The margin requirement for short (credit) vertical spreads is equal to the difference between the strikes multiplied by the number of spreads. The credit received from the spread can be applied to the margin requirement.
 

example

For example, in order to open the following spread, you would need $65 in your account (the margin requirement less the credit received).

  • Sell to open 1 ABC 100 Call for $1.40
    • 1.40 x 1 x 100 = +$140
  • Buy to open 1 ABC 101 Call for $1.05
    • 1.05 x 1 x 100 = -$105

Margin Requirement: (101 - 100) * 1 * 100 = -$100
Net Credit: $140 - $105 = +$35
BP Effect: -$100 + $35 = -$65