**Proficiency Bonus is never added to damage rolls. Period.** 

Proficiency bonus is only added to attack rolls, ability checks, and saving throws for which you are proficient. Damage rolls will only ever add the appropriate modifier for the stat which was used to make the attack. However many spells, especially cantrips, do not add your casting mod to the damage. 

**Firebolt requires you roll 1d20+INT+proficiency vs. AC and deals 1d10 damage** 

>Make a ranged spell attack against the
target. On a hit, the target takes 1d10 fire damage. A
flammable object hit by this spell ignites if it isn't being
worn or carried. - PHB p. 242

**Spell attack = roll a d20 and add prof and mod. Spell save means target rolls defense.**


Each spell's description indicates whether you need to make an attack roll, or the target needs to make a saving throw, or both.
>**Saving Throws**

>Many spells specify that a target can make a saving
throw to avoid some or all of a spell’s effects. The spell
specifies the ability that the target uses for the save and
what happens on a success or failure.
The DC to resist one of your spells equals 8 + your
spellcasting ability modifier + your proficiency bonus +
any special modifiers.

>**Attack Rolls**

>Some spells require the caster to make an attack roll
to determine whether the spell effect hits the intended
target. Your attack bonus with a spell attack equals your
spellcasting ability modifier + your proficiency bonus.
Most spells that require attack rolls involve ranged
attacks. Remember that you have disadvantage on a
ranged attack roll if you are within 5 feet of a hostile
creature that can see you and that isn’t incapacitated
(see chapter 9). - PHB p. 205


**There are two types of attack spells requiring d20 rolls, melee attack spells and ranged attack spells.**

The difference is that melee attack spells will not be made at disadvantage when next to an enemy combatant whereas ranged spells would be (and all ranged attacks for that matter).