No, you've got it. Unfortunately this is common when it comes to the various standards and online guides/calculators, especially at the 'boundaries' (e.g. the engineering toolbox table has 4AWG supporting up to 120A, whereas your expected load is 125A). The BlueSea calculator uses the ABYC (American Boating and Yacht Club) standards. Are these better or more specific to those used for the engineering toolbox tables? I don't know. I do know that the engineering toolbox tables tend to be more pessimistic, which means that you end up with a safer, better performing system. The manufacturer's datasheet for the cable being purchased should always be your authority.
Sounds good, the 25% is a ballpark figure, just something above your expected load (125A) else suffer nuisance activation.
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Sounds good, per point (2.).
@wrybread comment above is perfectly valid and anything above the required ampacity is basically safe so you are then making judgement calls based on potential future scenarios, cost/budget, aesthetics etc. Some people are simply not aware of the currents flowing in various system elements e.g. the PV-side current is almost always significantly lower than the battery-side of an SCC - but people think they should use the same cable and, sometimes, waste a lot of money, especially if their PV array is quite a way from the SCC.
Assuming you mean, "10AWG wire", sounds good.
OK
Absolutely not.
Like any cable, you should determine the fuse as the LOWER of (a) the ampacity rating of the cable and (b) the expected load +25%.
The Orion 12/24|15 (360W) has a nominal output of 15A but a maximum output of 25A, giving a maximum input of 50A (ignoring losses). So for alternator side (input) you should use a
minimum of 10AWG (according to
here) but you also have to also consider the length of cable from the alternator to the Orion AND the fact the cable is going to be inside a hot ambient temperature area. Assuming a 12' round-trip distance 8AWG cable carrying 50A will get you to a 3.14% voltage-drop (according to
here) but I would recommend taking that up-one (temperature) to 6AWG. 6AWG cable has an ampacity rating of 95A, expected load is 50A +25% = 62.5A, so fuse at 62.5A (or closest to it e.g. 60A.)
Battery side is at 24V and should be expected to carry a maximum of 25A so a
minimum of 12AWG cable would be required and, I'll assume only a short distance from the Orion to your battery, say 5' round-trip, so using 12AWG cable you already have a <3% voltage drop at 0.83%. 12AWG cable has an ampacity rating of 34A, expected load is 25A +25% = 31.25A, so fuse at 31.25A (or closest to it e.g. 30A).
Having the battery cable support the maximum current your battery is capable of delivering (without shutting down) makes a lot of sense for future-proofing your system, but it's a balance, I mean, 2/0AWG cable is more expensive than 1/0AWG cable. Your fuse selection should always be, however rated for your system as it is now, they can be upgraded later if needs be.
Edit: Corrected typo.
