Can the Faerie Fire spell be oriented in a small space to not affect allies?

Question

Many AOE spells use a point as their origin, and from this point, area is calculated. Faerie Fire doesn't state where spells start. Often, AOE spells are cast in rooms near walls to avoid friendly fire, so part of the spell is negated by the wall, and friendly creatures are safe. Can Faerie Fire be cast this way?

Let's say caster and friendly creatures are in a room measuring 20 by 20 feet. The caster wants to use the Faerie Fire spell. This spell can affect the whole area, including caster and friends. Can caster cast spell partially "inside wall" to avoid affecting friendly creatures? 

Answer 1

Faerie fire creates a cube:

Each object in a 20-foot cube…

A cube’s point of origin is described in the rules for Spellcasting:

You select a cube’s point of origin, which lies anywhere on a face of the cubic effect. The cube’s size is expressed as the length of each side.

No other restrictions are given on the area’s orientation, so you can orient the spell’s area however you need to in order to avoid allies, assuming you can draw a single line between all of your allies and all of your enemies: use that line as the base of the cube’s face, with the area of effect projecting toward the enemy. The presence of a wall doesn’t affect your ability to choose a point of origin on the near side of the wall, it only stops faerie fire from extending to any point beyond the wall.