FIXTURE-UNIT TABLES

Fixture-unit tables are often used when sizing drains and sewers. The table in Figure 4.1 is an example of a table that expresses the maximum number of fix­ture units allowed on pipes of various sizes and with various amounts of fall. Before we go on, look at the category for 3-inch pipe, at a V4-inch per foot fall. It says that you are allowed 27 drainage fixture units. But, notice the lit­tle number 2 next to the number of fixture units. That number indicates a note or exception. When you look at the bottom of the table, you will see that the note tells you that not more than two water closets can be carried on a 3-inch pipe. There are exceptions, but if you stick with this rule, you can’t go wrong. This is one of the tables that I was telling you about earlier.

Diameter of pipe		Fall in	inches per foot
(in)	‘/ні	%	‘/4	‘/2
2			21	26
2 Vi			24	31
3		202	272	362
4		180	216	250
5		390	480	575
6		700	840	1000
8	1400	1600	1920	2300
10	2500	2900	3500	4200
12	3900	4600	5600	6700
15	7000	8300	10,000	12,000

‘Includes branches of the building drain. The mini­mum size of any building drain serving a water closet shall be 3". “Not over two water closets.

FIGURE 4.1 ■ Allowable fixture-unit loads. (Courtesy of McGraw-Hill)

Fixture-unit value as load Minimum size factors of trap (ini

Fixture type

Bathroom group consisting of water closet, lavatory, and bathtub or shower Bathtub (with or without overhead shower) or whirlpool attachments Bidet Combination sink and tray Combination sink and tray with food disposal unit Dental unit or cuspidor Dental lavatory Drinking fountain Dishwashing machine domestic Floor drains Kitchen sink, domestic Kitchen sink, domestic with food waste grinder and/or dishwasher Lavatory Lavatory Lavatory, barber, beauty parlor Lavatory, surgeon’s Laundry tray (1 or 2 compartments) Shower stall, domestic Showers (group) per head Sinks Surgeon’s Flushing rim (with valve) Service (trap standard) Service (P trap) Pot, scullery, etc. Urinal, pedestal, siphon jet, blowout Urinal, wall lip Urinal, washout Washing machines (commercial) Washing machine (residential) Wash sink (circular or multiple) each set of faucets Water closet, flushometer tank, public or private Water closet, private installation Water closet, public installation

6 2 lVi 2 Nominal l’A 3 1 Vi 4 Separate traps 1 Vi 1 1% 1 l’A ‘A 1 2 lVi 1 2 2 l’A 3 lVi 1 Small P. O. l’A 2 Large P. O. l’A 2 l’A 2 l’A 2 l’A 2 2 3 3 llA 8 3 3 3 2 2 4 l’A 8 Note 6 4 Note 6 4 Note 6 3 2 2 Nominal 1 Vi 3 Note 6 4 Note 6 6 Note 6

FIGURE 4.2 ■ Fixture-unit ratings. (Courtesy of McGraw-Hill)

The information in Figure 4.2 is representative of what you might find in your local codebook. This is the type of table that assigns specific ratings for fixture units on given fixtures. In cases where a known fixture is not listed, an­other type of table, like the one in Figure 4.3, is used to assign ratings for fix­ture units. Before we get too many tables in front of us, let’s go over the three that you’ve just been introduced to.

The table in Figure 4.1 is easy enough to understand. If you find the size of the pipe you are working with, you can quickly ascertain the number of fix­ture units allowed on the pipe at a given grade. When you know the number of fixture units and the grade of the pipe, you can tell what size pipe is suit­able. For example, a 4-inch sewer that is installed with a grade of one-quarter of an inch per foot can handle up to 216 fixture units, and that’s a lot of drainage. Upgrading to a 6-inch pipe with the same grade will allow you to load the pipe with 840 drainage fixture units. That’s all there is to that table.

The listings in Figure 4.2 are compre­hensive and easy to understand. For ex­ample, a residential toilet is assigned a fixture-unit rating of four. A typical lava­tory has a rating of one fixture unit. Do­mestic shower stalls are rated for two fix­ture units. If you add this up, you find that the three normal bathroom fixtures total a rating of seven fixture units. How­ever, if you look at the top of the list, you will see that a bathroom group that con­sists of a toilet, lavatory, and bathtub or

shower has a rating of 6 fixture units. Wait a minute, that’s one fixture unit less than the individual ratings for the same fixtures. What gives? In this case, as­suming that all of the fixtures were being placed in the same bathroom, you could use the lower of the two ratings. Why? Because it is assumed that not all of the fixtures will be being used simul­taneously if they are confined to a single room. The use of a table, like the one in Figure 4.2, makes sizing drains a lot easier.

There may be times when the fixture that you are seeking a rating for will not be listed on a fixture-unit table. If this is the case, you can use a table, like the one in Figure 4.3, to assign a rating for fixture units. For example, a fixture with a 2-inch drain that is not otherwise listed would be

FIGURE 4.3 ■ Allowable fixture units based on trap size. (Courtesy of McGraw-Hill)

Maximum no. of fixture units that may be connected to:

Diameter of pipe" (in) Any horizontal fixture branch14 One stack of 3 stories or 3 intervals maximum More than 3 stories in height Total for stack Total at one story or branch interval VA 1 2 2 1 Г/І 3 4 8 2 2 6 10 24 6 2 Vi 12 20 42 9 3 20“ ЗО" 60” 16" 4 160 240 500 90 5 360 540 1100 200 6 620 960 1900 350 8 1400 2200 3600 600 10 2500 3800 5600 1000 12 3900 6000 8400 1500 15 7000 ‘Does not include branches of the building drain. "Not over two water closets. ’Not over six water closets. 4509f less for battery vented fixture branches, no size reduction permitted for battery vented branches throughout the entire branch length. ”The minimum size of any branch or stack serving a water closet shall be

3".

FIGURE 4.4 ■ Maximum fixture units. (Courtesy of McGraw-Hill)

rated for three fixture units. A 4-inch drain would carry a rating of six fixture units. Pretty simple stuff, huh? Some tables, like the one in Figure 4.4, deal with different piping arrange­ments. For example, the table in Figure 4.4 allows you to rate any horizontal branch stacks for multiple-story buildings and branch intervals. Notice that

Diameter of vent Number of wet-vented fixtures stacks (in)

1 or 2 bathtubs or showers 2 3 to 5 bathtubs or showers 2Vi 6 to 9 bathtubs or showers 3 10 to 16 bathtubs or showers 4

FIGURE 4.5 ■ Vent sizing table. (Courtesy of McGraw-Hill)

FIGURE 4.6 ■ Trap-to-vent distances. (Courtesy of McGraw-Hill)

several of the ratings are marked with exclusions. This is the type of detailed information that you must be on the lookout for.

Suppose you are concerned about sizing a vent stack that will accommo­date wet-vented fixtures? No problem, just use a table like the one in Figure 4.5. This table is so simple that it needs no explanation. Now, what if you need to know how long a trap arm may be? Refer to a table like the one in Fig­ure 4.6 for the answers to your questions. Depending on trap size, the size of the fixture drain, and the amount of fall on the trap arm, you can choose a maximum length quickly.

Take a look at Figure 4.7. It is a riser diagram of a branch-interval de­tail. It is sometimes necessary to break a drainage system down into branch intervals for sizing. If you need to do this, you can refer to this drawing for a clear understanding of where branch intervals break and what they are. Figure 4.8 shows a stack with two branch intervals. To size a system like this, you must apply your sizing techniques to each individual branch and to the stack.

Vent Through Roof

3rd Branch Interval

2nd Branch Interval

1st Branch Interval

Soil Stack

Cleanout

Building Drain

FIGURE 4.7 ■ Branch-interval detail. (Courtesy of McGraw-Hill)

TRAP SIZING

Trap sizing is a simple procedure. All you need is some basic information and a sizing table. If you know your trap size, you can determine the fixture-unit load that is allowable. When you know the number of fixture units that will be placed on a trap, you can decide on a trap size. There’s not much to it. Fig­ures 4.9, 4.10, and 4.11 show limits for fixture units on traps in the three main plumbing codes. If you notice, two of the codes have the same ratings, but one is more liberal than the other two. Remember to use your local code when doing actual sizing.

FIGURE 4.9 ■ Zone Two’s fixture — unit requirements on trap sizes. (Courtesy of McGraw-Hill)

FIGURE 4.10 ■ Zone Three’s fixture-unit requirements on trap sizes. (Courtesy of McGraw-Hill)

FIGURE 4.11 ■ Zone One’s fixture — unit requirements on trap sizes. (Courtesy of McGraw-Hill)

THE RIGHT PITCH

Having the right pitch on a pipe is neces­sary when complying with a plumbing code. The amount of pitch, or grade, on a pipe can affect its allowable length and fixture-unit load. You can use the tables in Figures 4.12, 4.13, and 4.14 as examples of how a local code might put rules in place for you to follow. The tables are easy to understand and use.

SIZING BUILDING DRAINS

Sizing building drains is simple when you have a sizing table and some basic information. Refer to Figure 4.15 for an example of a sizing table for a build­ing drain. In this example, all pipes are based on a pitch of one-quarter of an inch per foot. A 3-inch pipe can carry up to 42 fixture units, but not more than two toilets. Tables like this one should be available in your local codebook.