... it would be quite acceptable to break gauge just for the measuring device...
Sure, but if you
do decide to open
one of the heavy current-carrying cables, you can insert into the cut cable a short length, say three or four inches (100mm), of three or four parallel-connected 10AWG insulated wires. Feed one or all of these smaller gauge wires through the hole in a Hall-effect current-measuring coil. You can pass just one, two, three, or all four 10AWG wires through the sensor coil. Leaving the remaining 10AWG wires outside the coil, the total current passing through the 0AWG wire will divide evenly among the smaller 10AWG wires if these wires are parallel-connected with a low-resistance connection technique.
To make a low-resistance transition from a single 0 gauge wire to several parallel-connected 10 gauge wires, I would get two pieces of copper buss bar, secured on opposite sides of the current-sensing coil (perhaps bolted to a plastic or Lexan polycarbonate base), and with cable clamps
* attached to secure the wires. Feed one, two, three, or four 10AWG through the Hall-effect current measuring coil. You could get by with passing just one of the wires through the Hall-effect current measuring coil, measuring one third (if using three wires) or one fourth (if using four wires) of the actual current in the 0 gauge wire. The remaining two thirds or three fourths of the total current flows through the two or three 10AWG wires that do not thread through the current measuring coil. You do need really good, low-resistance, parallel connections of the 10AWG wires at the copper buss bars if you don't want to, or cannot, pass all the wires through the sensor coil.
*Actually, I would go further and crimp a wire ferrule onto each wire. Drill holes in the buss bars to accept the ferrules. Drill and tap holes perpendicular to the holes just drilled to accept threaded machine screws, or bolts, or set screws, or whatever that will tighten against the ferrules. If using a threaded bolt, you could add a jam-nut to make sure it stays tight. Set screws are available with nylon inserts that help prevent them from backing out.
@dpuklicz Don't try to cheat and not use the ferrules, for example by tinning the bare ends of stranded wire with solder. Solder creeps under pressure and then will require periodic re-tightening of the connections to the buss bar to maintain low-resistance parallel connections. It's okay to tin the wire with solder
after insertion into the ferrule and
after the ferrule has been crimped (preferably with a "star" crimp). This extra tinning operation isn't necessary with a proper crimp on the ferrule. Always used stranded wire, never solid wire, with crimped connections, whether ferrules or ring, spade, or forked terminals.
As noted above, you can divide your 0 gauge cable into a short length of several parallel, smaller-gauge, cables for the purpose of measuring current. If you take this far enough, you could (for example) use the original Allegro current sensor suggested in post #2, except I would use one that is recommended for new designs and not mounted on a cheesy Chinese circuit board claiming to be adequate for 100 amperes. Look at this
150 ampere bidirectional model from Digi-Key. You can solder directly to the pins and insulate the soldered connections with shrink tubing. The three smaller terminals for +5V, Signal, and Ground can be twisted, stranded, insulated 24AWG hook-up wire several feet long connecting to the Arduino. No circuit board and no extra op-amps necessary for interfacing to Arduino analog input port.