 feet from the main panel in the shed to the subpanel in the cabin.

The specs of the Magnum MS4448PAE Pure Sine Inverter/Charge that I am using are:

• Continuous power: 36A @ 120V or 18A at 240V
• Surge power (5 sec): 71A @ 120V or 35A at 240V
• 1 msec surge current (amps AC): Line-Neutral: 120, Line-Line: 70
• 100 msec surge current (amps AC): Line-Neutral: 75, Line-Line: 40
• 5 sec surge power (real watts): 8500
• 30 sec surge power (real watts): 6000
• 5 min surge power (real watts): 5400
• 30 min surge power (real watts): 4800
• Continuous power output at 25 C: 4400 VA (L-L)

I intend on running individual conductors, not an actual cable. I know about NEC Table 310-16 which gives the max amperage for each wire size, but I still have questions.

he owner's manual which has these statements:

• AC wiring must be no less than 10 AWG (5.3 mm2) gauge copper wire.

• The wire sizes recommended in this manual are based on the ampacities given in Table 310.16 (in conduit) or Table 310.17 (in free air) of the NEC, ANSI/NFPA 70,for 167°F (75°C) copper wire based on an ambient temperature of 86°F (30°C).

• The overcurrent protection device must be a circuit breaker or a fuse, and be properly sized and branch circuit-rated for the wire it is protecting and the appliances being powered. When in Standby mode, the full AC continuous pass-thru capacity of the MS-PAE inverter/charger is 30 amps for each AC leg (AC HOT 1 and AC HOT 2). To obtain the 30-amp continuous pass-thru capability, each AC HOT input to the inverter requires a 30-amp continuous duty rated double-pole breaker 1, which corresponds to a minimum cable size of #10 AWG 2 in conduit.

1. Should I wire for 1-phase or 3-phase? I do not expect any 240 V loads. But maybe the wire sizes for 3-phase are smaller and therefore cheaper in the end? And maybe 3-phase to future-proof?
2. Am I sizing based upon the surge current or the continuous? I would suspect the surge current.
3. Assuming 71 amps total, is each conductor sized to handle 71 amps?
4. Aluminum or copper?
5. Bonus question, what size breaker should be in the shed main panel for the feeder? The 30 amp breaker referred to in the owner's manual is for the input (pass thru).

Answer 1

Wire it up for 120/240, no difference in wiring: #10 AWG THHW and a double pole 30 Amp breaker in the main panel. Manual calls for copper wire only.

Answer 2

#1 you are asking about 120V, vs. 120/240V split-phase. 3-phase is not on the menu, you won't run into that until you get into industrial.

120V results in twice the voltage drop for the same amps. But since you need to double the amps to do the same work, it actually means four times the voltage drop! Having two banks of 120V loads balanced yields best results.

Split-phase gives you the option of 240V. Now let me ask you this: where are you going to get water from? Because ALL immersion well heads are 240V. That is because they have to run hundreds of feet down a well, and they'd rather have 5% drop than 20% drop!

#4 aluminum, no question. Where people get in trouble is they go "I use #12 copper for 20A, #10 copper for 30A, #8 copper for 40A, #6 etc." and they just get on autopilot like that.

Heck no! For the price of 10/3 copper, you can get 2-2-2-4 aluminum which is good for 90A. Why in blazes would you ever not do that?

#2 mooted by #4.

#3 mooted by #4.

#5 30A is more than plenty. If you look at the trip curves of the breaker and compare that to the inverter's short-time output, you see the 30A is more than tall enough. If the breaker needs to protect the inverter from overload, use a 20.

You will need as a bare minimum #10 copper wire (30A) , which is the same price as 2-2-2-4 aluminum (90A).