2002 NEC Annex D Example D1(b)

[eprice]

I have a question about this example that I'd like to get some comments on, just to see what I am missing. Referring to the table in the example showing loads on line A, Neutral, and Line B. In particular the loads for the three 115-V appliances: the air conditioner, the dishwasher, and the disposer. The table shows a net imbalanced (neutral) load for these three of 20 amps. Why would it not be 18 amps? The way these appliances are arranged, the largest imbalance among them would occur when the two appliances on line B were running and the air conditioner on line A is not running.

[ June 11, 2003, 06:54 PM: Message edited by: eprice ]

 

[charlie b]

Re: 2002 NEC Annex D Example D1(b)

The ?Neutral? column of this table is not showing a net imbalance (despite the fact that neutral current is the net imbalance current). The Table is showing the amount of neutral current for which we must account, in the selection of a neutral conductor size. It is based on 100% of the phase currents for most of the household loads, and on 70% of the phase currents for two specific loads (range and dryer).

 

[eprice]

Re: 2002 NEC Annex D Example D1(b)

Let me try to phrase my question a bit differently.

220.22 says that "The feeder or service neutral load shall be the maximum unbalance of the load detrmined by this article. The maximum unbalanced load shall be the maximum net computed load between the neutral and any one ungrounded conductor...."

Looking at the table in the example: I understand where the 61 amps of neutral current comes from. It is as you say, 100% of the phase currents for the lighting and small appliance load and 70% of the range and dryer loads. I understand why the 230-V air conditioner contributes no neutral load. It has no load between neutral and an ungrounded conductor. What I am questioning is the neutral load shown on the table for the 115-V air conditioner, the 120-V dishwasher, and the 115-V disposer. The "maximim net computed load between the neutral and any one ungrounded conductor" to put it in NEC language for these three appliances would be the 10 amps + 8 amps = 18 amps on line B rather than the 12 amsp + 8 amps = 20 amps shown in the example. The 12 amps and the 8 amps are not on "any one ungrounded conductor".

 

[charlie b]

Re: 2002 NEC Annex D Example D1(b)

OK, now I understand your question, and it has taken me some time to work out an answer. I think this is another example of an Example that gives clues without giving us explanations. Let?s look at three such ?clues.? First, in deriving the 61 amp neutral load, Example D(1)(a) did not tell us which circuits were connected to Line A and which to Line B. Secondly, Example D(1)(b) connected the disposer to Line B (why not A?). Finally, Example D(1)(b) showed the 115V A/C Unit and the dishwasher in the same row (why not on two separate rows?), and disregarded the neutral contribution of the smaller of the two loads. Now, what can we infer from these clues?

We must first realize that this type of calculation should be done in the office, long before an electrician shows up at the site to wire the circuits. If the Example showed the disposer on Line A, you would not have questioned the addition of the 12 amps to the 8 amps. But nothing in the code compels the electrician to connect the disposal to Line B. Similarly, nothing in the code compels the resident to run loads in such a way as to maintain a balanced loading. Since we don?t know during the initial calculation which loads will be connected to which Line, and since we never know exactly how the resident will be running the loads, the calculation must account for the ?worst case.? That forces us to size the conductors for the ?worst case.? But what is the ?worst case?? It is all loads connected to a single Line, and therefore the neutral carrying load from every single circuit. That is why the example includes the neutral?s 8 amps (i.e., from the disposer connected to Line B), even though it is not associated with the same ?one ungrounded conductor? that is shown to be carrying the 12 amps (i.e., from the A/C unit connected to Line A).

That only leaves us with the question of the A/C Unit and the dishwasher. Why does the Example not show both the 12 amps and the 10 amps? Why does it show only the larger of the two? I think the asterisk after ?air conditioning unit? (and the related footnote) represents an inadequate attempt to explain the design intent. I also think that the code is using this Example to acknowledge that there will be some efforts to balance loads, so that we need not count each and every load individually. In other words, it is no longer strictly a ?worst case,? but rather more like a ?worst-case-with-some-sanity-included? type of case. Let me expand the Example to list a total of 7 specific 120v loads (instead of the three ? the 12 amp, the 10 amp, and the 8 amp). Let me add four 120v loads (without actually naming the equipment), as follows: a 7 amp, a 6 amp, a 5 amp, and a 4 amp. I think the Example would have us pair the 12 and the 10, and show only a 12 amp contribution to the neutral, and then pair the 8 and the 7, thus showing another 8 amp contribution to the neutral, next then pair the 6 and the 5, thus showing another 6 amp contribution to the neutral, and finally list the 4 amp load separately, thus showing another 4 amp contribution to the neutral. The neutral contribution resulting from these 7 loads would be 12 + 0 + 8 + 0 + 6 + 0 + 4 = 30 amps.

[ June 16, 2003, 05:29 PM: Message edited by: charlie b ]

 

[eprice]

Re: 2002 NEC Annex D Example D1(b)

Charlie,

Thanks for the reply. Sorry it has taken me so long to get back to this, but I've been out of town, and then dealing with all of the stuff that piled up on my desk while I was gone It looks like you put a lot of effort into this and I feel a little guilty to have put you to so much work for a simple matter of curiosity on my part.

Yes, you hit upon what was bothering me. Why did the example choose to lump the 12 amps and 8 amps together for the neutral load rather than 10 and 8 (which is what we would get if the electrician placed the appliances on the lines in the best way and in the way shown in the example), or as you mentioned, 12 and 10, or even 12 and 10 and 8?

So, to rephrase what I think you said, the example isn't showing us THE way to do it, but one of several possible ways, one in which some allowance for "worst case" is made, without getting ridiculous?

That makes sense but there are still some things that bother me about that explaination. (BTW this is a rather academic discussion and if you haven't the time, don't feel obligated to reply ).

I believe there is a requirement in the code that the electrician balance the loads in the panel as closely as possible between line A and B, but I can't put my finger on it right now, and in fact the examples make that presumption as far as sizing of the conductors for line A and line B is concerned. In example D1(a), the 14,550 VA of neutral load wouldn't really be 60.6 amps of 240 volt load. It would be 121.25 amps of 120 volt load divided evenly between line A and line B. If it were really 240 volt load, it would result in no neutral load. The neutral load of 60.6 amps would occur if the resident turned on all of the load on one line and none on the other. In all respects of these examples other than the air conditioner, dishwasher, and disposer neutral load, the examples seem to make the presumption that the electrician balanced the loads evenly between line A and line B, then the resident turned the loads on in the worst possible way: all of the loads on one line and none on the other.

In example D1(b) in order to get the 12 amp and the 8 amp loads simultaneously on the neutral, they would need to both be placed on the same line, line B for example. But in that case, the load on line B would be 106 amps, not 104. and the load on line A (assuming the 10 amp load were placed there) would be 97 amps, not 99.

If we can make the presumption that the electrican did the best possible job of balancing loads when calculating the line A and B loads, why can't we make the same presumption in calculating the neutral loads? I understand the reason for assuming the resident turned the loads on in a very unbalanced way.

 

[charlie b]

Re: 2002 NEC Annex D Example D1(b)

I don?t think there is an NEC requirement that the electrician balance loads on all panels. Nor would I want such a requirement. Can you imagine an inspector red-tagging an installation because the loads were out-of-balance by 1%?

All I can offer is that the design process is sometimes ultra-conservative, so that a less-conservative choice on the part of the builder would not result in an inadequate, far less an unsafe, installation. The designer should not presume that the electrician will do the BEST POSSIBLE job (my emphasis) of balancing loads. Design for worst-case, and the real case will be ok. It?s not a great answer, but I think it?s on target.