# What is the use in downsizing feeders to 83% of the feeder breaker



## MotoGP1199 (Aug 11, 2014)

Saves money. Most people don't run everything on their house at the same time. You will find out that most houses are pulling like 40-60 amps on a 200 amp service. Not only that look up at the service conductors coming from the POCO, usually they are undersized as well.


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## HackWork (Oct 2, 2009)

DansElectric said:


> Men, I'm a humble first-year apprentice, so please permit me to ask a novice-level question.
> 
> I understand that according to 2017 NEC 310.15(B)(2), feeders of a residential dwelling can be sized to 83% of the feeder OCPD (main breaker) rating in a dwelling unit.


 ONLY if it is power the entire dwelling unit.

So you can downsize the service and you can downsize feeders that power the entirety of a single family house, or each apartment of a multi-family house. But you can't downsize the feeder for a subpanel that is only powering part of a dwelling.



> However, I don't see the point in doing this. Why not simply size it to 100% of the main breaker? It doesn't really save that much money in wire costs, does it? Because, the feeder really isn't that long. It's only being run between five and 30 feet.


 By your description you are talking about the service I assume. (Different than a feeder).

Why should we use a larger wire size than necessary? 4/0 can handle 200A, or else they wouldn't allow it. 



> Wouldn't leaving it at 100% allow for future expansion if the home owner wants to remodel the house or add on to it later?


 NO, not at all. That size conductor is rated to carry that amount of current.



> Then, a new feeder wouldn't have to be pulled in. The owner could simply have another breaker added to the main panel, because there would be some room for future expansion left over.


 A new feeder never has to be pulled.


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## HackWork (Oct 2, 2009)

MotoGP1199 said:


> Saves money. Most people don't run everything on their house at the same time. You will find out that most houses are pulling like 40-60 amps on a 200 amp service. Not only that look up at the service conductors coming from the POCO, usually they are undersized as well.


I believe that 4/0 can in fact handle 200A perfectly safely. If it couldn't handle that much current, then the NEC would not allow us to use it. 

There is excessive wiggle room ("safety buffer", what have you) put on conductor ampacity allowance. 310.15(B)(2) simply allows us to use a smaller wire while still being inside of that buffer.


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## Malywr (Jan 23, 2018)

How about 2/0. 200A service for dwelling: Example Annex D7


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## varmit (Apr 19, 2009)

The 83% sizing of service conductors takes into account several factors to arrive at an allowable wire size:

Few homes use anywhere near the current rating of their main breaker(s).

Residential loads are quite cyclic.

Many breakers will trip if loaded to 100% of their rating continuously.

The utility side of the service is only sized to accommodate the anticipated load. For their purposes, the size of the main breaker is irrelevant. 

Most residential services, by default, use standard size main breakers (200 or 400, or 2 x 200 amp mains). This is a savings in that it saves a detailed load calculation and uses generic sized service equipment. This is why you rarely see 150 amp, or other non standard services these days.

The savings in material and labor is only a peripheral benefit.

This method has been used for many years and only in very rare situations, has caused problems.


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## oldsparky52 (Feb 25, 2020)

The PoCo has 7 services (each 200-amp, houses in the 1300 - 1400 square foot range) on a 50kva transformer. 30 years later still good to go.


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## 99cents (Aug 20, 2012)

DansElectric said:


> Men, I'm a humble first-year apprentice, so please permit me to ask a novice-level question.
> 
> 
> 
> ...


You might be a first year apprentice but you’re wiser than a lot of guys with a license. Allowing for future expansion at minimal cost is what good electricians do; sometimes you can make a few extra bucks off of it and it also shows the customer that you think ahead.


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## HackWork (Oct 2, 2009)

99cents said:


> You might be a first year apprentice but you’re wiser than a lot of guys with a license. *Allowing for future expansion* at minimal cost is what good electricians do; sometimes you can make a few extra bucks off of it and it also shows the customer that you think ahead.


But what he is talking about doesn’t allow for future expansion. He’s talking about an exception in our code that allows us to use smaller gauge wire in certain situations. Using the normal size wire will not give them any future benefit.


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## 99cents (Aug 20, 2012)

HackWork said:


> But what he is talking about doesn’t allow for future expansion. He’s talking about an exception in our code that allows us to use smaller gauge wire in certain situations. Using the normal size wire will not give them any future benefit.


Damn, what a waste of what is normally another one of my thought inspiring and insightful posts.


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## emtnut (Mar 1, 2015)

HackWork said:


> But what he is talking about doesn’t allow for future expansion. He’s talking about an exception in our code that allows us to use smaller gauge wire in certain situations. Using the normal size wire will not give them any future benefit.


It's based on the load calc (If it's the same as up here), so technically it would be allowing for min 34A of expansion wouldn't it ? ... unlikely to ever be useful, but ....




Assuming 4/0 spontaneously ignites at 187.66667 Amps :biggrin:




Noteworthy, is that many O/H 200A services up here are fed from Hydro on #2 ACSR AL.


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## HackWork (Oct 2, 2009)

emtnut said:


> It's based on the load calc (If it's the same as up here), so technically it would be allowing for min 34A of expansion wouldn't it ? ... unlikely to ever be useful, but ....
> 
> Assuming 4/0 spontaneously ignites at 187.66667 Amps :biggrin:
> 
> Noteworthy, is that many O/H 200A services up here are fed from Hydro on #2 ACSR AL.


#3 CU has an ampacity of 100A, #4 is 85A

The exception will allow us to use #4 for a 100A service or feeder that powers an entire dwelling unit.

If you use #3, you are still limited to 100A. There is no benefit to using #3, no future expansion.

While typing that I realized that you might have meant that someone can come in later and add something which would bring the calculated load above 85A. I don't see that ever happening, it's just not realistic. Calculated loads are just wild ass guesses, especially in residential. If someone is going to add something later, I can't see them wishing that the original electrician installed #3 wire. They would instead upgrade the feeder/service, which is what should be done if you are that close to it's max. 

What you say has merit, but in real world situations I just don't think anyone has ever used the full size conductor for future expansion. We know that #4 can in fact handle 100A or else they wouldn't have the exception in the first place.


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