NEC in the Facility: Load Calculations, Part 1

Requirements for branch circuits, feeders, and services each have their own Articles (210,

215, and 230, respectively). The NEC also contains an Article providing requirements for

outside branch circuits and feeders (225).

When performing the load calculations for any of these circuits, you must follow Art. 220.

The requirements of Art. 220 are modified or replaced in other Articles, for specific types of

loads. Table 220.3, which occupies nearly a whole page, lists these loads and the additional

Articles to apply.

The typical industrial or commercial facility has load-side distribution at 480V, with 480V

transformer(s) center-tapped to provide 277V for lighting. Branch-circuit panel supply

transformers step down 480V to 120V. The NEC lists these and other typical voltages as

nominal voltages to use for load calculations (unless otherwise specified) [220.5].

"Nominal voltage" is sometimes a point of confusion. Suppose you want to add loads to an

existing panel and must do load calculations. So you measure 115V on a snap-in breaker at

the panel. Should you use 115V for your calculations? No; the nominal voltage is 120V. If

you look at the transformer nameplate, it will probably say 208Y/120V.

Another point of confusion is what to do with fractions of an ampere. The NEC permits you

to round off to the nearest whole number [220.5(B)].

When designing power distribution for a facility, you must assume a lighting load no less

than that in Table 220.12 [220.12] if the table specifies your occupancy. Should you use

this table for work in an existing facility?

Probably not. The word "general" in Table 220.12's title is a good clue. Are you working

with "general" lighting loads that you must calculate by floor space, or do you have a

lighting plan designed to support the purpose of the building and the equipment in it?

What if your facility is industrial and thus not listed in Table 220.12? Should you "play it

safe" and just select the largest Table 220.12 value? No. Industrial buildings tend to be

built for specific purposes, with lighting loads already known. Table 220.12 provides a

minimum lighting load value to use where actual lighting loads aren't known.

Use a Table 220.12 "unit load" value only if

Your facility is one of the specified occupancies.

You don’t have lighting load data.

If you have lighting load data, don't be surprised if your actual "unit load" (total lighting

load divided by total area) greatly exceeds the Table 220.12 value. But if the Table 220.12

value exceeds your calculated value, look for a calculation error and review the lighting

design before continuing.

When calculating branch circuit loads, what's the maximum load you’re allowed? The limit

is this: The total calculated load on a given branch circuits can't exceed the rating of that

circuit [220.18]. When calculating, don't forget to multiply continuous loads by 125%.

But suppose you're designing a fabrication shop. The tenant owns a single arc welder and

needs to use it all over the shop. So you run a single branch circuit with 10 receptacles on

it. Since only one receptacle will be used at any time, what's the calculated load? Answer:

It's the load presented by that single arc welder. This assumes there are controls

(administrative or otherwise) in place to ensure there won’t be a second arc welder

(perhaps a rented one) also in use.

The principle is "load diversity," which the NEC mentions in several places [e.g., 310.15(B)

(3)] but doesn't define. Account for load diversity so that you don't grossly oversize the

conductors.

A common example for residential applications is the fact that you aren't going to run air-

conditioning and electric heat at the same time. Therefore, you don't need to size the

service to accommodate both loads; you need to accommodate only the larger of the two.

Only when two loads are non-coincidental (or, ideally, mutually exclusive) should you

consider them diverse.

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.

To avoid confusion, follow the NEC sequence when performing your circuit calculations.

You'll see this sequence in each of the Annex D examples. You'll also see it in the structure

of the Chapter 2 articles.

Getting circuit calculations right is a multi-phase process. In Phase 1, you describe the

application. Look at each of the Annex D examples, and you see a short paragraph that

does exactly this. Don't mistake this as being purely for the sake of laying out the

background for the story problem. It's really an illustration of what information you need to

collect on the application before you start calculating loads and sizing conductors. Begin

with the one-line diagram.

Think of load calculations as Phase 2 of a multi-phase process. If you look at each of the

Annex D examples, you'll notice a recurring methodology. The designer first characterizes

each load as either continuous or noncontinuous, then lists it under the appropriate

heading. When this is done, the next steps then become simple to execute:

1. Add up the noncontinuous loads;

2. Add up the continuous loads, then apply the 125% rule; and3. Add that to the first sum.

The caveat on these load calculations is you need to check them against the load tables in

Articles 215 or 220 (as appropriate).

Each of the Annex D examples illustrates a different application. The first steps are always

the same:

Describe the problem (based on the one-line diagram)

Characterize the loads as either continuous or noncontinuous Determine the calculated load

No matter what the application, the subsequent steps depend upon correctly completing

these first steps.

The first five Annex D examples are for residential applications. Even if you do only

commercial and industrial work, carefully reading these is helpful. The same principles

apply as for industrial and commercial. The difference is with residential your load

calculations include specific circuits such as those for ranges and clothes dryers.

Annex D3 is a commercial example, in this case a storefront. It provides a fairly simple

illustration of the sequence of calculations that end with feeder conductor sizing. As for

load calculations, it provides the twist that the actual connected load is less than the Table

220.12 load. You have to use the larger of the two values, so in the case of Example D3 you

must use the Table 220.12 load.

Annex D3(a) throws in two twists, not just one: 

1. Demand factors

2. Motor loads

For industrial applications, you'll always have motor loads and it's unlikely you won't need

to apply demand factors. Study D3(a) thoroughly.