Getting the E in MEP to Work in the United Kingdom and Beyond

Getting the "E" in MEP to Work in the United Kingdom and Beyond

Gary Ross – Capita SymondsLtd.

MP5397 The IEE regulations (a.k.a. BS7671) are used in over thirty countries worldwide. In this class, we 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.

Learning Objectives

At the end of this class, you will be able to:

Explain how to take a national standard and apply it to Revit MEP
Use electrical design in Revit MEP
Describe how customization in Revit MEP 2012 was performed
Customize Revit MEP to suit your local or company standards

About the Speaker

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.

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

Electrical 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 millimeters.

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 wat was available in previous verisons.