#8 wire outside

[WaterlooMarc]

Does it need to be in conduit?

Bought a house with an inadequate tankless water heater in the master bath and I need to run another circuit to the location in order to upgrade. Oddly (to me anyway) the main panel for the house is on that same exterior wall so I could run it pretty easily outside and come in where I need it to. If it doesn’t require protection I think I could hide it under the siding near the the ground. If it does not conduit I’ll probably just bury it and still go the exterior route.

[srv52761]

Depends on the insulation and the AHJ in your area.

There are two things that need to be considered.  
   Is under siding considered as a damp or wet location.  It has weep holes to eliminate moisture.  Many ahj would say yes.  You would need conductors suitable for that and UF cable comes to mind.
   Is it protected from physical damage.  Code uses ubiquitous terms such as "likely".
In the interior under drywall you would need steel plates if less than 1.25" as it goes through studs.  But people are always pounding nails and screws to hang stuff indoors.  Seldom see that through siding.  And I have never seen that in the space of the siding corner posts.  But the code leaves it open to interpretation by the ahj.

In the end, not much in the code prohibiting it, and it is done.  But the code allows the AHJ to weigh in.

Note, this refers to vinyl and wood siding.  I recall reading in the code someplace that metal siding may be different, but not sure where.  I will look for it later.

P.S.   you said run it under the siding near the bottom.  Where you would come out of the house from the service panel, would that be below ground?  If so, I would highly discourage that.

[WaterlooMarc]

Thanks for the reply. It’s wood siding.

[crokett™]

I would bury it in conduit.

[TDKPE]

I would bury it in conduit.

So would I.  Or tuck conduit up under the eaves or along the bottom of the siding.

If it's PVC conduit, don't forget expansion joints.  That stuff moves a lot with temperature changes.