NEC in the Facility: Load Calculations, Part 4

Oct. 23, 2012 | Electrical Construction and Maintenance

To calculate feeder or service circuit loads, the minimum you can use is found in 220.40 (the first subsection of Art. 220, Part III).

When calculating feeder or service circuit loads, what's the minimum you can use? The answer is in 220.40 (the first subsection of Art. 220, Part III).

The total calculated load on a given feeder or branch circuit must be at least the sum of the loads on the branch circuits this circuit supplies (as determined by 220 Part II). But you must adjust this sum by any applicable demand factors required by 220 Part V and permitted by 220 Part III. If you're using the "Optional Calculations" of 220 Part IV, use the applicable demand factors contained in Part IV rather than those in Part III. You'll find five demand factor tables in Part III, but only two such tables in Part IV.

The Informational Note in 220.40 refers us to examples D1(a) through D10. These examples are immensely educational. However, this numbering can be a bit disconcerting. For example, there is no Example D7. Among other anomalies, you'll find:

D3 and D3(a) but no D3(b).
No D2, but D2(a), D2(b), and D2(c).

While this numbering system appears to defy logic, a closer look helps reveal why it's that way. D1(a) and D1(b) both address one-family dwellings. So do D2(a) through (c), but with a specific variation. In our next issue, we'll look at D3.

Discuss this Article 1

Jacobo (not verified)

on Oct 23, 2012

To make a branch or service/feeder calculation there are a lot of parameters to be taken into consideration.. It’s tedious, time consuming and prone to errors. I was wondering how is it possible that in the era of IT, an electrical engineering project dealing with the above subject, these calculations are still made manually. I know that there are a few fine software packages . They are focused basically on power systems. However, in the field related to the NEC, specifically Articles 210, 215 and 220, practically they don’t address it according to the requirements of the Code. To the best of my knowledge, there is not a professional software program in the market that is dedicated to feeder calculations based on the NEC, and calculations are still made MANUALLY.

I agree that students must learn and understand the Code and make feeder calculations manually. But in the real world, in the professional work, efficiency is a must. For example, nobody makes manually a simple calculation of 123 x 321. However, to make a load calculation for the feeders and service for a Project similar to the one shown in the NEC Annex D, Example D(4), there is no “calculator” to help. If there is one, please let me know.