13 June 2012
The IEE regulations (a.k.a. BS7671)
are used in over thirty countries worldwide. In this paper, I will
explain how we customized Autodesk® Revit MEP® to suit these
standards. You will learn how you can extend this customization to
suit your company's needs. We will also cover new features in Revit
MEP 2012, and through case studies from work at Capita Symonds, we
will demonstrate how you can finally make full use of the "E" in
Revit MEP.
Intent
There have been many whitepapers, AU classes
and training sessions on the electrical functionality of Revit MEP,
and so this session will not in particular cover again those same
items. The intent of this session will be to inform the users of
Revit MEP, who sit outside the US and need some form of
localisation done, based on their local standards.
The author has completed some customisation to
the standard electrical template, based on the UK requirements in
BS 7671 (also known as the 17th Edition of the IEE (IET)
regulations). This work was competed whilst working for Autodesk,
in preparation for the 2012 release, and is now available to all in
the standard Electrical template for 2012 in the UK. This template
is named Electrical-DefaultGBRENU.rvt.
This paper and the class that accompanies it,
intends to show how to make customisations, and to explain settings
given in the UK template. In doing so, the Author will give real
use cases of how the Author’s new employer - Capita Symonds - is
using the electrical tools to their best advantage.
Introduction and history
The main reason the Author became involved in
customising the standard Electrical Template was in answer to
requests from Revit MEP users around the UK, Europe and the Middle
East for a more localised set of parameters that would better suit
their needs.
Up until the 2012 version of Revit MEP, the
electrical template was very much a straight copy of the United
State’s version of the same, and as such it was pretty much
unusable as a design template.
Quite apart from the basics of differences in
voltage, regulations and standards, the very cables (or wires as
Revit calls them) are quite different in the US, compared to those
from a nation using the BS 7671 standard. This prompted the Author,
a Mechanical Services Engineer and CAD and Revit user by trade, to
delve into the murky depths of what the BS needs, and how far Revit
can be customised to suit those needs.
This required a more in depth knowledge of the
Revit template itself, a basic understanding of BS7671 electrical
requirements, and for a mechanical engineer, a masochistic
nature.
The BS7671
standard“BS 7671 is one of the major standards of British
Standards which handles requirements for electrical installations.
British standard 7671 Requirements for Electrical Installations is
the national standard in the United Kingdom for low voltage
electrical installations. Besides United Kingdom, a large number of
countries in and out of the continent; base their electrical
installation regulations on BS 7671. The countries that have
embedded British standard 7671 into their national standards
include Mauritius, St Lucia, Sierra Leone, Sri Lanka and Uganda
among others. Several countries continue to perform electrical
installations under the standard.
Publishing of electrical installations regulations in the United
Kingdom has been a perennial activity since 1882. IET, formerly
known as IEE, since publishing their 15th edition in 1981; have had
the electrical installation regulations in all the editions
thereafter, correspond to the international standard IEC 60364.
Today, the regulations are broadly based on the European Committee
for Electrotechnical Standardization (CENELEC) harmonization
documents. The standards or regulations therein, therefore, share a
significant technical similarity with the current electrical
installation regulations of other European countries.
In 1992, IEE Wiring Regulations became British Standard BS 7671,
Requirements for electrical installations. The standards are
maintained by the Joint IET/ BSI Technical Committee JPEL/64, the
UK National Committee for Wiring Regulations. BSI is an NGO
organization while Wiring Regulations are non-statutory. There is
an On-Site Guide reference for building-site electricians which
contain information not available in BS 7671:2001.
(www.bs7671standard.com,
2011)
E
lectrical Settings
The main work for the customisation all
happens within the Electrical settings part of the Revit interface,
which is found on the Manage tab, under MEP settings. As you will
see later we will work with Load Classifications and demand
factors, but we will access these from within Electrical settings.
In the examples below, both the original and the customised
settings are shown, original on the left, and new on the right.
General
In the general section, most of the original settings are fine, and
indeed down to personal preference, but the circuit naming needs to
be set to L1, L2, L3 rather than A,B,C as the purely alphabetic
system is no longer used in BS7671 countries.
Wiring
Wiring settings again are very much user
preferences, and not related to any particular standard, and so are
not discussed here further.
Wire Sizes
Wire 
(or cable) size tables are fundamentally the amount of Amps (herein
called “Ampacity”) that can be carried by a certain material (e.g.
Copper) at a certain temperature, when sheathed in a certain type
of insulation.
The “old” template on the left above, only
shows typical U.S. standard wire types, with U.S. standard
insulation types, and again based on U.S. standard safe operating
temperatures.
Fundamentally different between the US style
and the BS7671 style is the naming of the wire itself, with US
sizes showing Standard Wire Gauge (SWG) numbers whereas the UK
standard uses Cross Sectional Area (CSA) sizes in square
millimetres.
Interesting enough to mention here is that the
SWG system is actually based on the British system of Piano wire
sizes, and so is actually more British than the CSA system, but we
digress.
As it happens, Revit uses neither of these to
actually size the wires, but in fact uses the diameter column, and
where new wire sizes were added, their respective diameters have to
be calculated and entered similarly.
The BS 7671 document gives “Current Carrying
Capacity” figures for cables of varying materials, temperatures and
insulations, which have been translated to “Ampacity” figures, and
new materials etc added.
To make things as simple as possible, the
naming of each individual material now accurately matches the table
headings in the BS document. For instance “Copper table 4D2A Ref
Method E 1 two core cable with CPC” as a material directly relates
back to table 4D2A in the standard and that table reflects current
carrying capacity at the safe temperature. In addition, these are
broken down to represent that cable type, installed in standard
“reference method” installation techniques, such as clipped direct
or in metallic conduit. CPC denotes a circuit protection cable,
also known as the earth cable.
For example:
This excerpt from the British standard shows table 4D2A, and the
highlighted section would be a single material type in the wire
types list.
Also of note here is the ambient and conductor
operating temperatures (top right) which match both the ambient
temperature (in Revit general electrical settings) and the
operating temperature (in Revit wire sizes).
Correction Factor
These are factors that change the current
carrying capacity or ampacity of the cable as the ambient
temperature changes. As the ambient temperature approaches the
operating temperature of the cable, the factor reduces and so
therefore the ampacity of the cable reduces. At the time of
production, no specific information about this correction factor
was available, and so no factor was shown.
Ground Conductors
In the old template, the materials available
for the ground conductor are copper and aluminium. In the new UK
template allowance need to be made for the installation method and
so the list of materials is copied from the wire types above.
In the UK there are two apparent ways in which CPC’s or earth
cables are sized. One is to downrate them by 10% from the ampacity
of the live (or hot) conductor. The other is to step down one cable
size from the live conductor, and it is this method that has been
used in the 2012 template. We would urge electrical engineers to
check that this suits their personal choice.
Wiring Types
All of the above factors and settings come
together to form wiring types. The wiring types given in the 2011
template are typical US cable types. The 2012 UK template however
gives cable types available in the UK and Europe, and these are
named after the standard Pirelli codes, the reference (or
installation) method and the number of cores running together. For
instance:
Using this naming convention, the user should
be able to identify the cable by its known type, to enable it to be
ordered from schedules, its ampacity at given operating conditions,
to ensure it is relevant for the intended purpose, and its
installation method, so again this can be called off and scheduled.
All of this is referenced back to BS7671, so that an observer can
understand the regulations that have been applied in order to
choose this cable.
A typical schedule from these items could
therefore be:
Showing cable (wire) lengths with different
installation types having different labour rates – which are then
multiplied by the overall length of cable.
Load classifications and Demand Factors
The 2011 list of load classifications is and
was complete enough for most designs, and therefore the only
additional work needed was to set up demand factors specific to the
UK and assign these to the existing load classifications. There is
little documentary source for these demand factors, as the current
BS document seems to infer that the engineer should use his/her own
experience to evaluate the demand factors.
For that reason reference was made back to the
previous version of BS7671 which did give some guidance. The
interpretation of table H2 from the previous version can be seen in
the UK prefixed demand factors in the 2012 version of the template.
Engineers are advised to check these assumptions for their own
needs:
Distribution Board Schedules
All of the above can then be included in the design, and the
outputs can be delivered via the Revit Panel Schedules (or
Distribution Board Schedules as we call them in the UK).
The BS7671 document gives its requirements for the items
required of the DB schedule, as does the electrical contractors
association NICEIC, and these have formed the basis of the design
of the Standard UK template for the panel schedules in the 2012
release. In addition to this various consultants such as AECOM,
Capita Symonds, WSP and Buro Happold were asked for their own
versions, to make sure every need was covered. Thanks is given here
for their input.
The standard template can be seen in the image below:
And the Capita Symonds customised version can be seen here:
These customisations are simple to carry out using the tools
found on the Manage tab, then Panel Schedule Templates, and then
Edit a Template.
Putting the customisations to work
Of course the proof, as they say, is in the pudding. Capita
Symonds is able to significantly improve turnaround of electrical
design deliverables and reduce potential errors that normally
wouldn’t have been picked up until the QA stage.
To show the improvement it is first necessary to provide a
benchmark, in the form of what would “normally” be done at a
“normal” design consultancy on a “normal” project.
Traditional method
a) Engineer studies
the project and lays out positions for assumed distribution board
locations.
b) They then start laying
out light fixtures, power outlets etc and will have in mind which
distribution board will serve which area.
c) Then they would add
circuit numbers to each item or group of items, and ask the CAD
team to creatre these drawings in CAD.
d) This process would be
repeated X times to fix any spelling (typically circuit references)
errors etc on the drawings.
e) At the same time,
the Engineer would start inputting their scheme into an electrical
design package such as Amtech.
f) Any design changes,
mechanical requirements (always late!) architectural changes etc
would then repeat through steps “c”, “d” and “e” above (with more
and more chance for error each time) until resolved to a point
where they can be issued as deliverables.
g) The drawings will then be
re-visited with any circuit changes required from the results of
the calculations.
h) A QA check
would then take place, removing any outstanding typos, engineering
errors etc.
i) Assuming no
more changes, the project can be delivered.
This whole process which has been around for
as long as CAD and drawing boards before that is amazingly slow to
operate, wide open to error and omissions and is almost a “how to”
guide for duplication and disjointed practice.
What is needed from Revit MEP therefore is a
method where most of this can be streamlined, and most if not all
of the openness to errors can be removed.
Capita Symonds method
- Using Revit MEP, the engineer lays out his/her Distribution
Boards (Panels)
- The engineer will then add in power outlets, lighting fixtures
etc.
- Curcuits are created and loinked back to local Distribution
Boards.
- These circuits are then sheduloed out on the DB (panel)
schedules.
- The engineer can add or remove items, change locations and
generally woork through the design, using Revit MEP’s panel
scheules as a basis for design.
- After the majority of changes have been made on the project,
the engineer wil do one calculation in Amtech, using the curcuit
numbering, lengths etc from Revit as a guide.
- These calcualation results are then compared to the Revit
model, and any adjustments can be made.
- The project can now be delivered.
Using this new process, the engineer is in ful
control of his/her design, and the amount of possible errors
generated by human error, CAD errors, and drawings not matching
schedulkes can be almost irradicated.
What then follows is a normal QA check, and we
have found that using this new system with Revit as its basis,
significantly reduces errors that would normally have to be picked
up by QA.
The outcome of that is that our engineers are
happier with their designs, and can be confident that they are
correct both in principle and on paper. Our Clients and downstrrean
design team get a bettr deliverable product and fewer errors
carried forward through the design, and of course this leads to a
smoother running project with fewer misunderstandings and potential
litigation that comes from that.
Conclusions
It can be seen from the above that making the 2012 UK Electrcial
template more applicable to local standards has enabled engineers
at Capita Symonds and others to work almost their entire design
through the use of Revit MEP. This leads to fewer errors passing
downstream, and overall improved effiiciencies.
As the wire sizing calculation in Revit MEP is
not working to BS7671 standards at present, it can only be used so
far, and currently a separate calculation has to be made.
It is hoped that in the future sowtares such
as Amtech and Cymap will link to Revit MEP better and enable futher
improvements to be made to this workflow.
As it is though, its’s a great step forward
from what was available in previous verisons.
About the author
Gary.ross@capita.co.uk
Having worked at Autodesk for nearly four years as MEP
specialist and customer adviser, Gary Ross now works for Capita
Symonds, a multidisciplinary AEC consulting engineering company
located predominantly in the U.K.