Why Cash-Neutral Long–Short Portfolios Can Exceed Margin

Summary

The document describes a paper-trading problem in which a cash-neutral long–short equity portfolio repeatedly runs out of excess liquidity when target positions are converted into share orders. The trader scales portfolio weights by a high fraction of account value, calculates target shares using recent closing prices, and submits the differences from current holdings. Small day-to-day changes in account value do not prevent margin problems, leading the trader to suspect that low-priced stocks may require more margin.

The reported resolution is that the broker assigned a 100% margin requirement to most of the traded names, leaving effectively no excess liquidity. This illustrates that cash neutrality and stable account value do not guarantee that positions fit broker margin rules. The account is a paper-trading example, and the excerpt does not explain the broker’s margin model, how requirements vary by security, or what reserve level would be sufficient. Traders need to check instrument-specific margin requirements when sizing and routing orders.

Key ideas

• Cash-neutral target weights do not ensure that a portfolio meets margin requirements.
• The example converts target dollar exposures into share quantities and trades the difference from current holdings.
• Broker-specific requirements can make certain stocks consume substantial excess liquidity.
• In the reported case, a 100% margin requirement on most names explained the order rejections.

Tags

# L/S cash-neutral portfolio exceeds margin


# L/S cash-neutral portfolio exceeds margin












I am testing out a systematic, cash-neutral, long/short strategy in a paper trading account with Interactive Brokers. Each day, an algorithm tells me what my target portfolio should look like in terms of relative position sizes. So, I multiply these target percent positions by 97% of my account value to yield target dollars amounts. I then divide these dollar amounts by the last close in order to get the target number of shares, take a difference between my currently held positions and these target positions (in shares), and send the corresponding orders to the broker. The problem is that I constantly run out of Excess Liquidity when I try to place these orders (i.e. I exceed my maintenance margin). This happens even though my account value generally does not move by more than 0.5% in a day (it is a diversified, cash neutral portfolio). I trade a lot of low price stocks (under 5 and 2.5 dollars so these might have increased margin requirements) but I feel there must be a bigger reason why this keeps on happening. Should I try keeping more cash in reserve, i.e. target positions to 90% of account value instead of 95-97%? Or am I completely off base and the problem is elsewhere? Any advice is appreciated. Thanks!

UPDATE: The mystery has been solved. For most of the names I trade, my broker (IB) happens to have a special margin requirement of 100%, which effectively reduces the account's excess liquidity to 0.

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This summary was written by Stratmill's research agent from the original; it is not a copy of the source.