Is "margin of safety" defined/specified anywhere - as in, how would I know that four breakers totaling 160amp is ok, but what about four breakers totaling 200amp?

(For a panel with a single main breaker, I can see how this question is irrelevant. But for a split-bus panel, I can't see what prevents someone from significantly overloading the panel.)

I know that it's very unlikely for that much current to be flowing for very long, but still, doesn't the electrical code have all kinds of rules that address situations that are even *less* likely? I'm not an electrician, so I don't actually know the answer to that...

[Maybe this was something that was never addressed in the code, and since split-bus panels are now a thing of the past, it never will be! I wonder what I could point to if an electrical inspector were to deem this unsafe, due to the risk of overload. Oh well. Any electrician would simply say "replace the panel", but that's a lot to pay for something that may have no benefit, especially since a new panel would need to go in a new location to satisfy code, not the stairwell it's currently in.]

You do not do load calculations in most circumstances by adding up the breaker sizes. Even if you were to actually draw 160A all at once from the panel (near impossible) the panel and your service can handle it. Service cable ampacities are different than those in use for branches and feeders.

While split bus panels aren't common these days, they're not banned and there's no reason for an electrician to recommend replacement just because it is a split bus. Now some of the manufacturers who made these are suspect so certain of these I'd get rid of because the breakers are likely defective but that's got nothing to do with the split bus.

Ok. I'd just think that at least one of those breakers would need to trip before the total current through panel exceeded its 125amp rating, in order to protect the panel itself from an overcurrent situation. It seems analogous to me to a situation where you've got a (modern) main panel rated at 125amp, but you've put in a 160amp main breaker. Or a 20amp breaker on a circuit with 14-gauge wire.

The possibility for damage due to overcurrent is small, but real, and I'm surprised there's nothing specific in the code that addresses this. Maybe somewhere in the code there's a general rule to "follow the manufacturer's instructions", and the manufacturer addresses this total-"main"-breaker-rating question in the manual to the split-bus panel. That is, maybe the manual itself says something like "don't exceed 160amp in the top portion of the panel". If only I could get my hands on that 50-year-old manual...

I'm mostly just asking for the sake of curiosity. I'm really not concerned about an actual overload situation; just trying to figure out if/how the electric code addresses this specific issue.

Actually it is not analogous to that.

Panel loads are based on load analysis and some mathematics to compute what can safely be handled. And the fact that the loads in residential are not continuous.

Fine, I'll agree that it's not analogous, but only because the split-bus panel doesn't seem to have any physical OCPD that's designed to limit the current flowing through the panel. (In the 2 situations I said were analogous, there is an OCPD, by design; it's just not properly sized.)

Let me ask the question in a different way: What does it mean to say that a split-bus panel is rated at 125amp? What, specifically, does that mean I should not do, in terms of loads and breakers? What might an electrical inspector look at (again, in terms of the amperage of those top-section breakers) and say, "Woah - you can't do this!"?

What makes you think the panel is rated at 125A? Even main breaker panels can typically handle more than what the main OCD is rated at.

If you want to see if you're overloading your service or any particular branch circuit, you need to do a proper load analysis, which is largely more than we can explain in a post here. For most big appliances, you follow the appliance manufacturer's recommendation (or use 80% of the load specified). For lighting, you'll have to compute what the loads could be. For receptacles in residential use, there's no legal limit as to how many you can have.

So I ended up calling someone at Square D, and they talked through some of this with me, and as you've suggested, a load analysis really is the only way to be sure that there's no (or minimal) risk of an over-current situation. This leaves open the possibility that a homeowner or an un-skilled electrician could overload the panel relatively easily. To address this, and to simplify/eliminate the math burden of a load analysis, the 1981 NEC prohibited the installation of a split-bus panel without a main breaker ahead/upstream of the panelboard of a rating no greater than the panelboard's rating, with some exceptions. (NEC 408.36 addresses the issue currently.) However, for existing installations, installed prior to the 1981 requirement, a load analysis is the only way to ensure safety since there is no upstream breaker.

The NEC in 1981 and even now doesn't require that and 408.36 doesn't say that. You obviously missed "EXCEPTION #1"

So, you're saying that NEC 408.36doesn't "address the issue" (the language in my previous post) of overcurrent protection for split-bus panels? My reading of Exception #1 (which I most certainly did not miss) is that it's specifically addressing the question of split-bus panels in the general context of overcurrent protection. I'd even go so far as to say it "obviously" addresses the issue, but that would sound awfully snarky. :)

Is this what you are looking for?

230.90

Exception No. 3: Two to six circuit breakers or sets of fuses shall be permitted as the overcurrent device to provide the overload protection. The sum of the ratings of the circuit breakers or fuses shall be permitted to exceed the ampacity of the service conductors, provided the calculated load does not exceed the ampacity of the service conductors.

http://freenec.com/T57.html

I'm say Exception #1 allows split bus panels. Your assertion that they are illegal without another main upstream is not supported by the current (or prior) codes. A split bus panel that meets the definition of the service disconnect is still legal to install (though I don't know why one would).

I said that they were "prohibited ... with some exceptions". From my reading of the exceptions to 408.36, you can install a new split-bus panel only if:

1. there are no more than 2 main breakers (Exception #1); and
2. those 2 breakers have a combined rating not greater than the rating of the panelboard (Exception #2).

That 2nd exception is what protects the panel from an overcurrent situation.

In my current panel, there is no such protection; the combined rating of the main breakers (160amp) is greater than the panel's rating (125amp). Only a load analysis (or an experienced eye) would ensure that I'm not overloading the panel and risking some kind of meltdown (see my original post).