The general rule is one trap per fixture, but combination fixtures are treated as a single fixture when their compartments are close enough. Under the IPC, when may a combination fixture be served by a single trap?
- When one compartment is no more than 6 inches deeper than another and the waste outlets are no more than 30 inches apart
- BWhen one compartment is no more than 12 inches deeper than another and the waste outlets are no more than 60 inches apart
Twelve inches of depth difference and 60 inches between outlets are both roughly double the real allowances. The numbers are plausible enough that they catch anyone who remembers the form of the rule but not the figures.
- CWhen the sink is in a dwelling unit rather than a commercial kitchen
Nothing in this rule turns on whether the sink is residential or commercial. Commercial three-compartment sinks are the most common application of the allowance, so the distinction gets the exception backward.
- DNever, since each compartment of a multiple-compartment sink requires its own trap
Requiring a trap on every compartment would outlaw the continuous waste arrangement that every commercial kitchen uses. It also confuses the one-trap-per-fixture rule with a prohibition that does not exist.
Why A is correct
Six inches of depth difference and 30 inches between waste outlets are the two conditions, and both must hold. Within those limits the compartments behave as one fixture and a single trap serves them all, which is why a standard three-compartment sink with a continuous waste is legal on one trap.
What this question is testing
Whether the candidate can hold a two-part exception with two separate dimensions, and whether they connect it back to the general one-trap-per-fixture rule and the prohibition on double trapping.
On the job
Three-compartment sinks, mop sink and drainboard combinations, and multi-bowl laboratory sinks all live under this rule. It matters most when a fabricator delivers a sink with a deep soak compartment beside two shallow ones, which can put the depth difference over the limit and force a second trap. Checking the submittal against the two dimensions before the sink is set avoids reworking the waste piping in place.
Memory technique
Six inches of depth, thirty inches apart, one trap.
Exam tip
Exceptions with two dimensions require both to be satisfied. Check the depth and the spacing separately.
Where to look it up
Chapter 10, the fixture traps subsection and its exceptions. UPC differs on this point — follow the code your state has adopted.