Table 310.20....Your Service Drop.

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frizbeedog

frizbeedog

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Oregon
What size is your service drop?

Ampacity is ampacity, right?

I know, I know, They don't work under the NEC.

Light my path folks. :smile:
 
frizbeedog said:
Ampacity is ampacity, right?
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Yes.

But I know of few reasons why the power company lines are smaller then we use.

1) The power company knows that the NEC load calcs are bogus, they result in a service much larger then really needed.

2) The power company knows from many years of data what a single family home of a certain size will really draw. I have heard the power company figures the real load of a dwelling unit to be about 50% of the NECs service calcs.

3) They are not limited to 75 C terminations.

4) They really don't care the conductors or the equipment are overloaded for short periods of time.

5) Primarily there work is outside and up high, if it was to overheat the danger is minimal and they have great 24/7 fix it service.

If the power company sized their conductors and transformers to the NEC all of our rates would have to increase to cover the costs. Just the wasted power running larger transformers that are underloaded would be costly.
 
iwire said:
1) The power company knows that the NEC load calcs are bogus, they result in a service much larger then really needed.
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You have my attention now.

What secret do they possess....about load calcs?

You brought up points I had not even considered. Call me naive. ;)
 
frizbeedog said:
You have my attention now.

What secret do they possess....about load calcs?
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No secrets just real life data. The power companies have been providing power for a long time, if anybody was to know the real loads of a typical dwelling unit it would be them. :cool:

Heck the power company could check the KW for a feeder that serves 100s of homes and do some averaging to come up with pretty good numbers. I don't know if they actually do it that way just suggesting what they could do.

IMO Table 310.15(B)(6) is a direct result of even the NEC admitting that the Article 220 calcs have a lot of room in them.

If it was up to me to come up with the NEC load calc formulas you can bet I would be conservative as well. :smile:

Hopefully Charlie E will jump in and add and correct me. :smile:
 
iwire said:
IMO Table 310.15(B)(6) is a direct result of even the NEC admitting that the Article 220 calcs have a lot of room in them.
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Well, that table has been debated here lately, but not with that spin. :cool:


....that I know of.
 
frizbeedog said:
Well, that table has been debated here lately, but not with that spin. :cool:


....that I know of.
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If I was king that table would disappear and the load calcs would be brought into reality. :grin:
 
iwire said:
If I was king that table would disappear and the load calcs would be brought into reality. :grin:
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Interesting....because 310.15(B)(6) is really not an ampacity table.

I feel less naive by the minute. :cool:
 
frizbeedog said:
. . . because 310.15(B)(6) is really not an ampacity table. . .
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If a wire under a set of circumstances (individual dwelling units, services and feeders, 120/240-volt, 3-wire, 1?, etc.) and giving the service sizes for a particular size conductor, wouldn't that be an ampacity table? :)
iwire said:
. . . Heck the power company could check the KW for a feeder that serves 100s of homes and do some averaging to come up with pretty good numbers. . .
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Bob, that is exactly what we do. We set up a demand meter on a transformer and demand meters on each customer served by that transformer to get a relationship between the energy usage and the demand for a particular type of customer. This is done over long periods of time so we can develop an equation that will work for most installations of a certain rate and time of year. We have a couple of dozen installations spread over our system so we can achieve some semblance of accuracy and base our TLM (transformer load management) program on the data.

iwire said:
. . . IMO Table 310.15(B)(6) is a direct result of even the NEC admitting that the Article 220 calcs have a lot of room in them. . .
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Agreed, I believe that table and Table 220.88 were done with the help of EEI (Edison Electric Institute). You might want to check this statement out since I am not 100% sure of my memory. :)
 
The POCOs do use real data instead of calculated data.

We have tables we use to estimate transformer and service drop sizes based on home size, gas vs electric heat, etc.

We also have similar load data for many businesses
 
charlie said:
If a wire under a set of circumstances (individual dwelling units, services and feeders, 120/240-volt, 3-wire, 1?, etc.) and giving the service sizes for a particular size conductor, wouldn't that be an ampacity table? :)
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I see your point...every conductor's ampacity is based on a particular set of circumstances.

However...310.15(B)(6) aside for the moment.

If I were to feed that same dwelling from a remote service with overhead conductors, my wire would have to be larger than that run by the Serving Utility feeding the very same dwelling.
 
iwire said:
I did not say 24/7 make it safe service but at least in my area if the power is out the power company shows up quickly and in any sort of weather.
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Your right, that was unfair of me. FPL does have pretty decent outage response time.
 
charlie said:
If a wire under a set of circumstances (individual dwelling units, services and feeders, 120/240-volt, 3-wire, 1?, etc.) and giving the service sizes for a particular size conductor, wouldn't that be an ampacity table? :)
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I would call it a service rating rather than an ampacity rating.

I suggest that the term ampacity be kept to mean 'The current which, under stated ambient conditions, a conductor can carry _continuously_ without exceeding specified temperature limits.'

A conductor with ampacity X might be used in a situation where it sometimes is overloaded with current Y > X, but that doesn't increase the ampacity.

Just IMHO.

-Jon
 
Frizbeedog, go get your amprobe on your service and tell me you really need a 200 amp service. Its so overkill its not funny....


I have a 200amp service, yet I can run my entire house off a 7500 kw generator... Hmm, something does not add up...
 
stickboy1375 said:
I have a 200amp service, yet I can run my entire house off a 7500 kw generator... Hmm, something does not add up...
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Damn, my lab can sometimes strain a 150kW genset. I'd hate to have to pay the gas bill on a 7500kW beastie ;)

-Jon
 
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