CUPERTINO — About 200 union plumbers and steamfitters gathered outside Apple’s under-construction spaceship campus Thursday to protest what they said were unfair wages being paid by a contractor.

Members of UA Local Union 393 claim a contractor on the job is not paying plumbers the “prevailing wage rate” as prescribed by the state’s Department of Industrial Relations.

“There’s workers out here not being paid the right wage for the work being performed, and that’s not right,” said Bill Guthrie, a union spokesman.

Guthrie said the group gathered outside the construction site about 3:20 a.m. At the peak of the job action about 200 workers protested outside the main gates to the site.

By 8 a.m., about 70 protesters remained outside an entrance on N. Tantau Avenue. Protesters at times were blocking the entrance, and at least once stopped a pickup truck from driving into the work site.

There were early reports of traffic being affected by the protesters. The Santa Clara County Sheriff’s Office sent 10 deputies out to the area to “protect the protesters’ First Amendment right” and make sure “kids can get to school and people can get to work,” Sgt. James Jensen said.

The construction manager for contractor Preston Pipelines of Milpitas said the dispute revolved around the union’s view on the high-density polyethylene plastic pipe the company is laying at the new Apple building. “What they’re saying is that this type of pipe that we’re installing is special and therefore should be paid at a higher wage rate, and we’re saying that it’s not that special,” construction manager Dan Condon said. “It’s pipe that goes in the ground. We’ve done it before.”

Condon said he believed the difference between what Preston Pipelines was paying workers at the site and what the union demands is about $20 an hour. “We’re a union contractor, and it’s really kind of disappointing that one union is treating a union contractor like this, and that Apple’s being affected and that the public’s being affected,” Condon said.

Preston Pipelines has lately had about 15 workers at the site, where it has been working for more than a year, with an average labor force of around 25, Condon said.

Apple said the dispute was an issue for the contractor, and the company would not comment.

Staff writer Ethan Baron contributed to this report.