Amid ongoing debate in Washington about a corporate tax overhaul, some of the largest companies in California are paying well below the required income tax.

Of the 25 largest companies in the state, measured by revenue, roughly half reported that they paid close to the full tab or more for last year. But more than a third reported an effective tax rate of less than 25 percent - considerably less than the required 35 percent U.S. income tax rate for corporations. 

While these companies are all likely complying with U.S. tax laws, various loopholes make it possible to skirt the country's nominal corporate income tax rate.

Amgen of Thousand Oaks and Sempra Energy of San Diego have the lowest effective rate - 13 percent. Amgen, a biotech company, hasn't reported a tax rate above 20 percent in the past three years.

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"Residents of California paid $440 (each) to make up for the taxes that are avoided by corporations and wealthy individuals through the use of offshore tax havens," according to a recent report by the California Public Interest Research Group. The group calculated the estimate by dividing proceeds the state would have received from income that was instead shifted to offshore tax havens - $11.9 billion per year - by the number of filers in the state.

Besides Amgen and Sempra, eight other companies - including eBay, Hewlett-Packard, Google, Northrup Grumman, and Apple - reported a tax rate of less than 25 percent.

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