Entertainment and event professionals have been helping turn the Principality Stadium into a 2,000-bed hospital to treat coronavirus patients.
Theatre and lighting technicians who usually build festival sites and TV sets are among those helping to put power into beds and create kitchens.
Company director Tom Feierabend said they were asked because they construct festival sites and suchlike at speed.
He had been "waiting for the call" since China started building hospitals.
The 30-year-old, working as a freelancer at the site, said: "When a hospital needs to be built in a stadium that's not just a big empty room, it's the theatre, TV, and events industry that steps up."
That equipment, staff and knowledge was there "because the arts industry exists", he said.
"If there weren't theatres and music festivals in the UK there would be no power in these field hospitals, there would be no catering in these field hospitals," he said.
"There would be no field hospitals because there wouldn't be the equipment and skills required to mobilise and build a small town from scratch in 10 days."
Mr Feierabend, from Cardiff, said the fifth floor of what is now named the Dragon's Heart Hospital was complete.
"TV and film set companies are building back boards for beds, theatre technicians are running in power for beds, events crew are building marquees and putting down floors," the director of T&M Technical Services said.
Festival electricians were building power distribution systems while catering firms were building kitchens, feeding workers, and gearing up to feed staff and patients.
"Site management teams are managing the flow of equipment and staff, coordinating ID badges, safety, communications, and directing resources where they are needed," Mr Feierabend said.
T&M has previously lit up the exterior of the Senedd, worked with the Sherman Theatre and the National Theatre of Wales.