Commercial kitchens should be independently considered when it comes to drainage pipe sizing.

Commercial kitchens can have some of the most complex plumbing installations in a relatively small area. By the time a plumbing contractor has received the plans for a commercial kitchen, there has been a lot of discussion and coordination between the architect, plumbing engineer and kitchen designer. When everything goes well, the discussion and coordination normally proceed smoothly. When they do not, the coordination may be less than hospitable. The problem is, you may never know just how well cooperation is between all parties.

That is why, for any commercial kitchen, a contractor should go in with his eyes open. For larger installations, changes can be an ongoing occurrence.

Backflow Protection

One of the common complaints from plumbing contractors is that the plumbing engineer misses a number of backflow preventers in a commercial kitchen. That is not necessarily true. In most cases, the kitchen designer and architect changed the equipment or system and never informed the engineer. Sometimes the owner contracts with a detergent company that comes in after the architect, engineer and kitchen designer have completed their work. The plumbing contractor is the last avenue to see the potential cross connection.

Knowing that the kitchen installation can have numerous cross connections, it is important to be vigilant in observing all connections to the potable water system. Certain commercial kitchen equipment and appliances have built-in backflow preventers or air gaps. However, it is always a good idea to check the equipment or appliance to make sure backflow protection is provided and is in accordance with the plumbing code.


The standard method for sizing a drainage system is by fixture units. In a commercial kitchen, using drainage fixture units may not work.

Fixture units size the piping system based on the probability of simultaneous discharge. There is a general assumption that there will be a time interval between the use of each fixture. For example, even when lining up for a water closet, it takes just over one minute, on average, to use the fixture. Hence, there is about 65 seconds between flushes in a worst-case condition.

In a commercial kitchen during peek times, the discharge from the fixtures may be continuous. Food prep sinks may be constantly running. Similarly, dishwashers may be continuously discharging. Add in the pre-rinse sink, kettles, potato peelers, etc., and you have a much greater quantity of water flowing in the drainage piping.

When the plumbing engineer does the sizing analysis, you may find that the drain is much larger than you anticipate. There is a tendency to think that the engineer screwed up by oversizing the drain. Contractors often accuse engineers of being too conservative and always making the drain too large. Everyone knows that bigger is not better. In many instances, bigger is worse in drainage pipe sizing.

Before jumping down the engineer's throat, consider that the engineer has extensive experience in the design of commercial kitchens. The engineer's experience in prior installations may be the reason for the larger size pipe.

A mistake plumbing contractors sometimes make is to value-engineer the drainage piping in a commercial kitchen. You can save money for the owner by lowering the drainage pipe size to a smaller diameter based on a calculation of the fixture unit value of all the fixtures in the kitchen. However, in making that change, you assume all the liability for the change. If the drain is undersized, the drainage system could fail.

To put this into perspective, I had the opportunity to investigate a commercial kitchen that prepared 30,000 meals a day for a hospital complex. When I reviewed the plans, I thought the engineer was crazy. The main drains for the commercial kitchen were 6 inches in diameter. I immediately took out my plumbing code and started running numbers. The drains could have been 3 inches or 4 inches in diameter. There never was a call for a 6-inch drain in the entire system.

As part of my investigation, a camera was run down the drain. But I experienced a problem. During normal food preparation time, you could not run a camera; there was so much flow that we could not get a clear indication of the performance of the drain.

It was decided to run the camera down the drain at 3 p.m. This seemed logical, since the lunch meal would be over and it would be prior to the start of the dinner meal. As the camera ran through the 6-inch drain, it appeared that something was wrong. The 6-inch pipe was flowing full - not half-full flow, full flow.

That prompted a visit to the commercial kitchen to see what they were doing. At that hour, the kitchen operation was completing the cleanup of the lunch dishes, pots and pans. In addition, the preparation for the dinner meal was ongoing. The discharge of water was obvious from many fixtures and appliances. Apparently enough so that the 6-inch drain was flowing full.

The design that I thought was crazy was, in fact, undersized. The main drain could have easily have been an 8-inch drain. Remember, the fixture unit sizing method would have indicated a 4-inch drain.

This was simply a wonderful example of why commercial kitchens must be independently considered when it comes to the drainage pipe sizing. So when you see too large a pipe, ask the plumbing engineer why. They should be able to prove to you why the drain is the size they have shown on the plans. If the drain looks too small, I would also recommend that you inquire with the plumbing engineer.

Your best bet is to always install it in accordance with the engineer's design.