What is a six-unit trap?

I'm remodeling and adding a second floor bathroom. King County (Seattle) plumbing code says that a 3" stack can accommodate 48 DFUs, which is more than enough for me. However, there is a footnote that states:

"Only four water closets or six-unit traps allowed on any vertical pipe or stack..."

I only have 2 toilets connected to the stack. But I'm confused by "six-unit traps". At first I was reading this as limiting you to "six trap arms" connected to the stack (which would be more than I have planned to connect to the stack -- if you count the toilets). But, now I'm thinking that a "six-unit trap" is something else altogether (like a 4" trap?) and hopefully I don't have to worry about it. Google wasn't much help so I thought I'd ask here.

Anyone?

Here's the reference (see footnote "d"):
http://www.kingcounty.gov/healthservices/health/ehs/plumbing/unitvalues.aspx

Thanks.

A six-unit trap is a 3" trap.
Your shower uses a 2" 4-unit trap.

Size of Trap and Trap Arm, Drainage Fixture Unit Values (DFU)

702.1 for Maximum sizing.

1-1/4" 1 unit
1-1/2" 3 units
2" 4 units
3" 6 units
4" 8 units

From Table 7-3 of the 2009 UPC

Thanks for the response, Terry. But I'm still a bit confused, because the first table on the King County page I referenced lists a toilet as 3 units and a shower as 2. What am I missing?

Is there a difference between "fixture units" (which the King County page is defining) and "trap units"? If so, I appreciate the values you provided, but does King County have that documented somewhere? Or is it just in the UPC?

Thanks.

I don't know why you're making this so confusing for yourself.

There is no unit-trap, no six-trap, no whatever else. Only fixture units.

The chart comes from the 2009 UPC Code book that Washington State uses.
I don't know if Canada uses code books, but since the homeowner is Seattle_Steve and has listed the King County web site for information, this will be good information.

You don't have any 3" p-traps in your home, so I wouldn't worry about it. You normally only see a 3" trap in a commercial situation.
These are max limits for the trap sizes.

Ok, thanks Terry. That's good enough for me. (Yes, I'm in Seattle.) Thanks.

in the "old days" toilets WERE a 6 unit fixture, but with the low GPF, it has been reduced.

Ah, now I get it. That totally makes sense hj. Thanks!

What they are saying is that toilets and showers are "excpetions" to the general rule. They are a larger trap, but with typical volume much less than an application using a 2", or 3", trap in a "regular" way. Just for example, a washing machine places a much bigger load on a 2" trap than does a shower. A shower, nominally, delivers 2.5 gallons per minute. A washing machine can potentiall deliver 10 times that amount