When a load is supplied by a 2-pole breaker, the current leaving the source on one breaker pole will pass through the load, then return to the source via the other breaker pole. In one sense, it is the same current, so it makes no sense to add the two numbers. When you are dealing with 2-poles from a 3-phase system, the situation is not so easy. You have current leaving one pole with one particular set of timing for its positive peaks, its zero crossings, and its negative peaks. The current that passes through the load and returns to the source via the other pole will have the timing of its peaks and zero crossings ahead of (or behind) the other set by one third of a cycle (i.e., 120 degrees out of phase).
The way to do the math is to convert everything to units of KVA, then add them arithmetically, then convert back to current. For example, if on one 2-pole load you measure 18 amps, you count that as 18 amps times 208 volts, or 3744 VA. Do the same for all other loads on the panel, keeping clear whether you are talking about a single pole load (multiple amps times 120 volts), a 2-pole load (per my example), or a 3-pole load (multiply amps times 208 volts times the square root of 3). When you get a total VA, and if you want to calculate a ?total? current, you divide the total VA by 208 volts, then divide again by the square root of 3.
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