Archive for October, 2008

Delta High Leg Can Now Be the “C” Phase

Tuesday, October 28th, 2008

Summary: 2008 NEC allows the high leg of a delta high leg system to be labed as the “C” phase instead of the “B” phase.

A change to the 2008 NEC now allows the high leg of a delta high leg system to be labeled as the “C” phase instead of the “B” phase as was the traditional phase designation.  The high leg must be identified by an orange color (it was often referred to as a red-leg delta) or by other effective means and is usually the B phase. However, to accommodate meter configuration the high leg is permitted to be the C phase where metering is part of the switchboard or panel board. The Code change in this section requires legible, permanent field marking of the switchboard or panel board. 

Below is the traditional arrangement of the high leg delta system with the “B” leg as the high leg.  The high leg has a phantom 208 volts phase to neutral which is seldom used.  The difference between the first and second diagrams is the designation “CLOSED” and “OPEN”.  This refers to whether or not the delta is formed with three transformers (CLOSED delta) or two transfomers (OPEN ansformer.

The 2008 NEC now allows the high leg to be the “C” leg, which then allows the first two busses in a switchboard to be across the 120/240 volt side of the delta.  Single phase meters can then connect to these two busses without having to bridge the center bus.











Hotel Bath Exhaust Subducts

Saturday, October 25th, 2008

Summary: A subduct is a duct inside a vertical exhaust duct routed between floors of a building.  The subduct replaces the role of a fire/smoke damper at much lower cost.  This article presents a discussion of issure surrounding the design and installation of a subduct exhaust shaft.

I could write a small book about subducts, but here is the short version.  You either got to this web page because you don’t know what a subduct is, or your got here because you do and you are looking for more information about them.  For the rookie, a subduct is simply a trick of duct construction to avoid a fire/smoke damper in an exhaust duct routed between floors of a building.  As we all know, any shaft up through a building must maintain the fire rating of the floor assembly,  So if you have a single duct routed in a shaft and there is an opening at each floor, the fire rating must be maintained.  Since fire smoke dampers are expensive, the code offers an alternative as follows: provide an inner duct boot with a minimum 22 inch upward extension inside the main duct and maintain a constant negative pressure on the duct.  The idea is that fire and smoke will not climb down the boot if exhaust air is being drawn up the main duct.  It seems reasonable and I’m sure it has been tested.  So why is this worth a blog entry?

Here is a photo of a real subduct before installation. 

The reason I am writing this entry is because the code has so much room for interpretation, and the methods of constructing a subduct can vary significantly, with vastly different performance and cost implications.  Let’s start with the subduct construction.  The best construction is a sheet metal main duct with sheet metal subducts.  The cross sectional area of the main duct is sufficiently large so that the subduct does not excessively restrict the airflow from below.  Since the airflow is greatest that the top of a building, the air speed at the top subduct is a critical design point.  The speed of the airflow around the top subduct should not exceed 400 feet per second.  One trick is to reduce the size of the subducts at the upper floors to allow more free area in the main duct at the top where the airflow is greatest.  This also has the benefit of accomplishing some degree of self balancing since the static pressure is greatest at the upper floors.  Regardless, balancing dampers are recommended at all floors.  The bottom floor balancing damper may be omitted since the balancing process should start with that damper fully open anyway.

A common VE recommendation is to eliminate the sheet metal duct and utilize the fire rated shaft as the main duct.  The subducts remain as sheet metal elements.  There are two major issues with this approach.  First, there is always concern that the shaft material will develop mold, especially with the moist air of the showers in a hotel.  The counter argument to this issue is that mold-resistant paint can be applied to the inside of the shaft walls.  Also, since the shaft is always in exhaust (note, subducts are not allowed for supply systems), any mold will be exhausted.  Also, since the exhaust air is moving 24/7, there might be a slight moisture build-up for a few minutes, but the moisture will rapidly be removed by the constant airflow.  This may be true for dryer climates, but I would have some concern in the humid areas of the country.

The second issue is the integrity of the shaft which is generally gypsum board.  The construction challenge is to get the shaft sealed so it does not leak.  One can argue that the leaks will simply come from the same place as the bath air, but that could potentially rob the lower floors of proper air exhaust.  As for my opinion, I prefer to have a sheet metal duct liner.  But in dry climates on hotels four floors or less, I allow the shaft to act as the duct. 

The code also says that the airflow will be continuous in the subduct shaft.  So what does continuous mean?  First, we all agree it means the fan runs 24/7.  But what about when there is a power failure?  Some jurisdictions require subduct fans to be on emergency power.  To take it a step further, some jurisdictions require airflow monitoriing with a trouble alert at the fire control panel.

Hotels with attics:  This next comment is about subducts in regard to buildings with attics.  With a flat roof, the subduct shaft terminates at a curb on the roof and the exhaust fan is installed at that curb.  However, if there is an attic, where does the shaft terminate?  It is not uncommon to see the shaft terminate at the upper floor ceiling with a fire damper installed at the attic floor.  From that point unprotected ductwork is routed within the attic to a common exhaust fan.   So here is my question: If the fire damper is activated, the exhaust air stops, is the subduct still code compliant?  For this reason, I have always extended the subduct shaft through the attic to the exterior of the building.  If you do not want to install the exhaust fans on the sloped roof, then simply install the exhaust fans inside the shaft in the attic with a fire rated access door.  For the sake of the maintenance staff, remember to have the architect provide a catwalk to each fan.

Mushroom fans or Utility fans:  There are two types of fans that can be used for a subduct exhaust.  The first is a simple mushroom fan mounted on top of the roof curb.  This is good if some type of acoustical treatment is included in the neck of the fan curb.  A simple sound lined baffle is my favorite detail.  Remember to have enough height on the curb that the subduct at the top floor bathroom does not stick up above the curb. The other type of fan is a small utility fan mounted a few feet away from the roof curb.  This installation allows a horizontal section of ductwork which can contain vibration isolation and a sound trap.  This is probably the best installation, but it costs more than a mushroom fan.  And regarding direct drive vs. belt drive for the fans, I like belt drive for balancing, but direct drive is nice for maintenance.  A typical hotel with belt drive exhaust fans generally has at least one exhaust stack not functioning due to a broken belt.

Tall Buildings:  What about really tall buildings?  A subduct cannot be balanced for more than about 20 floors.  If the shaft is very tall, keep the shaft large and the airflow slow so it behaves as a plenum rather than a duct.  If the building is more than 20 stories tall, install multiple sections of duct with no section of duct serving more than 20 stories.  Another approach is to provide an exhaust fan in each guestroom as a “pusher” at each floor.  This is a typcial Hyatt design standard.  I have seen this done, but the code really does not address this approach.  I can imagine a code reviewer arguing that the fan could force smoke from one floor to the next and defeat the subduct smoke control concept.  Hyatt specifies that that pusher fan be relatively weak compared to the exhaust fan.  So one could argue that the pusher fan would not push smoke from one guestroom to another.  Since Hyatt does this as a standard approach, it appears plan reviewers are not chanllenging it. 

I recently found a manufacturer of pre-fabricated subduct inserts that have a profile to minimize air resistance.  See this product at

Multiwire Branch Circuits

Monday, October 20th, 2008

Summary<: 2008 NEC requires a common handle tie or multi-pole breaker.

The 2008 NEC has a new requirement for multiwire branch circuits.  The new requirement is for a common handle tie or multi-pole breaker rather than separate single-pole breakers.  For example, devices that are wired with a common or shared neutral can no longer be served from single phase breakers.  The breakers must have a handle tie or be a mult-pole breaker.  The motivation for this added requirement in the NEC is to assure that all the energized conductors which may be present at a device or outlet box are degenergized during maintenance or fault. 

So as a designer, what do you show differently on the plans?  One approach is to consider this just a code issue that the electrician must address.  However, the practical issue is the purchasing of the breakers.  If the proper mult-pole breakers are not purchased, then the only means of being code compliant is to field install handle ties.  Some plan reviewers have begun looking for the panel schedules to indicate the multi-wire branch circuits.  Design Master will have this feature incorporated in the next release.

Making 3D BIM Work For You

Wednesday, October 15th, 2008

Summary: The following is a step by step introduction to designing and communicating ideas with 3D BIM.

There are many different ways to approach designing systems for a 3D BIM coordination model. I have found that the best way to create a 3D model is to start by designing in 2D, the way you normally would. Once you have the basic design in, all you have to do is add a third dimension. By breaking up the drawings by floor and by system you are giving yourself the flexibility to both have a 3D model which can be exported and a 2D plan which can be plotted. Splitting it up in this manner also helps with file size. Working with a model of the entire building will slow down all but the fastest computers. Another benefit to having both your 2D and 3D plans in the same drawing is that when you have to make a change, you only have to make it to one drawing.

The process of constructing a 3D model obviously begins with designing it, but at some point you are going to want to see how it fits together with everyone else’s models. The first step to is to export the 3D work on each floor of each system. If you are using Design Master you would click on the export 3D button. AutoCAD users can use the WBLOCK command to export their work. Next, you need to compile all the systems of each individual floor together into a single model. To do this I created a file for each floor of the building and started XREFing in the exported models which corresponded with that floor. Once this is done, you are left with a full MEP system separated into floors. The next step, is to go into each floor and adjust the orientation of the XREFed models so they all line up at the insertion point. Then adjust the ‘Z’ coordinate so the model is elevated to it appropriate location in the building. You should be left with a full MEP model for each floor, which, when compiled into a model of the whole building will snap together seamlessly.

One practice that is becoming quite common is group meetings between the different trades involved in creating a building coordination model. The building coordinator, who created a single presentation model comprised of each trades model, will share the model with all the designers involved in the project. This allows the different trades to see what each other have created and to work through conflicts in the design. The meeting can either be held in a single physical location or over the internet with a service such as GoToMeeting or WebEX. Either way, a projector is generally used to show the 3D model which is being manipulated in a BIM program such as NavisWorks JetStream. The participants in the meeting should also have 2D plans on the conference table for quick reference. If you are partaking in the meeting from a remote location you will need a speaker phone to join in the discussion.

Once the meeting is complete, you should have a list of changes you will need to make to your model before the next coordination meeting. This process will repeat until each trades model fits together without any conflicts. Then the physical building can be constructed.

Introduction to BIM

Wednesday, October 8th, 2008

Summary: Introduction to the concept of Building Information Modeling in contemporary engineering.

In the visions of the future, 3D-BIM starts at the concept phase and is carried through to the construction documents and beyond. That is the future, perhaps. For today, we still “design” using a range of tools including arm waving and architectural flimsy, you know, that tracing paper architects always have. I recall watching an architect working with a client over a set of plans. The architect grabbed some flimsy and drew a rectangle, laid it on the plans, and asked, “A room about this size?”. He then tilted the room at an angle and exclaimed, “In fact, this is how it should be”. The client nodded in concurrence and the design process continued.

Someday the computer will be as fast as flimsy, but it is not there yet. But as the design moves from flimsy to CAD, the process turns from “design” to “construction document” creation. And at some point it is appropriate to add detail to the elements toward being a 3D-BIM. I’m sure the creators of Revit and ArchiCad would cringe to hear me say anything but that the process should begin immediately, but that is why we have blogs, so people like me can share real life experiences.

The fact is that there is still both a culture of transition and a practical basis for that culture that does not match full 3D-BIM at the start of a project. I don’t think anyone should apologize for this condition. Don’t be thinking that you are somehow failing to keep up with technology just because you find it easier to think in stages. The human mind can only solve so many problems at once. In fact, don’t we often teach the concept of breaking a tough problem into smaller, easier problems, and solving each one separately. So rather than trying to think of door swings and egress path lengths at the start of a concept design, an architect draws big mass blocks and space planning diagrams. The door swing and lock set details would just get in the way of some really challenging issues. Likewise, working in 2D removes one dimension from the problem during a system layout. The third dimension can be added later after the engineer has the plane solution figured out. When you think of it in terms of dimensions, and time being the fourth dimension, you would never try to solve all four dimensions simultaneously. The fourth dimension is called “means and methods” for the contractor, and engineers are not suppose to address that issue. Of course, at some point it is good to think about that dimension, because sequence is of huge concern to the contractor. But even within design/build contractor organizations, there is generally a separation between the design phase and the shop drawing (third dimension) phase. And then further separation between shop drawings and project sequencing. (fourth dimension)