Is this an illustration of a crown vent? I don’t see what that has to do with an S-trap discussion.
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What does that mean? If you read the article this formula has nothing to do with the difference between a P-trap and an S-trap.
If the distance between the trap weir and vent is less than 2 pipe diameters it’s called a crown vent.
I’m still waiting for someone to come up with a real formula for what is a S-trap. It must come from a code book.
Yes. The discussion is about an island vent. S vent was added in later. My example shows one way to vent it. You could add an AAV or a circuit vent.
“ASSE 1051 - Type A devices may only be used as individual vents for a single fixture drain. ASSE 1051 - Type B devices may serve as branch vents for one or more individual vents, including the dry vent connecting to a common vent, wet vent or circuit vent.”
There isn’t a single way to do this. That’s all I trying to point out.
What modern code book is this from?
You could add a AAV but can you describe to me how to add a circuit vent to the OP’s illustration?
I was attempting to find a definition and contribute. I’ll be sure to follow the rules moving forward. SMH
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I’m looking for a specific definition of a S-trap from a code book only. Anything else is an opinion or speculation.
Not without seeing an iso
Well, maybe they need to inform us all!
This is all I see in the IPC so far:
1002.3Prohibited traps.
The following types of traps are prohibited:
- 1.Traps that depend on moving parts to maintain the seal.
- 2.Bell traps.
- 3.Crown-vented traps.
- 4.Traps not integral with a fixture and that depend on interior partitions for the seal, except those traps constructed of an approved material that is resistant to corrosion and degradation.
- 5.“S” traps.
- 6.Drum traps.
Yup it’s prohibited but was is the definition of a S-trap. Does it have specifics like a crown vent? Looking for code book only definitions.
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You got it.
Here is a close one: (from link)
It is not stated directly in the International Residential Code (IRC), but as a reference to a prohibited type of vent at P3105.3: “Crown vent prohibited. A vent shall not be installed within two pipe diameters of the trap weir.” Essentially, a trap arm that’s too short creates the equivalent of an obsolete crown vent
What is the minimum trap arm length? (howtolookatahouse.com)
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Crown vent Daniel 
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But I guess thats as close as we get with code, since it talks about 2x diameter.
Interesting stuff, glad you brought that up!
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Daniel it’s food for thought as the IPC and UPC have no definition for a S-trap but they will tell you it is illegal.
Just trying to get those that say it is or it isn’t an S-trap to think.
There’s a clear definition of a P-trap with dimensions in both code books.
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There is no definition in the plumbing code. IPC or UPC. S trap is just common lingo. It is basically a non U-shaped piping arrangement.
Beat me to it
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So in my opinion the OP picture does display an S-trap. Prove me wrong 
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So now you have me curious what you were taught
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Agreed. Was it an opinion? He was one of Illinois 5 state inspectors so I’m going to lean towards his teaching.