The answer to your question is a bit complicated, and let me first suggest two alternative solutions.
I gather you have 14/3 running from the switch box to the fan location, and it would be difficult to add the 2 additional wires you need for the signal switch. And I understand you have a separate constant power source that you can easily extend to the new fan location.
The first alternative would be wireless. You wire up the fan like previously, but you don't have the extra wires for the signal switch. Without being able to suggest a specific product, I'm sure it would be possible to add a wireless receiver and relay at the fan, with a wireless transmitter in the bathroom. Effectively a remote switch since you can't run the extra pair of wires to the switch location.
The second alternative would not be wireless. So you grab constant power elsewhere as you plan (meaning you can only kill power to the fan via that breaker), and you still have 3 wires going to the fan. Those would be neutral, switched hot for signal, switched hot for LED. The LED could be connected directly to the neutral and the switched hot. The signal switched hot and neutral would be connected to the coil of a relay that take the place of the signal switch. The result is that each neutral will only carry current from the hot(s) in its cable.
The second solution is what I did for my install, using a relay from Relay-In-A-Box (RIB)
One question is whether the constant power you'd be extending to the fan location is on the same circuit as the 14/3 or not. If so, no problem; if not you'd have 2 circuits powering the same piece of equipment. That means those two breakers should be side by side and connected with a handle tie, so a single action disconnects power to the fan. [Pretty sure that is an NEC requirement, but it's certainly best practice.] That could be avoided via the use of a second relay, for the LED light, and RIB has dual relay products.
Hey Wayne, you always have some good ideas!
Yes, your assumptions are correct. The constant power NM was run there when the roof sheathing came off (surprise: your cathedral ceiling members aren't actually attached to the original rafters better fix that before attaching the new roof!). But at the time it was a very quick (and unexpected) endeavor, and I missed that the existing two switches were run through a single 14/3 vs (2) 14/2... duh.
The vent fan is in a cathedral ceiling assembly, and not very easy to route wires there. I think I found a way I could get a couple wires there (from the adjacent under construction area), though would need to locate that momentary switch in an alternate location, I risk messing up the insulation, and fingers crossed the fan removes fine from the duct/electrical adapter (and that i can reach a knock-out to pass the wire into the j-box. After some thought, it may work... just a lot of 'maybe's attached to that.
Yes, I was checking into the first alternative you suggest, though ideally the device would go into the J-box, and the access hole is awful tiny... maybe 2"x1.5". The relay idea is really great, though unless I modify the enclosure... I will run into the same issue with the wires (getting-to the exterior of the fan adapter j-box and getting the wires actually into it.
The new constant power wire I ran to the 3 fans (in 2 adjacent baths) is currently run near a sub panel, where the existing wiring also has a breaker. The thought being j-box it just above the sub-panel so they are on the same branch circuit. I definitely would not want to run if they were not sharing the same branch circuit, given the potential for one disabled circuit to be energized by the other.
I had someone share 300.3(B) was the 2017 NEC reference for conductors needing to run together, though I found there is an exception:
300.3(B)(3) Nonferrous Wiring Methods. Conductors in wiring methods with a nonmetallic or other nonmagnetic sheath, where run in different raceways, auxiliary gutter, cable trays, trenches, cables, or cords, shall comply with the provisions of 300.20(B). Conductors in single-conductor Type MI cable with a nonmagnetic sheath shall comply with the provisions of 332.31. Conductors of a single-conductor Type MC cable with a nonmagnetic sheath shall comply with the provisions of 330.31, 330.116, and 300.20(B)
300.20(B) Individual Conductors. Where a single conductor carrying alternating current passes through metal with magnetic properties, the inductive effect shall be minimized by (1) cutting slot in the metal between the individual holes through which the individual conductors pass or (2) passing all the conductors in the circuit through an insulating wall sufficiently large for all the conductors of the circuit.
So, I am thinking NM-B may fall under that exception. Ultimately, I may need to just suck it up and do the additional work (and then I can run both lights). Thanks!!