# Parallel Conductor Overcurrent Protection

#### ckdckd

##### Member
Does NEC 2011 article 240.4(B) apply to conductors in parallel? I can not seem to find an exclusion regarding parallel conductors, but I thought there was one.

A specific example would be if two parallel runs of 500 KCMIL (75 degree C) are permitted to be protected with an 800 Amp breaker, since the combined ampacity is 760 which does not correspond to a standard breaker size and the next standard size is 800?

I know 240.4(B) allows me to protect a single set of 500 KCMIL (380A at 75 degree C) with a 400 Amp breaker, but for some reason I think I remember seeing something prohibiting it for parallel conductors.....I just can not find it now. So, I'm wondering if it was code or just something somebody said but without code backup.

#### charlie b

##### Moderator
Staff member
Yes you can protect a pair of parallel 500s with an 800 amp breaker. But keep in mind that you can't use a pair of parallel 500s if the load is over 760 amps.

The code does allow us to parallel conductors. We generally take that to mean that paralleling a conductor that has an ampacity of 380 amps with a second, identical conductor, gives us a combined ampacity of 760 amps. But I don't believe there is anything in the code that explicitly says this.

#### augie47

##### Moderator
Staff member
It has been discussed previously in the Forum and, if memory serves me correctly, there is no direct wording to that effect as Charlieb states. You might look at 310.4 and 310.15(b)(2) which, IMO, supports the practice.

310.4?

#### ckdckd

##### Member
That makes sense.
Thanks.

#### Carultch

##### Senior Member
Does NEC 2011 article 240.4(B) apply to conductors in parallel? I can not seem to find an exclusion regarding parallel conductors, but I thought there was one.

A specific example would be if two parallel runs of 500 KCMIL (75 degree C) are permitted to be protected with an 800 Amp breaker, since the combined ampacity is 760 which does not correspond to a standard breaker size and the next standard size is 800?

I know 240.4(B) allows me to protect a single set of 500 KCMIL (380A at 75 degree C) with a 400 Amp breaker, but for some reason I think I remember seeing something prohibiting it for parallel conductors.....I just can not find it now. So, I'm wondering if it was code or just something somebody said but without code backup.
It is expected that you will treat parallel conductors as a single conductor for with combined ampacity and combined (1/resistance, i.e. conductance) for voltage drop purposes, provided that they are 1/0 and larger, and built in as identical of a manner as practical. Length, metal type, insulation type, raceway type, termination manner, conditions of use, etc.

They can either share a raceway, or be routed through separate raceways that are as identical as practical. If they share a raceway and if you are using multiple raceways, there has to be the same number of sets in each. You couldn't use two sets in one, and one set in the other.