If you’ve ever seen a fire hydrant gushing water onto a city street, then you might be able to get a good idea of the type of pressure that’s behind the flow. But the minimum pressure required for a hydrant is actually regulated by National Fire Protection Association (NFPA) 291, which sets the standard at 20 psi. This is the number used during fire flow conditions.

“Fire flow” refers to the rate of available residual water supply that can be used from a hydrant system in order to support firefighting operations. The pressure requirement is in place because the success of firefighting efforts doesn’t just depend on accessing an unblocked hydrant,  but also getting enough force and volume of water as it’s being used. If the pressure is too low, hose streams can lose reach and thus lose effectiveness. Regular hydrant maintenance can also include flushing dirty water out, to help ensure the water supply is reliable when firefighters need it.

Technically speaking, fire hydrants can operate below the 20 psi level, though some states do not allow it. Additionally, working below 20 psi is harder to do and could have a major effect on system components if negative pressure occurs in the main water line. Pipes could pull in contaminated water or even collapse. Because of this, 20 psi is regarded as an important safety standard and not a suggested guideline.

While NFPA 291 addresses water pressure during fire flow conditions, it also explains why fire hydrants are different colors. The color coding process indicates the available water flow of a hydrant based on measured performance. For example, blue hydrants typically have the highest capacity flow at around 1,500 gallons per minute (GPM). Green is next between 1,000 and 1,499, while orange and red both generally have lower GPMs. Because firefighters are familiar with the color codes, they know what to expect from a hydrant before connecting.

Determining the GPM of fire hydrants involves testing, as outlined in NFPA standards. During this testing, technicians collect flow and pressure data, which can be taken from more than one hydrant at a time. This is often the best way to get a baseline measurement of how the system delivers while under demand. This consistent process ensures that GPM numbers are based on actual performance and not estimates.

But in order to accurately measure fire flow pressure, the NFPA uses a fire hydrant’s static pressure. This is the pressure in a hydrant system when water is not actively flowing. When water begins to flow from the hydrant, the pressure drops and shifts to residual pressure, which is then used to determine real-world performance. Without the transition from static to residual, there would be no way to truly rate a fire hydrant’s actual operating performance.