I think that
@sunshine_eggo applied the "doubling" factor to account for the "peak output" capability of the MPP units Inverters, which are rated at double the continuous capacity for up to 5 seconds. But I disagree with his "extra safety factor" caclulation in class-T fuse size, and your original question asks about using 2 fuses on MPP input "48v connections. (
@sunshine_eggo replied to that exactly that question, but I strongly recommend that you add three fuses on the battery links instead.)
My Summary: Wire size 1/0 is sufficient for battery cables to the "48v" and "grounding" busses. 175A fast-blow "class T" fuses should be installed on three BATTERY "+" cables into bus bars,
not the "+" cables from the bus bar into each of two MPP charger/inverter units. If wire distances are short, then you are probably good with1/0 wire throughout, although the cable lengnths are important in calculating "voltage drop power losses (the waste heat from "voltage drop" power losses will get pushed into into components, after being generated within the wires).
Gory details follow:
The concept (using only two class-T fuses on each of
two Inverter/Charger connections from the bus, rather than using class-T on each of
three battery packs is wrong. The general consensus among most diysolarfoum experts (and some BMS manufacturers) is that fast-response Class-T fuses are needed to protect the parallel battery packs instead, especially for the case when a remaining active pack (or multiple active packs) become overloaded due to a BMS shutdown within an individual disconnecting battery pack.
In configurations with parallel battery packs, the BMS units themselves have been characterized (by a manufacturer of a really good one) as being "too slow" to respond to sudden and excessive over-current situations created by a short circuit or 'excessive' high-current event. For example, the batteries are becoming highly discharged while running the 120-VAC inverter in a boondock situation: Since these parallel MPP units cannot be programmed to shut themselves off from low voltage before BMS shutdowns should occur, one battery BMS may terminate discharge current early - leaving the other two, or maybe even just one, to support possibly high (excessive) current by themselves, either burning out the BMS units or harming the battery cells.
Instead, each individual battery pack should be protected by a class-T fuse, according to the either (A) maximum capability of the BMS unit and cells which it is supporting, or (B) a lower "maximum peak current" which you want to use as a protection limit in a non-error situation.
Situation A: A 16s battery pack of 120Ah should be built with a BMS unit capable of either 150 or 200 Amps of continuous power, with the BMS configured to trip with an overcurrent fault at a 'lower-than-maximum' current setting. Subjected to significantly higher 'peak' current amounts, they can remain undamaged for periods between 0.1 seconds and perhaps 10 seconds, depending on the peak current amount.
IMO, it is unwise to subject a BMS to 'peak current' exceeding it's continuous rating by more than 50% for even one full second. You can start burning out individual chips within the array of chips used for output power management, and once you begin to loose them they fall into a cascade of failures, which remaining functional chips becoming more overloaded and more likely to burn out with each new failure.
Similarly, but with perhaps less serious timing constraints, the LFP cells themselves must not be used at high current for very long. Most cells are rated for 150% or 200% "peak capacity", in comparison to their continuous ratings - but those peak specifications are hardly ever documented with specific lengths of maximum run time.
With good 120Ah cells, and a BMS of size "150A" or "200A", the BMS will becomes damaged before the cells do. You would size per the BMS Peak capability without damage (I SWAG these values to be around 225-250A for a 150A BMS, around 325-350A for a 200A BMS, varying according to the make and model of BMS). If you have a "200A" BMS supporting only 120Ah cells, the usual 'peak current' cell ratio should probably be applied first, creating a lower limit (e.g., 120Ah cells being red-lined at 2.0x continuous a 120A current rating is only 240 Amps, rather than the maximum capability of the BMS).
Situation B: Situation A is the highest possible limit for a particular BMS-controlled battery bank. As with most configurations, your setup should be instead limited (to a lower value) by the calculating the maximum allowable peak current of your devices. The the peak draw from both MPP units (150A * 2 units = 300A, for up to 5 seconds) is divided among 3 batteries, or at most 2 batteries with one battery offline.
@sunshine_eggo and I agree the result for 2 batteries
actually running to be approximately 150A on each, for up to 5 seconds.
- - -
But the ampacity of your alternative wire sizes (1/0 already in hand, 2/0 under consideration) are shown in a table here:
https://www.powerstream.com/Wire_Size.htm. The site characterizes those figures to be "conservative". An actual calculation of voltage drop and lost power depends on your wire lengths, ampacity in non-chassis wiring also depends on the characterstics of the insulation. The wire losses are HEAT, and the heat being generated over short distances of insulated wire will largely be conducted to to the BMS and cells at one end, and into the MPP units at the other. None of those components like heat. Using the "transmission" ampacity figure, 1/0 wire offers 150A , but you have no intention of using them that hard for more than 5 seconds.
For individual MPP units, a single set of 1/0 wires can support that continuously with zero headroom - with this constituting only a short term peak, that's probably OK. If the distance to the battery bus is long, I would want to use either double wire sets or switch to single wires of larger size 2/0. That's primarily to avoid heating up the components at the end terminals, rather than preventing wire burnout.
However, the MPP documentation of DC battery terminal connections is really weird, and suggest that only a 4-AWG wire is possible (due to the maximum lug size). I definitely hope that is not the case.
The corresponding battery cable loading, with 3 batteries being shared, is 50A continuous and 100A peak. If one battery fails, the peak loading would only go to 150A each on the other two (with 75A maximum continuous current). Your batteries are capable of more current, but should never be asked to provide more than that (even for the case of one battery "offline"). In your configuration, current significantly higher than 150A on any single battery constitutes an error condition which should be shut down by blowing the fuse (immediately). 1/0 battery cable is sufficient for worst case, and of course more than adequate for the case of all 3 battery packs in operation.
The class-T fuses should be installed on the battery "+" cables, and they should be size 175A. More than 25A of headroom, above and beyone the fact that you have already allowed for one battery pack going offline, makes very little sense to me. This size is widely available (offered by Blue-Sea, Bussman, Littlefuse, and - of course - Mersen). Larger fast-acting class-T fuses cost a lot more, in most cases, and would basically allow levels of current which you do not want to support. I disagree (in tways) with
@sunshine_eggo's calculation, in which he first provides 25% more heasdroom for wire size but then adds another 25% to the fuse size. I feel that
the fuse should not exceed the wire capability, by any amount, and also feel that using
a very conservative 'transmission' ampacity limit might not be a realistic assessment of power handling in your configuration: The '150A' loading probably can't occur for more than seconds, and the copper wire (a great conductor of heat) will be dissapating heat to its terminal ends. 90-degree insulation on 1/0 cable is extremely unlikey to burn in that situation, and the cable won't become damaged. The fact that 3 batteries packs should be supporting the load simultaneously is another factor in allowing for 1/0 battery cable connections.
