If I short 100 shares of a stock trading at 20$, what is my initial margin and my maintenance margin that I need to maintain with my broker?
I have a REG T margin account.
Depends on the stock involved, but for the most part brokerages allow you gain entry at 50%, meaning you can short twice the cash on hand you have.
Going forward, you need to maintain 30%, so on a $10,000 short, you'd have to maintain $3000 in your account.
Example, an account with $5000 cash - You can short $10,000 securities. Let say 100 shares of xyz at $100 per share.
After trade settles, you won't receive a margin call until your balance falls to $3000, probably right around the time xyz rises to $120 per share.
Riskier stocks will have higher margin maintenance requirements - leveraged vehicles like FAS/FAZ (triple leveraged) require 90% margin (3x30%) if they are allowed to be 'shorted' at all.