National Electrical Code Tips: Article 695, Fire Pumps
Sure, you install motor overload protection to prevent motors from burning
up. Under overload conditions, they open the circuit and thus protect the motor.
But unlike other motor loads, you want the fire pump to run no matter what. Even
if that means overheating the conductors or running the motor at locked-rotor
current. Saving the fire pump but losing the facility isn't much of a bargain.
Article 695 is geared toward protecting the facility, and that means keeping
power available to the motor under conditions that would normally require
interrupting the supply power.
This is a departure from the normal thinking in the NEC. It's critical to
understand this fact, or you'll set yourself up for misunderstanding and
misapplying Article 695.
Normally, you want to shut down equipment to prevent damage to the supply
conductors and/or the equipment itself. But not when that equipment is a fire
Keep in mind that this pump is what supplies water to the fire suppression
system. Obviously, you want your sprinkler system to have water when there's a
fire. And, of course, your fire brigades and/or the fire department are counting
on those fire hoses to be more than mere wall decorations.
So far, so good. If you understand what we've just covered, then many Article
695 requirements make perfect sense. For example, you don't want to install the
fire pump inside the paper goods warehouse where it could easily be destroyed by
the very fire you're trying to put out. Minimizing, or even eliminating, its
exposure to fire is a principle that needs no explanation.
The same thing goes for the requirement that it have a reliable source of
power. A little less obvious is the requirement that its wiring be independent
of other wiring, but when you think about it this also seems like a "no brainer"
Then, there's the other stuff. Things that just look wrong. Even, sometimes,
after you think about them a little bit. But when you go back to why that fire
pump exists at all, they also make sense. For example, you can tour many a fire
pump room and see a padlock on the disconnect. Usually, it's a "breakaway" type
that, if you apply sufficient force, can readily be removed. It's there to make
the point that you should not operate that disconnect handle. And, what's
counterintuitive at first glance, is that disconnect is locked in the closed
position. "I can understand making a disconnect lockable in the open position.
Great idea for maintenance. But lockable in the closed position?"
Yep. You don't want that fire pump disconnect left open, only to figure out
when the building's half burned down and several people are dead that, oops,
someone opened the disconnect. The same rationale behind this lock is why you
can't have an automatic means of protecting the pump against overloads.
Another thing about fire pumps. Run oversized conductors to minimize voltage drop and do what you have to do to acheive zero voltage imbalance. Voltage imbalance causes counter-EMF, which means the motor runs hotter. An example of preventing this is to put single-phase loads (e.g., pump room lights and heaters) on their own transformer instead of running some of the motor supply to a panel.
Let's address some
highlights of the Article 695 requirements:
- Notice how much space Article 695 devotes to "Power Sources for Electric
Motor-Driven Fire Pumps" [695.3]. It amounts to a full page. You can have
individual sources or multiple sources. You can generate the power onsite. This
subsection has many different requirements to cover many different ways of
supplying power. It's something you should read very closely before thinking
about where to site your pump and before you begin the specification process. It was heavily revised with the 2020 Code.
- You cannot use phase converters for the fire pump service [695.3(I)].
- Note that what's not mentioned in 695.3 (or anywhere else in Article
695) is the use of a soft-start. Adding one is a very good idea for a lot of
reasons, one of which is if it's properly set up it will enhance the "fire pump
must run" goal. But since it's not discussed in the NEC you can't just add it.
That would be sneaking something in. Well, it is discussed if you can consider
it a fire pump transfer switch [695.4(A)] and get the AHJ to go along with its
being used for that. Or, perhaps you can find a properly sized soft-start that
is listed for this use; contact your electrical distributor to begin that hunt.
- So can you put a soft-start in if it's not listed? Whether it's listed
or not, you need to discuss the design with the AHJ. You'll need to show a few
things about this design. For example, you're including a manual bypass in case
the soft-start should ever fail to start. Not that there's any serious danger of
this, but things do happen. And the soft-start itself should meet the same
conditions as those provided in 695.3, 695.4, 695.12, and 695.14. You don't want
it to be the weak link in the system. In fact, you don't want any weak links in
this system; that's a key difference between it and all other electrical systems
(overload protection is a built-in weak link that you can, presumably, quickly
restore by replacing a motor overload strip or similar action).
But don't forget that you must have a soft-start that provides enough oomph to
account for the fact that the fire pump is starting as a dead head load. Unless
yours is a fire pump design that doesn't. The major manufacturers provide
outstanding support on this aspect, so use that support to come up with a
specification that work best for the application. You aren't going to get
a truly optimal soft-start, you are going to just blunt the inrush problem so
it's not as severe.
The goal is to take advantage of the soft-start's ability to reduce stress on
the power distribution system, the fire pump, the fire pump conductors, the
water piping, etc. without impairing the delivery of the water. A slight delay
is OK, because there's already water in the system and the jockey pump will
maintain pressure for a short while even under high demand. Think of that jockey
pump as the battery backup to the emergency generator; it will hold the load
long enough for the generator to come up to speed. The fire pump can work in an
analogous fashion with the jockey pump instead of being slammed to 100%.
Make the design as robust as practical. It would also be good to think through
the signage and breakaway locks you're going to include in the design so there's
no chance of inadvertent shut-off.
- The goal of not interrupting power to the fire pump is addressed
repeatedly throughout Article 695. We see continuity of power stressed; it's the
title of 695.4. Then we read in 695.4(B)(2) that the overcurrent
protective device (not overload protection, these are two different things) must
be rated to carry the sum of all the fire pump loads including the locked-rotor
current (not starting current or running current) of the largest motor on the
circuit. And it must do that not with a time-delay function, but indefinitely.
This means a much larger breaker or fuse (and conductors) than what you'd use if
merely following Article 430 for a similar sized motor circuit.
- It gets dicey with disconnects, too [695.4(B)(2)]. They have to meet a
slew of requirements. All of these are aimed at preventing an unintended
interruption of power to that fire pump.
- If your fire pump operates at a voltage different from that of your
service, you'll need a supply transformer for that fire pump. Even if the two
match, there are utility transformers in the power grid distribution system so
don't let this intimidate you. That brings to mind another issue, which is if
you're going to connect a fire pump you should discuss this with the utility
first so they can make any needed adjustments to the supply. That's not an NEC
requirement, but it's good engineering. If you need a supply transformer, it's
good engineering to oversize it and take an efficiency hit than to have it be
very efficient and blow up on that hot August day when there just happens to be
a fire at your facility. See 695.5 for the minimum sizing requirements.
- The service conductors supplying a fire pump must be routed outside a
building [695.6(A)(1)]. Think about the logic behind this requirement; it doesn't make sense to route them through a burning building. And what
if you have an alternate supply? For example, two utility lines or a utility
line and an on-site generator? Well, be thinking worst case scenario and spend
the extra money to keep those as far apart from each other as practical. For
example, you have a supply on the east end of the facility and the alternate
supply coming in from the west end. If some maniac drives a truck of explosives
into that east end supply and takes it out, your alternate supply is still available.
- Voltage drop is an issue with fire pumps, because they place such a
huge load on any distribution system. The NEC has specs and requirements in
695.7, but the typical installation at any large manufacturing facility violates
these and nobody knows it.
You simply cannot start such a large motor across the line driving a dead head
pump and stay under a 15% drop. Sure, if you measure with a DMM you can probably show that you meet
this spec. But put a power analyzer on the distribution system almost anywhere
on site and you will see a huge drop. Almost to zero. It may last only a few
milliseconds, but it'll be there. And you'll also see the echo of this as the
system bounces back and forth seeking stasis. This is where a soft-start comes
in very handy.
- You can use EMT for fire pump control wiring [695.14(F)], but generally IMC is a better choice because it provides more protection than EMT and is lighter than RMC. Generally speaking, it's better to use a more rigorous type of
raceway than a less rigorous one for this application.
And though you can use some of the Chapter 3 flexible conduits remember that
they have maximum length rules and the relevant Chapter 3 Articles apply. Use
them where they are advantageous for such things as accommodating vibration.
Don't use them on the basis of "ease of installation." That's almost never a
justification for a flexible conduit, and we are talking about a fire pump here.
Make it rigorous.