A warehouse callback, a trouble on a notification circuit, and the previous company's tech was long gone. I opened the third floor can and found one pair of black and red, one pair of black and red, and a third pair that was also black and red. Three circuits. Same colors. No labels worth reading.
Nothing was wrong with that wiring from a code standpoint. That's the part that surprised me the first time.
What the code actually says
Here's the honest answer to the search: NFPA 72 does not assign colors to fire alarm circuit conductors. There is no table that says SLC is blue and NACs are red. If someone tells you "code says," ask them for the section number and see what happens.
What is code comes from the NEC, and it's about safety conductors, not signaling ones:
- A grounded conductor is white or gray (or marked white at the terminations in the cases the NEC allows).
- An equipment grounding conductor is green, green with a yellow stripe, or bare.
- Ungrounded conductors can't use those colors. Black, red, and blue are the usual picks on line voltage.
That matters for the 120 volt side of the system: the dedicated branch circuit feeding the panel or the booster supply. Use black for hot, white for neutral, green for ground, and don't get creative. Check your local rules on how the breaker and its disconnect get marked. I believe it's red marking and a "fire alarm circuit" label, but your AHJ may have a preference.
Everything on the low voltage, power limited side is convention, the project spec, or the manufacturer manual. The cable itself falls under NEC Article 760, which is where the FPL, FPLR and FPLP cable types come from. That's a listing and a jacket rating, not a color scheme.
What techs actually run
Conventions, not rules. They shift by shop, by region, and sometimes by foreman.
| Circuit | Common pairing | Typical wire | Example run |
|---|---|---|---|
| SLC (addressable loop) | Red and black, or a twisted shielded pair with its own jacket | 18 AWG shielded, or 16 | 640 ft loop |
| NAC (horns and strobes) | Red and black, sometimes red and white | 14 AWG | 180 ft, 1.2 A |
| Conventional initiating zone | Yellow and black, or red and black | 18 AWG | 240 ft |
| Panel AC feed | Black, white, green | 12 AWG | 60 ft |
| Aux or resettable power (24 VDC) | Orange or red with black | 16 AWG | 90 ft |
Treat that table as what you'll meet, not what you must do. I've seen blue and white SLCs on jobs where the spec demanded it, and red-jacketed cable with white and black conductors inside. Red FPL or FPLR jackets are very common because they let anyone pulling a ceiling tile know what they're looking at. Common, though. I wouldn't call it required, and some AHJs and specs insist on it anyway. Read the spec.
The thing that bites people is polarity. A NAC lands with a positive and a negative, and a loop has an in and an out. Two circuits sharing red and black means the color tells you nothing about which circuit or which leg. Use tags, not wire color, to tell circuits apart, and use the color to keep polarity consistent end to end.
Pick one, then keep it. Red as positive on every NAC for the whole job. That's it.
Don't copy a terminal diagram off a forum
I'm not printing which terminal a conductor lands on, because it changes by device model and revision. Generically: the supply side of a notification circuit goes in on the input terminals, the next device in the chain picks up from the output side, and polarity is observed throughout because the circuit is supervised in reverse. Open the installation sheet for the exact device in your hand. Five minutes there beats a forum post from 2014.
For the supervision piece, I covered the resistor side in end of line resistor explained, and the redundant return path side in Class A vs Class B fire alarm wiring. I won't redo either here.
Why the as-builts are where colors matter
This is the real answer to the question. Color code is only useful if somebody wrote it down.
On a job I finished last spring, the cleanest thing I did was add one sheet at the back of the as-built set: circuit name, conductor color for positive, color for negative, shield treatment, wire gauge, cable type, and the length of the run. Ten minutes. The next tech who opened a junction box didn't have to guess.
If your spec calls out colors, copy the spec. If it's silent, say what you used. "Positive is red on all NACs, SLC is the red/black shielded pair on the second floor riser" is a sentence a tech can act on at 2 a.m.
Gauge and length are worth recording while you're there, because they feed the voltage drop numbers in the voltage drop calculation, and the reviewer asking about it is a lot easier to answer if the as-built agrees with the submittal.
The short version
- The NEC controls white, gray and green. Everything else is spec, manual, or habit.
- NFPA 72 doesn't assign signaling circuit colors. Don't invent a standard.
- Red FPL or FPLR jackets are a common convention, not a universal rule.
- Pick a polarity scheme, hold it for the whole job, and write it on the as-builts.
Copying gauges and run lengths off my own as-builts into the voltage drop sheet and the submittal got old fast, so I built FireDeck (yes, me, biased) to stop doing it by hand.
Label your cans.