Neutral conductor sizing

Status
Not open for further replies.

anomaly

Member
I'm new at using the code so be gentle. We are working with line-to-neutral loads from a 480Y/277 system and based upon the load arrangement, we have a minor imbalance causing comparatively small currents in the neutral conductor. Can the neutral be sized based upon the calculated currents or must it be sized the same as the phase conductors?.......and where would I find this covered in the the code? Thanks for your help!
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
When you are calculating an imbalance, you are presuming that all loads, on all phases, will be running at the same time. But what if there are several Phase A loads running, and very few Phase B and Phase C? In that event, the imbalanced current in the neutral will be higher than your calculated value. That is a real event, as people living in or working in the building will be turning things on and off all day, and without thinking about what phase powers each load. That is why the NEC gives some allowances for a smaller neutral current, but only in limited cases, and only a limited amount of allowance.

In the 2005 NEC, the reference is Article 220.61.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
In a nutshell, you need to size the neutral for the maximum current it could possibly carry. You can ignore line-to-line loads, and the worst-case neutral load would be that of the greatest connected line-to-neutral among the three phases.

In other words, if (again ignoring phase-to-phase loads) A phase had 200 amps' worth of 1-phase load, B had 180 amps' worth, and C had 210 amps' worth, the neutral must be at least capable of carrying the worst-case 210 amps.
 

anomaly

Member
Wow.....I read 220.61 about six times and still couldn't extract exactly what I wanted to know...BUT...with your help.....it's clear now. Thank you all!
 
Status
Not open for further replies.
Top