May I run THHN above ground under a house unprotected or is a conduit required for this? The wires consist of 3 4AWG wires with an 8AWG ground going from a 60A breaker at the (100A) service to a newly built garage via underground wiring from the garage in 1.25" schedule 40 PVC and then under the house, which is raised on piers.
Any time you are running bare conductors in a dwelling unit that are not part of a cable system such as NM or MC, then they must be run in a conduit. Once you get to the exterior of the unit you could transition to a Direct burial cable such as UF or you could continue in a conduit system. The most common under ground conduit would be PVC and it could also be used under the house.
Special note: Most conductor insulation is multi rated but a straight THHN rating is not rated for wet locations it should also be labeled as THWN to be approved for buried circuits.
Remember you need Schedule 80 PVC where the conduit approaches the surface or in a place subject to potential damage. It is thicker pipe, but the joint surfaces are compatible with Sched 40. (it sacrifices inside diameter).
Single wires need to be inside a conduit or raceway their entire run. So no, you cannot direct-affix THHN wires. You have several options:
- Run conduit the entire way. Feel free to switch to EMT once you are out of the earth; you will get less contact cement on your fingers and you won't need expansion joints. Bonus: EMT is the ground path so you don't need that #8 ground. (which is fine by the way).
- Install a junction box, and splice to a multi-conductor cable for the rest of the run. This will require a Rather Large junction box, and Polaris connectors, and large entry hole, and large cable clamp for the cable - and the cost of all that might be worse than the EMT.
- The dreaded "cable in conduit" - run appropriate wet-rated cable the entire run, and go through the pain and suffering of cramming the cable through the conduit. Not a fan of this one; fair chance it will overwhelm your DIY skills and force you to pay an electrician a small fortune to pull the cable and finish the job (they won't just pull).
As an aside, your wet location needs THWN wires, but almost all THHN is dual-marked as both THHN and THWN-2.
Some other gotchas:
For a 100A feeder to a subpanel, you need #3 copper (better: #1 aluminum). Lots of people get tripped up by this*. If you are married to #4, you are limited to 70A.
#include standard "Aluminum is the right stuff for feeder" speech.
#include standard "compute voltage drop based on practical load not breaker trip, and don't compute based on 3%" speech.
At an outbuilding, your subpanel will need a shutoff switch. The cheapest way to get a shutoff switch is pick a subpanel that has a main breaker. You're not using it as a main breaker, capische? So don't match the main breaker to he feeder breaker. It's irrelevant and will force poor panel choices, like a 4-space panel for 60A or 12-space for 100A. You want more spaces than that -- really.
Spaces are dirt cheap when buying a panel. The biggest cliche in home subpanels is spending $400 too much on wire, but saving $40 getting a too-small panel, promptly running out of spaces**, and having to spend another $100 replacing the panel. You want enough breaker spaces that you never run out, because spaces are cheap before you install it, and expensive after.
*The reason this mistake happens is people drawing from a table intended for service entrances. This has been such an epidemic that the table has been removed from NEC.
** "circuits" don't count as "spaces", so a 12 space/24 circuit panel is in fact only 12 spaces. 24 relies on "double-stuff" breakers, which don't come in AFCI or GFCI, so such a panel is only 24 circuits in an imaginary world where those are not required. Here on Earth it's a 12-circuit.