E. C. Ernst, the big Washington-based electrical construction firm that filed a Chapter XI bankruptcy petition last Friday, blames over-expansion and overseas business for its financial problems.

In a statement filed in federal court in New York, Ernst said "expansion of the company, primarily overseas, is the primary reason for the difficulties necessitating the filing of the petition."

"The loses sustained by E. C. Ernst International have been especially severe," according to the affidavit filed by J. J. McCarthy, Ernst's corporate counsel and secretary.

The bankruptcy petition said Ernst has assets of $57 million and unsecured liabilities of $36.5 million.

The company said Friday when it filed for bankruptcy that the action was necessary because several banks had called due more than $20 million in loans and had attached $925,000 in Ernst's bank accounts.

When a company "goes Chapter XI," it is allowed to postpone repaying its creditors while its management reorganizes and works out a plan for repayment.

The petition filed by Ernst says its "difficulties and problems could be solved and (Ernst) will be able to propose an arrangement to its creditors providing for a distribution to them far in excess of any sums which would be available to them in a liquidation" of the company by sale of its assets.

Ernst is the second largest electrical contractor in the nation, the petition says, with almost 2,000 employes and a monthly payroll of $3.3 million.

The petition estimates Ernst will incur other expenses of about $4 million in the next 30 days and will generate enough cash to cover its spending during that period.

"The company has an extensive backlog of contracts and anticipates entering into new ones so that it might be profitable in the future."

Ernst lost $5.8 million on sales of $129 million last year and reported a loss of almost $7 million for the six months ended Sept. 30. The company had reported profits for the first three quarters of 1977, then revised its figures and posted the substantial loss.

The bankruptcy filing indicates Ernst's financial problems also revolve around disputed claims and additional work orders on construction contracts. Ernst's losses "would be reduced or even eliminated" if the company collected on all those claims, the petition contends.

A lawsuit filed against Ernst claims the company improperly reported the earnings from incompleted jobs as part of its profits.

The bankruptcy petition lists only one Washington firm, Maurice Electrical Supply Co., among Ernst's 10 largest creditors.

The biggest creditors, listed in order, are First National City Bank, New York; First Pennsylvania Bank, Philadelphia; Harris Trust Co., Chicago; Credit Suisse, New York; Barclay's Bank, New York; The Travelers Insurance Co., of Hartford, Conn.; the Internal Revenue Service; General Electric Supply Corp.; Westinghouse Electrical Supply, and Maurice.

The American Stock Exchange yesterday suspended trading in Ernst stock pending further developments in the bankruptcy case.