Some Abstract Questions On Article 220 Part III Load Calcs

[wwhitney]

I have been studying the standard load calculation for feeders and services, Part III of Article 220. I have a couple "big picture" questions and wanted to see if my preliminary answers (in parentheses) are correct:

1) Say I'm doing a Part III calculation for a specific feeder or service, and I want to make a spreadsheet with all of the building-specific numbers in it--square footage, nameplate data, etc. And I want to write a single spreadsheet formula that gives me my final answer, just using those raw numbers as inputs, one big formula. The formula will necessarily be specific to that calculation, as far as how many loads there are, whether you get to use the 75% factor for 4 or more appliances in a dwelling unit, the table look ups for any electric dryers/ranges in dwelling units, etc.

Can I always write that formula just using MIN(), MAX(), + and *, and never multiplying together two inputs that have units of amps? (I think yes).

2) Say I take a one line diagram (of a complete premises wiring system) that has the following properties:

- We have a node at any feeder splice, so each line segment has just two ends. (Panelboards are just nodes with lots of line segments connected to them)
- No loops. (The graph is a tree)
- The only power source is a single service. (The service is the root of the tree)
- Each branch circuit is a single segment terminating in a node that is labeled with all of the data required on the loads served (these nodes are the leaves of the tree)

Then for any feeder segment in the diagram, I can do a Part III computation of the load on that segment, just by looking at the part of the one-line diagram downstream (away from the service) from that feeder segment. The computation will depend only on the branch circuit data in that part of the one-line diagram (the leaves of that subtree). So I do that for every feeder segment in the diagram, and I label each segment with the Part III load. I can also label each branch circuit with its Part II load.

Now if I look at any node that isn't branch circuit data (not a leaf), it will have one upstream line segment (its supply) and 1 or more downstream line segments. If I add up the loads of all the downstream line segments, is the upstream line segment's load always less than or equal to that sum? (I think yes)

Cheers, Wayne

 

[MyCleveland]

Seems like you are proposing a One-Line diagram of sorts where the user enters the load VA values for various load type options you make available at each NODE or panel. These then will auto fill up stream (or down towards the root) at its serving NODE. This node would also need to be filled in for direct served items and then it auto fills its source....rinse and repeat...until you arrive at the main.

I don't believe I would try this for residential and commercial, but rather have a separate one for each.

You will need to build in error checks were cells could be left empty...data validation of sorts.

I believe the residential would be of greatest value...especially if you can provide options of single, double, tri-plex and Multi-Family.

 

[Dennis Alwon]

I don't see what the purpose of this project would be and honestly I don't quite understand what you are proposing. There are plenty of spreadsheets that consider 4 appliances or more at 75% but beyond that I am lost. Mike Holt's spreadsheet does many different things.

I changed some formulas on the dwelling to suit my needs but if anyone wants it I have made it available from my google drive https://docs.google.com/spreadsheet...ouid=105922773056837051482&rtpof=true&sd=true

Yes, I have permission to pass it around-- The range table is really cool. There are about 10 tabs in this workbook

 

[wwhitney]

This is not a project or anything I propose to do. These are more like thought experiments (the "abstract" part of the title). Each question is of the form "if someone were to do this, would such and such always be true, regardless of the details of the wiring system and the loads?"

A definitive answer to the second question would be something like "No, if you consider this example, then you'll see that this feeder with load A supplies only two other feeders with loads B and C, and indeed A > B + C." A less definitive answer would something like "I think you're right, I don't see any way that can happen."

Cheers, Wayne

 

[JoeStillman]

It sounds like either a passage from Leaf By Niggle (J.R.R. Tolkien (c) 1945) or the way Revit ought to work. Alas, it does not.

Most of the branch circuits in a dwelling unit are not relevant to the Part III or Part IV MF calculations. They are accounted for in the 3 VA/ft². And the diversity calculation you do for the individual dwelling units has no impact on the overall service size.

We do a lot of multi-family residential calcs in my office. We use one spreadsheet for the individual dwelling unit calculations and a separate one for the overall service. The overall service calc compares the Part III method with Part IV. Part IV almost always wins with the smallest MF service size. And the utility companies all scoff at our calcs and do their own. Theirs are a fraction of the size of ours.

IMHO (In my Heretical Opinion), resi load calcs are voodoo.