Nasdaq OMX Group will be allowing institutions and other securities firms to stipulate a minimum quantity of shares that they are willing to trade on one of its exchanges.

The electronic trading firm that that members of its “price-size exchange,’’ known as PSX, will be able to decide the minimum size of a trade they are willing to engage in. This will come some time in the fourth quarter.

“We have a number of major features coming into this product set in the fourth quarter,’’ Nasdaq OMX Group chief executive Robert Greifeld said Thursday at the Barclays Capital Global Financial Services Conference in New York.

“One is we'll have a minimum quantity, ‘’ Greifeld said, “in that you can put your 5,000 shares into PSX, you'll be able to put it in. And you will not get an execution unless you get, say, 5,000 if you put that as your minimum quantity or 3,000 or 2,000 or whatever you so decide.”

This has been “probably the single largest request we've had from the user base,’’ since the launch of PSX last fall.

Greifeld, at the time PSX was booted up in October 2010, called the “price size” approach “a fundamental change to market structure.”

All other U.S. exchanges at the time, including Nasdaq’s, use the “price-time” model, that gives priority to the time an order arrives, at a particular price.

The average trade size at that time was about 220 shares and Nasdaq hoped to see the average trade size on PSX go past 500 shares. The idea: Attract institutional investors, who want to trade in thousands and tens of thousands of shares, in a single transaction.

The minimum quantity stipulation would allow them to do that.

Nearly a year later, Nasdaq OMX says that PSX is “consistently trading more than 1 percent” of all shares in U.S. equity market.

“Driving this growth has been trading of Tape B securities with a record 2.79% matched Tape B market share on June 2nd and a total of 2.18% Tape B market share for the month of June,’’ it says.

Tape B records trades in NYSE Arca and NYSE Amex stocks.

Greifeld Thursday said that trading firms also will soon be able to route their orders elsewhere, if not fulfilled on PSX.

“We also will introduce routing capability into the PSX system in the fourth quarter, so that it's not an island by itself,’’ he said. “We'll take the standard INET routing and merge that into the PSX technology.”

INET is Nasdaq’s flagship electronic trading technology, acquired in 2005 from Instinet.

--This article first appeared on Securities Technology Monitor.