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Penny size is bullshit. Penny size applied to different nail types means a different length! Read this - [1]
Joist hanger names were called Tico nails [2].
- If double top plate - then 1.5" is not the limitation and 3" nails should be used.
Building code defines a 10D nail as a .148 nail, 3" long [3]
1" is 2D [4]. Note that penny size came historically from cost of the nails in pennies. So the length is not proportial to D size. [5]. The only thing we know is that the larger the D number, the larger the nail.
10D is 3" long - [6]
Joist hanger nails do not seem to have D size attached to them - this one is 1.5" [7]
This 8D is 1.5" long [8]. Is that an error? Chart says 8D is 2.5".
3.5" common nail, galvanized. 16D [9]

Penny Size of Nails