This is a three-part question. Rephrased it looks like the questions are:
- Does the National Fire Prevention Association, in NFPA72 or the NEC (National Electrical Code) allow wire nuts to be used to connect wires together inside a junction box for Class A or Class B wiring?
- Does the Authority Having Jurisdiction (AHJ) have the Authority to not allow wire nuts to be used in the junction boxes?
- What can be used as a substitute?
Are Wire Nuts Allowed by NFPA Code?
I have not seen any place in the "Code" that doesn't allow wire nuts inside of junction boxes for fire alarm systems.
The other part of the answer shows what the NFPA Code really is. It's guidelines published by a non-profit organization showing the absolute minimum requirements needed to make a safe fire alarm system. The published codes aren't law, the published codes don't cover everything, and by themselves, the published codes are books.
However, if anyone installing a fire alarm system does less than the minimum shown in the guidelines, from a practical point of view, the fire alarm system isn't safe. No government can overrule that practicality, and you need to know what the NFPA is saying to make a safe fire alarm system.
Does the Authority Having Jurisdiction (AHJ) have the Authority to Add to the Code?
We'll start out with just who is the Authority Having Jurisdiction (AHJ).
The AHJ is the government's representative - the fire marshal or fire inspector, the insurance company's representative - the insurance inspector, the building owner - the architect and engineers with the overall design of the fire alarm system, and so on. I'm paraphrasing, but in essence, the NFPA Code, in their definitions, defines the Authority Having Jurisdiction as each of these.
In the comments written by the NFPA (you need the Handbook to get the comments) installers have to listen to all of these AHJ's. The comments also go on to say that if there is a conflict between the NFPA Code and what any of these AHJ's say, the more stringent requirements will prevail.
Keep in mind that the fire marshal or fire inspector is trying to keep people and property safe. That's the motivation of the AHJ with the fire department.
My guess is that several times the AHJ has run into systems with wire nuts, where the wire nuts were badly installed. Maybe the wire nuts were not tightened enough, maybe one of the wires wasn't quite under the nut. (Many times, as a fire alarm service technician, I've had to find and fix these problems.)
Whatever the reasons for not using wire nuts, not using wire nuts is a more stringent requirement, and being more stringent, the requirement prevails.
What Can be Used as a Substitute?
At this point I assume the wires are already installed in the building. The only alternative I know of for wire nuts are terminal strips. Make sure the terminal strip being used is UL approved. Many aren't and using a terminal strip that is not approved by UL is not a good idea for a fire alarm system. (I have found that non-UL terminal strips fail a lot more often than UL approved terminal strips.) The AHJ might not even accept the terminal strips without the UL stamp, and if the AHJ rejects them after installation, the extra expense and work of installing non-UL terminal strips will have to be redone.
You can go to an electrical distributor in your area, possibly they have something that can be used. If you can't find anything there, catalog distributors like Digikey, Mouser, Newark, or other sources can be used.
Possibly, once you find a terminal strip, you could take a sample terminal strip in hand, and ask the AHJ if that is acceptable.
Just remember that all of us in the fire alarm business are bottom feeders. The NFPA Code is our guideline that we have to follow, but all of the AHJ's are above the NFPA Code and can add more rules for us to follow.
Good luck on the alternative to using wire nuts.
Can anyone who has figured out what to use as wire connectors in junction boxes let me know? Perhaps I can publish the other options to help others.