NEC question (SE vs feeder cable sizes)
Does anyone know why SE cables don’t have to be as large as those used for branch or feeder circuits?
Example: #2 AL is okay for 100A service, but a 100A branch circuit needs to be 1/0 AL
Just wondering
Does anyone know why SE cables don’t have to be as large as those used for branch or feeder circuits?
Example: #2 AL is okay for 100A service, but a 100A branch circuit needs to be 1/0 AL
Just wondering
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Replies
First the basics of all electric current ratings is based on heat generated and the ability of insulation to withstand that heat.
That is why you have diffent columns for different types of wire and derating for bundled wires or wires in hot areas.
Also there is something called IsquardT. Not mentioned in the electrical codes, but it the bases behind much of the engineering. It is the combination of the energy going into heating the wire and the leght of time. Because of the mass of the wire short spikes of over current won't cause much heating.
And most breakers have matching characteristics. They have a magnetic trips with is for large overloads, many times nominal current which trips in a fraction of a second. It is for faults (shorts).
And thermal, which is basically a heater and thermostatic switch, which can take a several minutes for say a 20% over load to few seconds for larger overloads.
This a 20 amp circuit with a 15 amp contractors saw does not trip when dring 60-90 amps for a couple of seconds during startup.
Now back to your question. First the special SE table is limited to residential uses. And the code writers realize that while the calculated demand might require a 200 amp panel that the aver loads are much, much less. And that large peak currents are only for a short time (IsquaredT).
Now in a commercail applications such as a store or restruant the lights, cash registers, kitchen equipment, etc is running aobut the same all day so that the load is much more constant and thus the special table is not allowed.
BTW, that is not just restricted to service entrance wiring (individual or cables), but can also be used on feeders with "similar characteristics" (not sure of the exact wording).
Typically where that would come in is if you had a serive entrance at the meter with outside panel you can have breakers there for say AC and hot tub, but then a feeder to an enternal sub-panel that would have all of the other lighting, applicances, receptacles on it. The SE table could also be used for that feeder.
That makes sense.
Thanks
Actually the branch circuit cables never need to be larger than the service entrance cables.
The proper (expensive) way to size service entrances and feeders is to do engineering. The equations are identical for both. The numbers you plug in may be different.
Never confuse the NEC with good engineering.
>>The proper (expensive) way to size service entrances and feeders is to do engineering. The equations are identical for both. The numbers you plug in may be different.
>>Never confuse the NEC with good engineering.
No offense intended but you are dreaming a pleasant dream. Never confuse common sense and good engineering with what the code requires. Else there's a red tag in your future.
PS: there are actually valid reasons why many service drops are undersized. I think it was Bill who laid them out -- in free air -- thermal mass, etc. Biggest reason is the utility doesn't have to comply with the code.
"Let's get crack-a-lackin" --- Adam Carolla
Unless you have an engineering degree you should not argue engineering.Any first year heat transfer book has all the answers to the question at hand.Around here engineering trumps the AHJ.
Where's "here", and what are your credentials? You have no info in your profile.
Are you an an engineer? If so, are you a practicing power EE? What's your direct experience in interpreting the NEC for day-to-day code compliance in residential construction?
To turn your remark around, unless you are a licensed electrical contractor or electrician doing residential wiring, maybe you should not argue NEC compliance for residential wiring.
In regard to the OP and the response that "branch circuits are never required to be larger than the service entrance conductors", I contend for a house, that is an incorrect statement. There are two tables for sizing conductors--one for residential SE conductors (Table 310.15(B)(6), and one for other circumstances (feeders and branch circuits) (Table 310.16). As the OP observed and Bill H explained, the conductor size for feeders/branch circuits is larger than for SE conductors.
All refs to the 2002 Code.
Cliff
around here you can build/install anything with zero regard to code IF an engineer stamps it...
here engineering trumps all codes...
all the inspectors/code wants here is a place to shift blame... if an engineer says it's ok and signs off... thats good enough...
Yep, true.
Around here, the engineer's calcs for any alternative will be reviewed by an engineer in the permitting dept of the AHJ.
I've been told by the Chief Building Official here (Sacramento, Calif) that they don't see many cases where an engineer has stamped an alternative to a residential NEC requirement.
Cliff
>>"Unless you have an engineering degree you should not argue engineering.
>>"Around here engineering trumps the AHJ.
Well OK then. Sorry to have questioned your highness.
BTW, an engineering degree doesn't actually do much for the AHJ. You actually need a license.
Unless you're willing to put your engineering stamp (assuming you have one) on the guy's electrical plans for free, maybe what you have to say is pointless. Interesting, but pointless.
"Let's get crack-a-lackin" --- Adam Carolla