My D&D group is new and we are all still getting used to playing the game and are running through the Starter Set adventure.
During this adventure we end up capturing a Wizard and we bound and gag him. My character being paranoid made sure to attach manacles to him and also to use two different lengths of 50ft rope to ensure that he would not be able to break free. During the return back to town the DM had him keep rolling strength checks against the manacles and rope and one by one he kept breaking through them and I ended up having to borrow the entire party's supply of rope to keep him secure because he kept breaking through them. By the time I was able to deliver him he was wrapped in 200 ft of rope and looked like more rope than man after breaking my manacles and two different lengths of rope.
I know that the rules say that the DC of Hempen rope is 17 but I still think that in any real world sense it is ridiculous to think that this Wizard can Samson his way through each individual rope while also being held by several other lengths of rope and a set of manacles. One would think that the weight of the ropes alone would keep him in check.
I guess my question is that is there any precedent for the DC being raised when you are attaching multiple different factors into keeping a prisoner secure or are you just supposed to roll each rope individually?
Edit: My DM has responded to my post and it looks like I didn't have all of the information originally
"I rolled 2 rolls per day of travel. This was his daily attempt to free himself from his bindings with disadvantage. He got 2 Nat 20's the day he broke the manacles and a 19 & 20 to break free of the rope. His Str modifier is -1, but 18 still clears the rope's DC.
He had disadvantage due to the leather armor and sheer volume of rope around him
He also had to break the manacles before he could even start trying to work at the rope, since you did put those on him first"
Best Answer
He must escape/break-free from each separately
However, there are only 2 restraints present: the manacles and the rope - the fact that you used 2 lengths (or 7 or 15) is irrelevant - you tied him up once.
He only gets one shot to escape and one shot to break each
Your DM was unfair; he was inducing Rolling to failure. The NPC can only attempt to escape from each binding once and only attempt to break each binding once. If he fails he is not agile/strong enough to do it trying again won't make him so unless circumstances change.
Allowing the NPC to do this means that he will succeed eventually (always assuming that it is possible for him to do so) thanks to how "rolling to failure" works out mechanically.
How I would rule it
The manacles have a DC20 Strength check to break and a DC20 Dexterity check to escape. The rope has a DC17 Strength check to break and a are an opposed check between his Dexterity and your Dexterity (Slight of Hand) [Sage Advice]. If you were wise, you would get someone to Help you tie him up to get Advantage.
Being tied up, I would give him Disadvantage on all of these because he is under the restrained condition. (See Appendix A).
So that's 2 chances to get out of each and he has to get out of both to escape.