Friday, April 17, 2015

Revit MEP - Circuit Length Calculation

I received an email asking if I thought it would be possible to use a Line Based family to calculate wire length for circuiting purposes. It was prompted by reading an earlier post about using them for Egress Path calculation and documentation. I think it's a reasonable approach for Egress Paths because there is no such tool or concept already in Revit.

Electrical circuits on the other hand do calculate their length already, granted it does so simplistically. It combines the X/Y/Z distances of the devices from the panel (and each other) to arrive at a total circuit length.


A friend some years ago sent me a PDF that showed a bunch (a couple shown above) of different equipment and device layout schemes he did to better understand how Revit arrives at a value for Circuit Length. What it shows is that Revit does nothing to factor in obstacles which force the wiring to go up or down or around the things that are more than likely going to increase the total circuit length.

To be fair...how could it really? How detailed is the model? How long do we want the software to interrogate the model, the linked models to do a better job? It's a bit like requiring all conduit to be modelled, even 3/4" conduit feeding single gang boxes in a wall when there isn't a single stud in the model. Like with so many things in life, it's about context.

It's one thing to put a single gang box on a wall to indicate one is required here and another thing entirely to run conduit to all the way to it when you can't be sure it can really go there. Or like showing a pair of outlets back to back on a demising wall in a residential project where fire rating and sound attenuation would require them to be separated by a minimum distance.

Some things are still conceptual (schematic) in nature even in fairly detailed models. If we start modeling every stud then perhaps it becomes more reasonable to expect more system detail too. Then again if I have to create a run of (15) 3/4" conduits across a wall or ceiling I probably ought to model that situation because it's going to take up a lot of space and everyone else ought to be aware of it.

It's also been my observation that no matter how good software gets at a task there are always outliers that dismiss its results as still wrong. That written, it does matter and Revit should improve how it does calculations and how much say we have in the outcome because the length it calculates is also used internally to contribute to other calculations like wire sizing and voltage drop.

Setting aside how circuit length should be calculated (it might surprise you to know that engineers don't always agree on this), I'd consider this approach if my task was to determine how much wire was needed, a little closer to actual requirements than might show up in the circuit properties. I'd create a schedule focused on Electrical Circuits.


What you see above: I created a couple panels and added a bunch of duplex outlets. I assigned them to circuits. The schedule's second column is Revit's calculated length, no more effort on my part than placing devices, creating circuits and choosing a panel.

I sorted the schedule by Panel and then Circuit, provided a header for Panel and footer for totals. I added a parameter for Length Factor (a number) and then a Calculated Parameter for Total Estimate (a length) which uses a formula of Length Factor * Calculated (column 2).

Then I consider each circuit's circumstance and enter in a factor that increases or decreases Revit's own calculation according to how easy or hard the actual wire run or routing will be...or at least what I think it will be. A factor of 2 doubles the length etc. This calculation doesn't factor into load calculations like for Voltage Drop...but then neither would using a line based family.

This doesn't account for situations that might share a neutral (or ground) or runs that are comprised of individual conductors in conduit (like THHN or THWN) or a wiring combined into a common jacket like Cerro's NMB (a typical household wiring product).

That's the risk/difficulty of automatic calculations, the closer you examine situations the more outliers there are. It can be a bit like arguing about something being 99.6 percent correct. If it costs another $1000 to get to 99.9 or 100%...is that difference worth it?

4 comments:

Jeff Hanson, Autodesk said...

Over the years i have seen a number of comments/threads/blog posts/etc...about the accuracy of takeoffs in Revit. Your last sentence about it costing anther $1000 to get the "last bit" of accuracy is a good point when it comes to estimating with Revit. The other thing I always think about is the way materials for a job are procured. To use your wire example, you probably buy/get charged for a spool of wire rather than by the inch or foot. Just another thing to consider when looking at the accuracy of the material estimates.

Steve said...

I've been asked if Revit can tell me how many roofing nails I need. When I respond with, "Sure you can divide roof area by a number of nails per/SF" I often hear, "but I need the exact amount".

They come on rolls (for nail guns) and in boxes of varying amounts (some as much as 5,000 to a box).

In my opinion I only really need to know if this is a 5 roll or 2 box job...not that we'll need 9,842 nails exactly.

Theo said...

I'v used a conduit in a project to see the length of some cables. These were the cables between different switchboards and panel boards. This because the dimensions of these cables can be up to a 5x300 mm2. A meter more or less at such diameters can coast a lot.

Anonymous said...

From an electrical designer's perspective, the only reason I typically need to estimate several circuits' lengths (in a schedule) is for voltage drop.

For this application, all that's really needed from Revit is the x and y length. I can input the estimated Z just by looking at the circuit. I haven't found a way to get x and y yet though.