Cardiff crushed a puffless Dragons to extend their winning run in this derby with the biggest victory of the lot.

Rhys Carre, James Botham and Tomos Williams all scored twice while Tinus de Beer kicked 10 points.

Dragons chased a late bonus but Cardiff's 18th successive league win over their rivals was never in doubt.

Such was the appetite for this Boxing Day derby that Cardiff had been forced to apply for permission to expand their capacity to 12,000.

Yet the full-blooded contest never materialised as the game rapidly declined as a genuine contest.

Cardiff simply outplayed Dragons in every single area of the game.

The front-row dominated, the second-rows were pacier, the back-row was more dynamic, the half-backs had more guile and the backs were simply better in everything they did.

For the Dragons, they will end the year at the foot of the United Rugby Championship and some questions will have to be asked following a capitulation such as this to their local rivals.

This losing run that stretches back to 2014 is becoming a plague for the Dragons, rendering even their best players clawless - or clueless.

They had barely touched the ball before Carre scored the opener inside three minutes.

Dragons made a contest of it for ten minutes when Keddie finished Aneurin Owen's burst but once Carre latched onto Tomos Williams' fizzing pass for his second, it was one-way traffic.

Ben Thomas brushed off Taine Basham with embarrassing ease for the third before Tomos Williams scored the pick of the bunch - a length of the field attack - to seal a bonus point after just 18 minutes.

It summed up the difference between the teams. Steff Hughes knocked on from a promising Dragons attack, Cardiff reached Williams' clearance in greater numbers to rob Ashton Hewitt.

Liam Belcher, a hooker, beat Basham with a dummy before Teddy Williams - a lock - beat opposite number George Nott with a sidestep before handing the Wales scrum-half a scoring pass.

Flanker Botham then capped his first appearance of the season following a hamstring injury with two tries in the space of seven minutes.

And by the time scrum-half Williams was waved through for his second - and Cardiff's seventh of the half - the bumper Arms Park crowd were barely cheering any more.

Only twice before in 22 years of the URC and it's previous incarnations had a team crossed seven times in the space of 40 minutes - Cardiff against Connacht in 2008 and Edinburgh at Caerphilly in 2002.

Dai Flanagan made three changes at half-time - he could have made far more - that included Dan Lydiate's return from injury but Cardiff still scored next.

Kieron Assiratti thundered over from 20 yards out to a raucous cheer as Cardiff hit the half century.

The rival scrum-halves were ticked off for their scuffle on the floor. It was the first time Dragons had shown any fight and even then it was half-hearted.

At 50-7, it was little wonder that Cardiff finally took their foot off the gas and Dragons somehow found themselves chasing a bonus point.

Their lineout drive saw Harrison Keddie grab a second before forcing Rhys Litterick into conceding a penalty try and yellow card on the hour.

But Cardiff, fittingly, had the last say when Mason Grady and Cameron Winnett combined to bully their way through four defenders for the full-back to claim the hosts' ninth try.

Cardiff: Cam Winnett; Josh Adams, Rey Lee-Lo, Ben Thomas, Mason Grady; Tinus de Beer, Tomos Williams; Rhys Carre, Liam Belcher (capt), Keiron Assiratti, Rory Thornton, Teddy Williams, Alex Mann, James Botham, Mackenzie Martin.

Replacements: Evan Lloyd, Corey Domachowski, Rhys Litterick, Seb Davies, Lucas de la Rua, Ellis Bevan, Willis Halaholo, Gabriel Hamer-Webb.

Sin Bin: Litterick (59)

Dragons: Jordan Williams; Rio Dyer, Steff Hughes (capt), Aneurin Owen, Ashton Hewitt; Cai Evans, Rhodri Williams; Rhodri Jones, Bradley Roberts, Lloyd Fairbrother, Joe Davies, George Nott, Harrison Keddie, Taine Basham, Aaron Wainwright.

Replacements: James Benjamin, Aki Seiuli, Leon Brown, Sean Lonsdale, Dan Lydiate, Dane Blacker, Will Reed, Jared Rosser.

Referee: Ben Whitehouse (WRU)

Assistant referees: Mike English & Gwyn Morris (WRU)

TMO: Wayne Davies (WRU).