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



Low Voltage Fault Tables

The following tables provide quick order of magnitude fault levels for a a range of typical low voltage situations.

What does N+1 mean?

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...

Gas Insulated or Air Insulated Switchgear

Various arguments exist around SF6 Gas Insulated (GIS) and Air Insulated (AIS) medium voltage switchgear. Recently we had to change a GIS design to AI...

Photovoltaic (PV) Panel - Performance Modelling

In an earlier note on the site [Photovoltaic (PV) - Electrical Calculations], the theory of solar (PV) cell calculations was introduced.  In particular...

Introduction to Cathodic Protection

If two dissimilar metals are touching and an external conducting path exists, corrosion of one the metals can take place.  Moisture or other materials...

Electromechanical Relays

Electromechanical relays have been the traditional backbone of electrical protection systems.  While over recent years these have been replaced by microprocessor...

Cable Insulation Properties

Cable insulation is used to provide electrical separation between conductors of  a cable.  During the historical development of cables, numerous types...

Always Use PPE

A lot of our members work in countries where PPE (personal protective equipment) is regulated or they work for companies/organizations which take employee...

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...

Difference Between Live and Dead Tank Circuit Breakers

A quick post in connection with an email question: Live Tank - the circuit breaker the switching unit is located in an insulator bushing which is live...

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