|NEC Article 695 - Fire Pumps
|Mike Holt for EC&M magazine
If you work with the connections or controls for fire pumps, you need to understand Article 695.
Today, you can expect to find a fire pump in almost any public building or factory. Whether your company is doing maintenance, retrofits, or new construction, someone on your team-and it could be you-is likely to work with the power sources or control equipment for fire pump motors. How can you know the work you do will ensure the public safety? Fortunately, Article 695 will tell you.
The Article covers the installation of:
Electric power sources and interconnecting circuits.
Switching and control equipment dedicated to fire pump drivers.
The article does not cover:
Performance, maintenance and testing.
Pressure maintenance (jockey or makeup) pumps.
Fire sprinkler system pumps used in one or two-family dwellings.
Power Sources for Electric Motor-Driven Fire Pumps.
Obviously, power to fire pump motor must be supplied by a reliable single source or by reliable multiple sources. The NEC codifies this in 695.3. Let's take a look at individual sources first, then multiple sources.
The NEC prefers an individual source, rather than multiple sources, for a fire pump motor. This source must be able to carry the locked-rotor current of the fire pump motor(s) and the pressure maintenance pump motors. It must also be able to carry the full-load current of any associated fire pump equipment. Finally, the single source is "reliable" only if the AHJ agrees that it is-review your design with the AHJ before implementation. The NEC allows for two individual sources:
Electric Utility Service. You can supply a fire pump with a separate service. Or you can supply it via a tap located ahead of-but not within-the same cabinet, enclosure, or vertical switchboard as the service disconnecting means. You must locate and arrange this connection in a way that minimizes the possibility of damage from fire within the premises. For example, don't locate it next to the production process but locate it well out of harm's way elsewhere on the property. If you put a tap ahead of the service disconnecting means, make sure it complies with 230.82(4). The service equipment itself must comply with 230.2 and 230.72(B).
On-Site Power Production Facility. You can supply a fire pump with an on-site power supply, such as a generator. As with the separate service connection, you must locate and arrange this onsite generation in a way that minimizes the possibility of damage from fire within the premises.
Sometimes, you just can't get reliable power from a single source. In such a case, you can obtain power from an approved (by the AHJ) combination of two or more sources, as spelled out in 695.2(B). You must locate and arrange each source in this combination in a way that minimizes the possibility of damage from fire within the premises. Two other considerations are generator capacity and feeder sources:
An onsite generator combined with other sources (or other generators) must have the muscle to start and run the motor(s) driving the fire pump(s) while supplying all other simultaneously operated loads. You can use automatic load shedding to comply with this requirement. You cannot use a tap ahead of the on-site generator disconnecting means.
What if you have a multibuilding campus-style complex with fire pumps at one or more buildings? If it isn't practicable to supply each one with an individual source, the AHJ can approve two or more feeder sources to act as one power source under certain conditions.
Continuity of power
[695.4] Protect fire pump motors from inadvertent disconnection. The requirements are fairly detailed, but here are two highlights. Remember, the goal is to ensure the fire pump runs if there's a fire:
The supply conductors must directly connect the power source to the controls.
You can install a single disconnecting means and an overcurrent protective device, if you meet certain conditions. One of those is supervision, such as an alarm system, to ensure the disconnecting means stays closed.
[695.5] Size the transformer to a fire pump motor for no less than 125 percent of the sum of the fire pump motor(s) and pressure maintenance pump(s) motor loads, and 100 percent of the associated fire pump accessory equipment supplied by the transformer.
The primary overcurrent protective device for a transformer must be able to carry indefinitely the sum of the locked-rotor current of the fire pump motor(s) and the pressure maintenance pump motor(s), and the full-load current of the associated fire pump accessory equipment when connected to this power supply. You cannot provide secondary overcurrent protection.
[695.6] Size conductors that supply fire pump motors and accessory equipment for no less than 125 percent of the sum of the motor full-load currents as listed in Table 430.148 and/or 430.150, plus 100 percent of the ampere rating of the fire pump accessory equipment.
Keep fire pump supply wires on the load side of the final disconnect and overcurrent protection entirely independent of all other wiring. Article 695 provides other requirements intended to protect these wires from damage from fire, structural failure, or operational accident.
Size the conductors for no less than 125 percent of the fire pump motor full-load current. Use Table 430.148 and/or 430.150, based on the 75°C terminal rating [110.14(C)(1)(a)] as listed in Table 310.16.
Example: Size a 25-hp, 208V, 3-phase fire pump motor at 125 percent of the motor's FLC as listed in Table 430.150.
74.8A x 1.25 = 93.4A, 3 AWG THHN at 75°C is rated 100A
You must also account for voltage drop. At the line terminals of the controller, when the motor starts, the voltage must not drop more than 15 percent below the controller's rated voltage. In addition, the voltage at the motor terminals shall not drop more than 5 percent below the voltage rating of the motor when the motor is operating at 115 percent of the full-load current rating of the motor.
Here's a 3-phase example. A 25-hp, 208V 3-phase fire pump motor is 175 ft from the service. The fire pump motor controller is 150 ft from the service. What size conductor must you install to the fire pump motor controller? Terminals are rated 75ºC. The answer: 3 AWG THHN. Here's the math:
CM = (1.732 x K x I x D)/VD
CM = Wire size, Chapter 9, Table 8
K = 12.9 ohms, copper
I = 404A (locked-rotor, Table 430.151B)
D = 150 ft
VD = 31.2V (208V x 15%)
CM = (1.732 x 12.9 ohms x 404A x 150 ft)/31.2V
CM = 43,396, Chapter 9, Table 8 = 3 AWG
If you increase ungrounded conductors in size, you must increase equipment grounding conductors (where installed) in size proportionately-according to the circular mil area of the ungrounded conductors [250.122(B)].
Here's a 3-phase example: A 25-hp, 208V, 3-phase fire pump motor is 175 ft from the service. The fire pump motor controller is 150 ft from the service. What size conductor must you install to the fire pump motor if it's 25 ft from the controller? Terminals are rated 75ºC. The answer: 3 AWG THHN. Let's walk through the math on this one, remembering that the operating voltage at the terminals of the motor can't drop more than 5 percent below the voltage rating of the motor while the motor is operating at 115 percent of the full-load current rating of the motor.
CM = (1.732 x K x I x D)/VD
CM = Wire size, Chapter 9, Table 8
K = 12.9 ohms, copper
I = 86A (74.8A @115%), Table 430.150
D = 175 ft
VD 5 % = 10.4V (208V x 5%)
CM = (1.732 x 12.9 ohms x 86A x 175 ft)/10.4V
CM = 32,332, Chapter 9, Table 8 = 4 AWG
Caution: For voltage drop, the 4 AWG wire is fine from the controller to the motor, but 695.6(C)(2) requires the branch-circuit conductors to be no smaller than 3 AWG.
Run like the wind
The fire pump must start and run, even at its own peril. You can't install controls that, for example, protect the motor from a phase-loss condition. It's better to burn up the pump on two phases than to let it stay cool while the facility burns. This concept is the basis for 695.14, which addresses control wiring.
Normally, we think of code requirements as a way to protect the connected equipment from burning up. In the case of fire pumps, that is concept is secondary to the larger goal. The larger goal is that the pump must run to protect people and other equipment with the fire-quenching water that sprinkler systems and emergency crews need to do their jobs. If you do your job with Article 695, you'll make theirs possible.