Hello, it can be too far gone to save and can be dangerous, it generally happens when the normal wind-up method is over the hand and round the elbow, this creates a natural twisting motion resulting in a liquorice stick cable, I normally coil my cables by holding in 1 hand and coiling with other,you can also give a slight twist in opposite direction to negate any twist that may get into cable, hope this will help, Bill

Vic. Throw it out, for the sake of $30 go and get one on a reel. or free wind as Woodie said

When they get a twist like you described the inner cables rub against each other leading to a possible short circuit, very common on floor polishers and the vacuum cleaner leads when the cleaners wrap the cords around the motor or vac barrel . Bunnings sell the 15amp leads rated for vans between $25 - $35 dollars already on a reel

When left on the reel they become a heat sink and will melt the cable to the drum . Had one that came into my office back when you could buy the alloy reels it was just a molten mass of copper and alloy. It was all because the operator didn't wind the cord fully and despite the fact that the load was under the rated 10 amps it generated enough heat for a melt down

20 M should be enough. 10 m is not enough.
Reminds me of a humourous incident in 2005. We are at Crystal cascades CP in Cairns and 2 new couples arrived, the crowd of course drag up chairs to watch the setup and add a commentary... so the 2 males are chalk and cheese one is a nice guy new to vanning travelling with his mate a bucket mouth. Newbies lead is too short by about a metre, I wandered over and suggested that he move his van a metre closer to the power pole but Bucket mouth says it's ok he has the makings of a short lead...... So BM cuts 3 metres off his own lead and puts a socket on it and puts a plug back on his cut end and the day ends round the campfire.....
We left Cairns and did some doodling around FNQ and bump in to this group again at Georgetown, this time no matter what BM did he could not get his van close enough to stay on the next site to his mate and use his shortened lead..... Just as nice guy is about to cut 3 metres off his lead (with side cutter) to presumably make up a lead for his mate I race over and pull his lead out of the power box and cop an earfull from BM who was most appologetic after I explained and then was greatly relieved when I told him that they already had a short lead that they made in Cairns. you could see the lights go on, the looks on their faces was priceless as it dawned on them..... "how did you know?" they asked.....

Having spent years in a recording studio where cables are worth a fortune i learned very early how to roll a cable. Like was said hold the lead with one hand (loop hand), plug towards you. Then make a loop of cable the size you want the coil to be with the other hand. Hang that loop in the loop hand. Then reach down and twist the palm of your hand around 180 degrees. Grab the cable and make a loop, place this in your loop hand. Then reach down normally and make a loop (same size) and place the loop in your loop hand. Rince and repeat.
We had 30 year old mic cables that were perfect. The chief Engineer told us that you should be able to low throw the cable out parallel to the ground and it should snake out in a straight line. BS I thought and then he did it ...over and over again.

So treat your cables well and they will give you years of great service.

Mick

Last year I was at a well known Tweed coast park, I was collecting the rubbish inside the van to walk down to the bins when a van backed in diagonally opposite me. I didn't take too much notice, but when I was walking back from the bins, the bloke said "hey mate can you help me here, I'm new to vanning this is our first trip - how do I plug this thing in ? " He was holding a 'funny orange tube' - name starts with A and ends in n. I walked over and explained to him that the Ampfibian he was holding was only when he wanted to plug his van into a 10A GPO at home - that all parks have 15A GPO's and the 15A van lead plugged straight in. He said " the bloke at the dealer told me I needed one, so I bought one'. He said to me 'you seem to know a bit about the Ampfibian" - I said I was an electrical engineer for over 35 years. Bouquets to the dealer for telling him he needed one, but brickbats for not explaining how to use it !

The other thing that will get the cord corkscrewing is not removing both connectors before rolling the lead up. If you have a lead with one captive end like A-Vans have, remove the plug from the pillar and wind the cord up from the captive end.