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I have a 5 gang switch box I'm trying to calculate how many "conductors" I can have in it. Basically my main question is, do wires that originate and terminate inside the box count as +1 each (wires in nuts/splices going to switch)? Or is it only wires that originate outside the box and come into it?

I have a 63in³ (2.75x2.5x9.25) box with 5x standard switches, I'm looking to replace with zwave switches. From my calculations I have (all 14ga):
+12 Hot/Neutral wires coming into the box (2 from breaker panel, 10 feeding the 5 lights)
+1 Ground (6 bare ground coming in/out but only 1 counts for this?)
+1 wire clamps in the box
+10 Devices ( 5 switches, counts as 2 each)
+3 Traveler wires (Do these count?)

So if it's that simple, I need at least a 54in³ and I'm good. But my question is what about the spliced short wires that originate and terminate in the box? So if all 6 of my neutrals (for the 5 switches and 1 live) are bonded together under a wire nut, then I run a neutral from that bond to each of the switches, that's +5 more spliced wires, do they need to get counted? And if I do similar for ground and hot that's another +10. Then I get into more fun dilemmas like max conductors in a wire nut and making splicing chains (my GB reds say only 5x #14).

It's a really ugly layout I wish they had split this up into different boxes, but I want to convert this whole thing so it can be put on my HA system. The current layout of 5x 2-wire switches has a lot less wires in the box so box fill isn't as much of a concern, but considering smart switches need both neutral and ground running to them as well it adds a lot more to that box. It's going to be really cramped with all the splices/wire nuts.

Anything else I'm not considering?

Thanks!

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Reading your question, it appears you are familiar with NEC Article 310.14 so lets not get into that. The biggest problem I have ever faced with multiple wire and devices was not meeting the box fill requirements but getting all that stuff into the box and mounted properly. So plan how you are going to splice and connect all of your conductors carefully. Bundle all of the common conductors together, try not to crossover too many conductors and plan to keep your splices in different areas of the box and not interfering with the installation of the devices.

One last thing, these boxes come in different depths, traditionally 1 1/2, 2 1/2 and 3 1/2. If it's not too late you might try to purchase and install the 3 1/2" variety.

Good luck.

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But my question is what about the spliced short wires that originate and terminate in the box?

Wires originating and terminating inside the junction box are not included in the minimum fill calculation.

Doesn't mean they don't take up space (see RME's answer for advice on good use of space and starting with as big a box as will fit).

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NEC 314.16.b.1 states conductors originating outside the box with conections or passing through are counted. Pig tails that connect a group of wires then connect to the switch or device are not added. With this said if you get crazy pigtailing with long pieces you can be under the box fill calculation but not able to install the devices.

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Pigtails don't count

Quoting 314.16(B)(1) (abridged):

A conductor, no part of which leaves the box, shall not be counted.

So, your pigtails don't count at all.

Only 1 ground counts

You are correct that only 1 of the equipment grounding conductors gets a fill allowance, this is as per 314.16(B)(5) (abridged as 250.146(D) isn't relevant):

(5) Equipment Grounding Conductor Fill. Where one or more equipment grounding conductors or equipment bonding jumpers enter a box, a single volume allowance in accordance with Table 314.16(8) shall be made based on the largest equipment grounding conductor or equipment bonding jumper present in the box.

But the travelers do count

However, since the travelers do leave the box, they do count against wire fill.

Get better nuts

You may wish to invest in some push-in type splicing connectors with 8 or 10 individual wire terminations in this case -- this will allow you to avoid fussing with daisy-chained wire nuts.

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