I have a pig tail made of type s cord with a dryer rec for one end and wire nuts for the heater end.. Using red wire nuts accepting 10 ga. Seems tight . Is this OK. Also should I use bx cable instead. I hope not because the so wire and 30 amp dryer plug were very expensive. I do not have a disconnect at the heater but the panel is in the same room/ but not is sight of the water heater. p.s. I use all of the strain relief cords with the s cords that are required. If you have different Ideas please let me know \.. Thank you Robert

If this is a male plug on the SO cord you added, there is no problem.

A plug and receptacle acts as a service disconnect.

Read the wirenut datasheet that comes with the package to see if it is rated to hold two #10 copper wires, or whatever quantity and size wires your splicing.