2% VOLTAGE DROP WHY NOT 3%

Status
Not open for further replies.

stuartdmc

Senior Member
In the 2002 NEC section 210.12 (II) Branch-Circuit rating FPN-4 states a maximum of 3% VD for branch circuits and an overall VD not to excide 5%. In section 215.FPN-2 it states the same thing for feeders.

My question is in what case is the 2% used for either the feeder or the branch circuit. Where in the NEC does it state the difference? Why do we generally use 2% for branch circuits? Why not 2% for feeders and 3% for branch circuits?
 
First, the FPN's aren't mandates by the NEC. I view them as strongly worded suggestions. At one time they were adopted as code where I practice.

I believe you can use up to 3% on either a branch circuit or a feeder, and up to 5% on the total and not have a problem. Some circuits can stand slightly more voltage drop and not be a problem, but I firmly believe this approach is generally a good idea.

Jim T
 
These suggestions are basically a ballancing act. You try to ballance the cost of the installation aginst the amount of energy wasted. You want to put the most money where it will do the most good. So in most cases, the 2% and 3% are a good allocation of copper.

I can think of two reasons why we shoot for a lower number on feeders:

1. In my experience, in most basic buildings, it is usually easier to get a lower voltage drop on the feeders. (Not always the case, but usually.) On most shorter feeders, voltage drop is not even an issue.

Branch circuits on the other hand, often have larger voltage drops. And it usually just isn't economical to use #8 wire for a branch circuit.

2. Feeders tend to have more continuous loads. The difference between 2% and 3% voltage drop occurs 24 hours a day, 7 days a week.

On the other hand, some branch circuits are seldom used (I'm thinking receptacles). Maybe only when the cleaning crew plugs in a sweeper. And then, you probably don't have the circuit fully loaded anyway. So its less efficient to use more copper for circuits that are less often used.

Steve
 
Status
Not open for further replies.
Top