Directors of Carborundum Co. today agreed to accept Kennecott Copper Corp.'s offer of $66 each for nearly 8 million outstanding shares, the president of the abrasives manufacturer said after a board meeting.

Carborundum president William H. Wendel also said he has been told by Kennecott president Frank R. Milliken that the Kennecott board also accepted the deal today.

Opposition to the proposal by several large Kennecott shareholders had been reported. The Kennecott board meeting was not open to the press.

Wendel said he now expects Kennecott to make the formal offer on or about Nov. 29 at the expiration of the waiting period required by the Delaware corporation law.

Meanwhile, the Federal Trade Commission, which had announced it would investigate the previous offer of $47 a share for Carborundum by Eaton Corp. of Cleveland, said it now will investigate the Kennecott proposal. Eaton withdrew its bid.

Trade circles said the Kennecott offer could amount to as much as $560 million if all Carborundum stock options held by insiders are exercised, and the hansome price would give Carborundum insiders every incentive to exercise the options.

The difference between the Kennecott offer for the shares actually outstanding and the price if all the options are exercised is about $65 million, but the insiders have to buy the stock before they can sell it to Kennecott.

The difference between the Kennecott offer for the shares actually outstanding and the price if all the options are exercised is about $65 million, but the insiders have to buy the stock before they can sell it to Kennecott.

A Carborundum lawyer told United Press International all the option exercising prices are below $40 a share and some probably are much lower. But he cautioned that the assumption Kennecott will pay the full $66 for all the optioned shares still was subject to negotiation. He also said large Carborundum stockholders such as the Mellon family could exercise at least as much influence on the transaction as the excutive insiders.

Arbitrage traders also stand to make huge sums out of the Kennecott offer. Some Wall Streeters estimate the arbitragers might make between $30 millio and $40 million because they have been buying Carborundum shares at prices of $35 to $45, aild more than 2 million shares have been reported sold.

The opposition by some large Kennecott stockholders is based on the $66 price, which they said was much too high for Carborundum stock, which was selling for only about $34 at the time of the Eaton offer. The $66 price was described as nearly twice above $50 a share after the Eaton offer - more than $3 over the Eaton of fering price value.

But spokesmen for First Boston Corp., which is representing Kennecott, defended the price, saying it would take at least that much to ensure the success of the offer. They pointed out that Carborundum rose above $50 a share after the Eaton offer - more than $3 over the Eaton offering price.

The transaction represents a windfall for most Carborundum stockholders. The stock never has sold for $66 a share and earlier this year was trading for less than half that - $31,625.