Branch circuits are the same for commercial and residential. No de-rating for the residential service calculation.
Help...I am confused. I have become so accustomed to applying demand factors to circuits because they were in a commercial application that I am not sure about a residential occupancy. What is the proper method of applying demand factors to a residential load? According to 8-104 a residence does not fall under the ruling for being classed as a continuious load. That seems to mean that for calculations of service the conductors or breaker do not have to be de-rated to 80%. Does this apply to branch circuits as well? ie if there is a known load of 50A is all that is required is a 50A conductor and a 50A breaker or does the derating to 80% apply so that a breaker would have to be sized at 125%? What about the conductor?
Branch circuits are the same for commercial and residential. No de-rating for the residential service calculation.
There is a difference between commercial and residential. In commecial conductor ampacities are reduced by to 80% of table 2. Also breakers are only able to be loaded to 80%. Both reductions are a result of the "contiuous load" rating applied in rule 8-104. In residential the "continious" rating does not apply as stated in 8-200 and is repeated in the loading for apartments in 8-202. I have been led to believe by some others as well as by inference of the rules that these rules refer to the intire "circuit" which includes the breaker, conductor, and even any enclosures in the circuit. There seems to be a definite difference in the treatment of residential loading and commercial loading of conductors and breakers. I am still not 100% sure on my interpretation so if you can help with any code rules that would speak against what I have said I would like to know them. Obviously I am not talking about the derating of calculated loads as this is covered in each section as it applies to the rules associatied with the load but rather whether or not conductors and breakers need to be sized at 125% because of a continuous load rating as a result of 8-104. I think what it all boils down to is whether or not residential occupancies (including apartments and multi-family dwellings) are deemed as continuous or non-continuous, for the application of 8-104.
My understanding of this issue is the same as that stated by TWH above.
Because 8-104 is a general rule in Section 8, it applies to all types of loads.
The only exceptions are 8-200(3) and 8-202(2), which apply only to residential occupancies.
And, they apply only to the service and feeder calculations, not branch circuits, and in the case of 8-202, only to that portion of the loads that are actually located in the dwelling units.
Load diversity is the reason why these exceptions apply only to the service and/or feeder calculations.
Even if a number of the branch circuits serve continuous loads, the chance of all of them being on at once is deemed to be remote.
With regard to the reason why a we need such rules, in other words, why a 100 amp rated circuit breaker, and 100 amp rated wire, isn't permitted to carry 100 amps continuously, I have my suspicions, but that is a subject for another discussion. [img]biggrin.gif[/img]
Ed
<font color="#a62a2a" size="1">[ October 10, 2004 08:54 AM: Message edited by: Electric Ed ]</font>
I think 8-200(3) is quite narrow in it's wording. In any event, a 4800 watt/240 volt heater running continuously will trip a 20 amp breaker as easily in residential as in commercial applications.
I guess it is hard to cover all the angles of a question in one post without it being very long winded etc. The "general" answer that the circuits are the same does not hold true. We see that especially, as was pointed out, when the rules are specific as opposed to general ie the point about the electric heat. However in some cases this still becomes confusing. At a code seminar last year the discussion arose about installations of ranges and dryers in occupancy's that were not dwelling units. In the dwelling unit the range, if rated at 50A or less, is allowed to be installed on # 8 conductors which are only rated for 45A with 100% loading. The same range, not in a residential occupancy, cannot be loaded like that. It now must have conductors rated at 125% of the kw rating. So my point being that there are a number of issues that need to be looked at before a conductor can be chosen. Continuous or non-continuous duty, location, environment, loading, type of load, etc to name a few. From my reading it seems that there are definite rules that apply to circuits in a residential occupancy that do not apply the same way elsewhere and vice-versa. I suspect that, like my first question was rather general, some of the post answers have been general as well. I am enjoying the discussion though and it helps in causing me to look at not just what I have to do to meet code code but why I have to do it that way. Thanks again for your various comments.
Which rule do you say allows branch circuits, in general, to be treated differently in residential than commercial?
Where does it say that you can put a 50 amp range on a 40 amp circuit? I hope that there is also a rule that the breaker won't trip, so supper isn't cold. Just because it's done, doesn't make it code.
<font size="2" face="Verdana, Helvetica, sans-serif">In section 8 rule 8-300. Subrule 1 says that a range of "12Kw or less". 12Kw @ 240 volts is 50A. The rule allows a demand to be used that reduces the requirement to 8Kw. Also subrule 3 specifically says that in other types of occupancies the demand is to be the actual rating.Originally posted by twh:
Which rule do you say allows branch circuits, in general, to be treated differently in residential than commercial?
In general there are rules in some sections that specify "in residential occupancies" or "dwelling units". These rules only apply to the type of occupancy stated.
Where does it say that you can put a 50 amp range on a 40 amp circuit? I hope that there is also a rule that the breaker won't trip, so supper isn't cold. Just because it's done, doesn't make it code.
A good explanation of the above can be found in the CE Handbook Vol 1 page 124 and 125.
So as you can see quite often a "50A" range may very well be only protected by a 40A breaker with 45 amp rated conductors.
Will the breaker trip? Not likley due to the fact that very seldom would all of the burners, two oven elements, receptacle, lights, timers etc be working all at the same time. Hence the demand allowed. So supper will most likley not be cold.
The handbook does a good job of explaining the difference between residential and non-residential loading in this particular area. Having read through quite a bit in section 8. I still think that (in residential occupancies), in ie the case of loads that are not specificaly delt with elsewhere, they could be considered as non-continuous just as the service is. If that is the case then section 8 seems to allow 100% loading on the conductor and breaker. See the explanation of rules 8-104,8-105 and the statement that says "conversly, if a load is not rated as continuous, subrules 4 and 5 do not apply, and the electrical equipment may be loaded at its rating regardless of the wiring method used". (page 92)
You got me on 8-300. I was wrong. However, the breaker does trip - even the replacement. I insist that supper will be cold, but only when the whole family is there.
If you are relying on the handbook, I can't respond. I don't have a copy, and I have never had a rejection based on the handbook.
Originally posted by Aspencade:
"If you look at the rules you will see that the equipment would not have to be rated for continuous use. If a load is non continuous then the equipment can be loaded to 100%."
Nowhere in my original post do I state anything contrary to your quote above.
My point was that unless covered by other areas of the code that state otherwise,
most residential circuits should be considered a continuous load unless you want to take the time to prove to the inspector that the circuit in question is non continuous as per rule 8-104(3).
As for the handbook, be very careful using it, although it is a good reference I have been told by people in authority (instructors and the inspection department people)
that it contains many mistakes.
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