# Residential service/feeder sizing, grounded conductor



## Willie B (Jan 31, 2020)

Just joined. I work these days with my know it all son. On the rare occasion he doesn't have an answer, he asks his father in law who knows everything even though he's WAY outside his field of expertise.


I've been sitting through school since the early seventies. Four years night school. Since then it's two days every three years. This question has never been answered: Why is there language in code about reduced size dwelling service & feeder conductors referencing 83%? 

Why is available SEU and URD ALWAYS reduced neutral? Language I find suggests this often/usually is not compliant. 

A 100 amp service gets #2 aluminum, but grounded conductor is #4 in almost all available cable.

#4 is good for 75 amps at 90 degree, but terminated on a 75 D rated terminal, only 65 amps. 

I can accept that load would rarely exceed 65 amps, but it might. As it has no overcurrent protection, it's out of character for the authors of code.

Willie B


----------



## Dennis Alwon (May 9, 2009)

It is quite common to use a reduced neutral in residence since many of the loads are 240V and don't require a neutral. You can actually reduce the neutral as small as the grounding electrode conductor size if the calculation allows it.

It is also possible to buy SEU with a full size neutral

https://www.mysouthwire.com/medias/...duct-specifications/h79/h97/8858402979870.pdf


----------



## Willie B (Jan 31, 2020)

Dennis Alwon said:


> It is quite common to use a reduced neutral in residence since many of the loads are 240V and don't require a neutral. You can actually reduce the neutral as small as the grounding electrode conductor size if the calculation allows it.
> 
> It is also possible to buy SEU with a full size neutral




As I read several editions of code the subtractions from 100 amp minimum are limited to cooking equipment, and more than 5 dryers. In the homes I work there are not 5 dryers, and cooking equipment being calculated at 80% does not explain.


----------



## Kevin (Feb 14, 2017)

I know you're in the USA, but here in ontario canada, we can use a smaller neutral on a service. A 200 amp service can use a much smaller neutral (IIRC it can be #4 aluminium... I would have to check again to confirm).

The reduced neutral is because the load _should_ be somewhat balanced. More so on a 200 amp service, it would be pretty difficult to get ONLY everything running on one half of a split phase service. Normally here a 200 amp service is installed for an EV charger, a hot tub or some other large loads.

The reduced neutral is permissible because of there are equivalent loads running on each half of the split phase they cancel out, which is more likely to happen on a service.

Where possible I will install a full size neutral though.

Sent from my new phone. Autocorrect may have changed stuff.


----------



## CoolWill (Jan 5, 2019)

The neutral only has the be sized to carry the unbalanced load, meaning the amount of current not cancelled by an equal current on the opposite leg.

The code makers are assuming that you have a well-balanced panel.


----------



## Cow (Jan 16, 2008)

In the 16 years I've been doing this, I've never seen a reduced service/feeder neutral showing signs of overheating.

Have you?

Have you ever measured the neutral current on a typical residential service?

If your answer is no to both of those questions, then I'd think you'd be at the point where it's time to consider maybe there is some thought process behind the codes allowance of reduced neutral in specific situations.

We don't do much residential, mostly industrial, commercial and ag and I reduce neutrals all the time where it makes sense. It saves money and reduces conduit fill, and the customer benefits from lower cost with no difference in how their electrical system functions.


----------



## A Little Short (Nov 11, 2010)

The 83% rule was added in the 2017 (I think that was the change year) It used to be in Table 310.15(B)(6). The table listed allowable conductor reductions for a certain size service.
The ampacity chart calls for a 3/0 for 200A but the table allowed a 2/0. The 83% rule is the same thing. If you take 83% of 200A you get 166A which would be a 2/0 conductor.
These allowances are only good for 120/240v single phase for dwellings.



The reason for the allowance is the code makers know that not every load will be on at the same time in a dwelling. Also, the ampacity charts are very generous as it is. So they know that a 2/0 will never see 200A or if it does it will just be for a short while and no damage will result.


You can be sure that if there had been a problem with these reductions we would have heard about it and the code would be changed to correct that.


----------



## Willie B (Jan 31, 2020)

Language I find is rephrased in different editions of code, net rules are consistent. Reduced ungrounded, or grounded conductors, but the only provision for further size reduction of grounded is for cooking equipment, or more than 5 dryers. As this is per housing unit, 5 dryers isn't a factor. The reduction for one range is insignificant. Many homes, mine for example don't have electric cooking equipment.

Willie B


----------



## HackWork (Oct 2, 2009)

I think your worries about voltage drop and how it's a destroyer of worlds is more important than this concern.


----------



## Dennis Alwon (May 9, 2009)

Willie B said:


> As I read several editions of code the subtractions from 100 amp minimum are limited to cooking equipment, and more than 5 dryers. In the homes I work there are not 5 dryers, and cooking equipment being calculated at 80% does not explain.


You are looking at feeders or branch circuits not services


----------



## Dennis Alwon (May 9, 2009)

This clearly states the neutral can e sized as small as 250.102(C)(1) which is almost the same as 250.66... my error above they made a new table for the bonding




> 230.31 Size and Rating.
> (C) Grounded Conductors. The grounded conductor shall
> not be less than the minimum size required by 250.24(C)
> 
> ...


----------



## Willie B (Jan 31, 2020)

Dennis Alwon said:


> You are looking at feeders or branch circuits not services


It applies to service conductors as well as feeders carrying full dwelling unit load. This can be the service conductors before up to six service disconnects, as well as feeders. It does not apply to feeders carrying part of a dwelling load.


----------



## Dennis Alwon (May 9, 2009)

Firstly you mentioned seu and 83%.. sounds like service to me. It does not sound like you want to learn but more like you want to argue. Maybe I am reading you wrong

Also it does apply to feeders only the neutral cannot be smaller than the equipment grounding conductor.



> 215(A)(1)
> 
> Exception No. 3: Grounded conductors that are not connected to an
> overcurrent device shall be permitted to be sized at 100 percent of the
> ...


----------



## Dennis Alwon (May 9, 2009)

Maybe we should start over and ask what you really want to know. Because I am confused on what you are looking for


----------



## Willie B (Jan 31, 2020)

To me it is a wholesale violation of code. I've been guilty more times than I want to be. 

Every edition of code I can find permits downsizing of ungrounded service, and feeder conductors supplying a single family, or multi family. I don't understand the logic.

Each code book addresses the further downsizing of ungrounded conductor. The calculations offered in code (NEC) are limited to cooking & dryer loads, Using those values there is not justification for the universal practice of sharply downsizing grounded. 

No argument, the odds of overloading neutral conductors are very small. 100, or 200, or 400 amps of load on one half of the single phase transformer aren't likely to happen. Still everywhere else in code odds aren't considered. Could it happen? Regardless of cost, code protects.

The reference to equipment ground conductor sizing is only prohibiting using dryer, and cooking factors from reducing grounded conductors from being smaller than equipment grounding conductors.

Yes I want to know. I believe this is a subject future code making panels should address. Argue: no.

I'm not arguing, asking for change.


----------



## Willie B (Jan 31, 2020)

Sorry, I don't know how to edit. In third paragraph I meant to say grounded not ungrounded.


----------



## Dennis Alwon (May 9, 2009)

Section 220.61 shows the calculation for service or feeder neutrals. Normally we can guess that downsizing a residential service one size we will never have an issue with it being load. The problem is when you want to downsize to the extreme... In those cases you would need a load calculation for the neutral. Annex D in the back of the book gives examples of calculating the neutral.

In some commercials jobs one must oversize the neutral...


----------



## eddy current (Feb 28, 2009)

Kevin_Essiambre said:


> I know you're in the USA, but here in ontario canada, we can use a smaller neutral on a service. A 200 amp service can use a much smaller neutral (IIRC it can be #4 aluminium... I would have to check again to confirm)


Actually we can go as low as a #6 neutral on a 200 amp service. 

@Willie B, At 75 degrees a #6 is good for 65 amps. There would have to be 65 more amps on one leg than the other in a residential service for the neutral to carry 65 amps. That would be very rare to have that much of a difference from one leg to the other and it would not be like that for very long. No need to have a larger neutral conductor.

Here in Canada if we use smaller ungrounded conductors for residential services as our code allows (200 amp service with 2/0 copper instead of 3/0 copper for instance) we must label the panel with the maximum continuous load (184 amps )


----------



## Kevin (Feb 14, 2017)

eddy current said:


> Actually we can go as low as a #6 neutral on a 200 amp service.


#6 aluminium or copper?

Sent from my new phone. Autocorrect may have changed stuff.


----------



## eddy current (Feb 28, 2009)

Kevin_Essiambre said:


> #6 aluminium or copper?
> 
> Sent from my new phone. Autocorrect may have changed stuff.


Copper. It only has to be as large as the bond and large enough to carry the unbalanced load


----------



## Dennis Alwon (May 9, 2009)

eddy current said:


> Copper. It only has to be as large as the bond and large enough to carry the unbalanced load


Interesting that you guys can go smaller than our bond of #4 for 200 amps. Maybe you all know something we don't...hahaha

Of course it is all based on the size of the conductors you use not the service size although they are usually related. If for some reason someone oversized the service conductors you may not be able to use a #4


----------



## Kevin (Feb 14, 2017)

eddy current said:


> Copper. It only has to be as large as the bond and large enough to carry the unbalanced load


That's what I thought. My post said 4# aluminium...

Sent from my new phone. Autocorrect may have changed stuff.


----------



## eddy current (Feb 28, 2009)

Dennis Alwon said:


> Interesting that you guys can go smaller than our bond of #4 for 200 amps. Maybe you all know something we don't...hahaha
> 
> Of course it is all based on the size of the conductors you use not the service size although they are usually related. If for some reason someone oversized the service conductors you may not be able to use a #4



Yes our bond for a 200 amp circuit is only #6 copper. Our grounding conductor for any size of service only has to be a #6 copper as well.

We do have the same rules about using larger conductors as you though so if we oversized the feeders for voltage drop or whatever, we must use the ampacity of the largest conductor to size our bond, not the overcurrent.


----------



## Willie B (Jan 31, 2020)

220.61(A) ....."maximum unbalance of the load".....

22.061(B) Permitted reductions.......ranges and other cooking equipment......dryers. ....an additional demand factor of 70%....

220.55 places a typical range at 80% demand factor. Does this mean an 8 KW range is factored at 80% or 6400 watts. Then, 70% of that, or 4480 watts?

For dryers: 220.54 places them at 100%. An additional factor of 70% for a 4500 watt dryer is 3150 watts? 

"maximum unbalance of load" is limited to the service disconnect overcurrent protection. Is it likely to happen? Absolutely not. You aren't likely to win at the lottery, but buy tickets. 

If my interpretation of 2020 NEC is correct I subtract 4870 watts from the maximum imbalance. In example, a 100 amp household service at 83% needs 83 amp conductors. With range & dryer, the neutral (center tap) conductor is reduced further by dryer & range another 20 amps.

I do sometimes see a failed connection of grounded conductor. twice, I have seen a failure mid cable of grounded conductor. When they fail there is significant damage. I'll say there is usually factors causing failure other than simple overload.


----------



## HertzHound (Jan 22, 2019)

Ranges and dryers have very small neutral loads. So after applying demand factors for the phase conductors, you can take an additional 70% deduction for the neutral conductor. Other straight 240v loads don’t have a neutral current to add and only get added to the phase conductors. 

You keep mentioning the 83% deduction. That deduction is only used for the optional method (short method) of calculating. You can size all the conductors to that. If you want to reduce it further, you would have to use the standard method (long method). As a rule of thumb you can get away with reducing it down one size like mentioned. If you wanted to really reduce it, you would have to use the standard method. 

There is no reducing it 83% for the phase conductors, and then another 70% for the neutral. Basically the neutral always gets sized by the standard method. 

I forget the rational of why they put the article about neutral sizing between the long (standard) and short (optional) method. Probably because no matter how you figure your phase conductors, the neutral is only figured one way.


----------



## A Little Short (Nov 11, 2010)

HertzHound said:


> You keep mentioning the 83% deduction. That deduction is only used for the optional method (short method) of calculating. You can size all the conductors to that. If you want to reduce it further, you would have to use the standard method (long method). As a rule of thumb you can get away with reducing it down one size like mentioned. If you wanted to really reduce it, you would have to use the standard method.
> 
> There is no reducing it 83% for the phase conductors, and then another 70% for the neutral. Basically the neutral always gets sized by the standard method.
> 
> ...



I'm going to have to disagree with you. The 83% reduction has nothing to do with standard or optional method of calculating service or feeder size. You use which ever method you choose to calculate the load, then when you know the service size, you use the 83% reduction for the size of the conductors. BTW, the reduction is in 310.15(B)(7) 1-4
You can use 220.61 to reduce the neutral in either the standard or optional method.



(7) Single-Phase Dwelling Services and Feeders. For onefamily
dwellings and the individual dwelling units of two-family
and multifamily dwellings, service and feeder conductors
supplied by a single-phase, 120/240-volt system shall be permitted
to be sized in accordance with 310.15(B)(7)(1) through
(4).
For one-family dwellings and the individual dwelling units of
two-family and multifamily dwellings, single-phase feeder
conductors consisting of 2 ungrounded conductors and the
neutral conductor from a 208Y/120 volt system shall be permitted
to be sized in accordance with 310.15(B)(7)(1) through
(3).
(1) For a service rated 100 through 400 amperes, the service
conductors supplying the entire load associated with a
one-family dwelling, or the service conductors supplying
the entire load associated with an individual dwelling unit
in a two-family or multifamily dwelling, shall be permitted
to have an ampacity not less than 83 percent of the service
rating.
(2) For a feeder rated 100 through 400 amperes, the feeder
conductors supplying the entire load associated with a
one-family dwelling, or the feeder conductors supplying
the entire load associated with an individual dwelling unit
in a two-family or multifamily dwelling, shall be permitted
to have an ampacity not less than 83 percent of the feeder
rating.
(3) In no case shall a feeder for an individual dwelling unit
be required to have an ampacity greater than that specified
in 310.15(B)(7)(1) or (2).
(4) Grounded conductors shall be permitted to be sized
smaller than the ungrounded conductors, if the requirements
of 220.61 and 230.42 for service conductors or the
requirements of 215.2 and 220.61 for feeder conductors
are met.


----------



## Willie B (Jan 31, 2020)

I wish I'd said that.


----------

