What does N+1 mean? 

By on

As a professional electrical engineer, people sometimes remind me that what I take for granted is a new concept for other people.  Following a recent admonishment, I should make more effort at covering introductory level topics.

The term 'N+1' relates to redundancy and simply means that if you required 'N' items of equipment for something to work, you would have one additional spare item.  If any one item of equipment breaks down, every thing can still work as intended.  For example if you need two UPS for a small data centre, you would install three to have N+1; if you have a water pump in your building your would install a second one for N+1; if you require three transformers to power your facility you would install four for N+1.

People sometimes use the term 'N-1' to mean the same thing (i.e. if you lose one item you can still work).  This is a less common.

This type of terminology is often expanded.  '2N'  (sometimes referred to as 1+1) means you have twice as many items as you need - in the data centre example you would install four UPS.  The pumping example is already 2N (in addition to being N+1).  Less common, but possible are things like 'N+2", '2N+1", etc.

A final word on topic levels.  I do try to mix my posts at varying levels so that anyone visiting the site will find something of value.  If while browsing the site you have an idea for a short post on a particular topic or  want some thing explained in simpler terms or in greater detail;  just let me know.  You can leave comments on related posts or use the Contact Us page to send in a request.  While there is no guarantees, I will try to address as many requests as I can.



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



myElectrical - Cable Sizing Tool Upgrade

Our IEE cable sizing was wrote a few years ago and had become rough around the edges. I thought it was time to give the tool a service. Unfortunately when...

UPS Battery Sizing

Various techniques exist to enable the correct selection of batteries for UPS applications.  The procedure described below is one of the more common. ...

Railway Electrification Voltages

This post is quick introduction and overview to different railway electrification voltages used in answer to a question sent in via email. While there...

Hazardous Areas – IEC and NEC/CEC Comparison

Depending where in the world you work, you are likely following one of two standards International Electrotechnical Commission (IEC) National...

A mechanical engineering paper, some history and memories

I was digging in my bookshelf and came across the 80th Anniversary Association of Mine Resident Engineers, Papers and Discussions Commemorative Edition...

How to refer fault levels across a transformer

Over the past year or so I've been involved in on going discussions related to referring fault levels from the secondary of a transformer to the primary...

Battery Sizing

This article gives an introduction to IEEE 485 method for the selection and calculation of battery capacity.

What is a rectifier transformer?

I've recently come across this question a couple times browsing the internet. Decided to give a quick answer here. A rectifier transformer is a transformer...

Dielectric loss in cables

Dielectrics (insulating materials for example) when subjected to a varying electric field, will have some energy loss.   The varying electric field causes...

Calculating Cable Fault Ratings

When selecting a cable, the performance of the cable under fault conditions is an important consideration. It is important that calculations be carried...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note