This page will provide some insight into how wire lengths get calculated in Revit MEP. Wire length is actually a property of a circuit, and can exist whether you draw wires or not.
When a circuit is first created in Revit MEP the wire length is always 0. It isn’t until the wire is connected to a panel that we are able to determine the length of the wire.
Once a wire becomes connected to a panel is where the “fun” of figuring how it’s length actually starts. The mechanism that we use to calculate the length is actually fairly simple, but is extremely non-obvious. In short we can define the rule as “Length is calculated between the connector on the device and the origin of the “panel” in X, Y, and Z directions only”. The next obvious question is what does that actually mean…
In the example below the “circuit length” is calculated to be 12’ 5 9/16”.
If I dimension the distance from the receptacle to the panel, however, I find that the receptacle is only 10’ from the panel.
If we look at an elevation view of the situation, we can see that there is also a “Z” offset between the receptacle and the “origin” of the panel.
We now simply add 10’ and 2’ 3 ¾” and get 12’ 3 ¾” which is fairly close to the circuit specified 12’ 5 9/16”. The other difference of around 2” is a difference in where the connector is on the receptacle and the origin of the panel. My dimensioning shows this not to quite make up the total 2” difference, but it is relatively close.
In this example things worked out relatively easily because all of my geometry was “aligned” with the X, Y and Z axis. There are certainly cases where this isn’t true at all like the one seen below.
In this case the actual distance between the device and the panel hasn’t changed from what it was previously, but if I check the length of the circuit it now reports 16’ 6” (and some number of 256ths that I’m not going to write down). The reason for this change is because we only calculate our lengths in the X, Y, and Z directions, not in a “as the crow flies” direction, so in this case the “length” is calculated as shown below.
That solves the simple cases where there is one device connected, but what happens when there is more than one device? The application simply starts at the panel and finds the closes thing that is on the circuit. From that point it finds the next closest thing on the circuit, and continues to repeat until it has found all of the things on the circuit using the same X, Y, and Z measurements above. A few examples to show the process are provided below:
Circuit Length approximately 77’. Measured plan distance approximately 71’ (the lights are mounted at 10’, so that’s pretty close to the extra 5’).
Circuit Length approximately 92’. Measured plan distance approximately 87’.
Circuit Length approximately 104’. Measured plan distance approximately 98’.
It's really just that simple.