From TDAmeritrade.
Position Minimum maintenance requirement
Long stock position valued at $4.01 per share or more 30% of the current market value of the stock
Long stock position valued between $2 and $4 per share $2 per share
Long stock position valued at $1.99 or less per share 100% of the current market value of the stock
Short stock position valued between $5.01* and $16.67 per share $5 per share
Short stock position valued between $2.50 and $5 per share 100% of the current market value of the stock
Short stock position valued at less than $2.50 per share $2.50 per share
*If the price of a security that has been sold short falls below $5 per share, the maintenance requirement is 100% of the market value with a minimum requirement of $2.50 per share.
This is typical of most, if not all, major brokers.
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