An AC condensate line must drain to an approved point through an air gap or air break into a trapped, vented receptor, never into a street, alley, or where it causes damage. Code sets the pipe at 3/4 inch minimum with a 1/8 inch per foot slope toward discharge.
Where the condensate line is allowed to discharge
The discharge point is the part homeowners get wrong most often. Condensate is an indirect waste, which means it cannot connect straight into a drain pipe. It has to end over an approved receptor through an air gap or air break. That leaves a physical break between the end of the condensate pipe and the standing water below it. The break is what stops dirty drain water, and the sewer gas riding on it, from siphoning back up into the air handler.
Approved receptors include a trapped and vented floor drain, a standpipe, a laundry or utility sink, or a mop sink. The receptor has its own P-trap that holds a water seal. That trap has to be vented so the seal is not siphoned dry. The 2024 Uniform Plumbing Code, Chapter 8, governs these indirect waste receptors. That is why this is a plumbing question and not a thermostat question.
What is not allowed matters just as much. The line cannot discharge into a public street, alley, or sidewalk. It cannot drain anywhere it creates a nuisance or damage, such as soaking a foundation, icing a walkway, or staining a neighbor's wall. Many older Phoenix homes route condensate to the outside. That is fine when it lands on grade or a splash area away from the house. But a line gushing onto a public way is a code violation, and a city inspector will write it up.
A common legal endpoint in newer homes is a tie into the home's drainage system at an approved receptor, again across an air gap. Tying condensate straight into a plumbing drain with no air gap is the classic mistake. It removes the safety break that the whole rule exists to provide.
The pipe size and slope the code requires
Two hard numbers control the pipe itself. The condensate drain has to be at least 3/4 inch across. It can never be smaller than the drain outlet on the equipment it serves. So if the air handler ships with a 1 inch condensate fitting, the line has to start at 1 inch and stay at least that size. It cannot neck down to 3/4 inch to match a fitting someone had on the truck.
The line also has to fall toward the discharge. Code requires a slope of at least 1/8 inch per foot. That works out to about a 1 percent grade. The International Mechanical Code states the rule plainly in Section 307: condensate drains must be sized and pitched so they "drain by gravity to an approved place of disposal." A line that sags, runs flat, or runs uphill holds water inside. Standing water in a warm attic is exactly where the next clog starts to grow.
Phoenix runs on the 2024 Uniform Plumbing Code for the receptor and the 2024 International Mechanical Code for the equipment side. Both were adopted in the 2024 Phoenix Building Construction Code with local amendments. The two codes work together. The IMC sizes and routes the condensate line off the unit. The UPC governs the trapped, vented receptor it lands in. The 3/4 inch and 1/8 inch per foot figures are the floor, not a number to shave down.
The attic and above-ceiling secondary drain rule
Phoenix puts a lot of air handlers in attics and above ceilings, and that location triggers an extra layer of protection. When a unit sits where a leak would damage the building below, the code requires a backup. The backup keeps a clogged primary line from quietly overflowing the equipment pan into the drywall.
There are two accepted ways to provide that backup. The first is an auxiliary (secondary) drain pan under the unit. It has its own separate drain line, routed to a conspicuous point. That point is usually a soffit or eave where a steady drip is hard to miss. The second option is a water-level shutoff, most often a float switch. It senses rising water and shuts the system off before the pan overflows. Either one is fine on its own. Many quality installs use both for extra safety.
This rule is the answer to a question Phoenix homeowners ask every July. Why is water dripping from the soffit on the outside of my house? That drip is usually the secondary pan doing its job. It is telling you the primary condensate line is clogged. It is not a roof leak. It is a warning that the primary drain has backed up and you are now running on the backup. If you see it, treat the AC as if its drain is blocked. Get the primary line cleared before the secondary path clogs too.
Why a clogged condensate line is the top summer AC water problem in Phoenix
A clogged condensate line is the number one summer AC water complaint in Phoenix, and the reason is biology plus heat. The condensate water is warm and the drain pan stays damp. So algae, slime, and mineral scale grow inside the line over a cooling season. Phoenix tap water runs hard, often 10 to 17 grains per gallon. That makes scale build fast and narrow the pipe until it plugs.
Once the primary line plugs, the water has nowhere to go. It backs up and overflows the equipment pan. On an attic or closet unit, that water finds the ceiling below. The result is the brown ceiling stain, the sagging drywall, and the mold that follows. The EPA is direct about the timeline: act fast, because mold can start to grow on wet materials within 24 to 48 hours. The fix is to dry the area and stop the moisture source, or the mold comes back. A condensate overflow is a plumbing-style ceiling leak. We cover what to do when you spot one on our water-leak-from-the-ceiling page.
Keeping the line clear is mostly simple upkeep. Confirm the line has the right slope so it actually drains. Clear it at least once before cooling season. Keep the trap and receptor in good shape so the system drains freely and stays sealed. A float switch is cheap insurance. If the line does plug, the switch shuts the AC off, and you get a warm house instead of a ruined ceiling. If your home also has a sump or subsoil setup tied into drainage, the discharge rules are similar. Our sump-pump-discharge-rules page covers where that water is allowed to go.
If the line is already overflowing or the ceiling is stained, shut the system off to stop the water. Then have the primary line cleared and the receptor checked. The water side of an air conditioner is a drainage system, and it follows the same plumbing code as the rest of the house.