What regulations apply to 48VDC wiring etc?
If the plan is to sell the product your designing, then odds are you'll want to certify your product with an ETL. Most products fall under IPC-61010, and exception would be if it's IT Equipment, Medical Equipment, ect. Use the specs for the product group that the to be designed product falls under. All of these specs are pay-walled, and are not cheap. I'll summarize parts of 61010 to give an idea of what is required, but it would be best to find a regulatory consultant.
Source: Electrical safety and isolation in high voltage
discrete component applications and design hints
What safety issues might arise?
The two main ones that I can think of are shorting and a user coming in contact with the wire.
If I remember right (not completely sure) voltages are not considered hazardous until after 60VAC or 75VDC. I've ran 48V on a PCB and through a cable to a heater and didn't have to take any special precautions for user safety but that portion of the product was in a user inaccessible enclosure.
Whatever voltage you have in the design, it will need to conform to certain spacing. PCB's and wire assemblies have different spacing as shown in the chart below. Pollution degree of the product (dirt makes arcing more likely) determines how far the wires or traces must be placed.
The user of the product needs to be protected against electric shock (see section 6.1.1 of 61010), during normal operation or a fault. If the hazardous voltage is not accessible to the user and requires a tool, then a warning sticker can be placed on the product to make it sufficiently safe.
As far as the tubing, I'm not aware of any requirements for the material for insulation. The main thing 61010 is concerned with is materials for voltages over 100V which are prone to arcing. That being said it would probably be a good idea to get a tubing that has some kind of IEC certification. I would think that it would need to be flame resistant.