Meter main feeder sized at 83%?

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Coppersmith

Coppersmith

Senior Member
Location
Tampa, FL, USA
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Electrical Contractor
Someone check my logic please.

I'm installing a meter main feeding a 125 amp subpanel (which is the original main panel) through a branch breaker (in the meter main). The subpanel is supplying the entire load of a one family dwelling. The cable going from the meter main to the subpanel is a feeder. Am I correct that this feeder can be sized at 83%? I'm a little confused because this is a subpanel.

310.15(B)(7) Single-Phase Dwelling Services and Feeders. For one family
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).

(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.
 
LarryFine said:
I'm thinking that you're okay as long as the feeder breaker is the only breaker in the MM.
Click to expand...

I'm not sure that matters. The meter main is a distribution panel. Let's say they built a second dwelling attached to the first and added another 125 amp breaker in the meter main to feed this second dwelling. Each dwelling would have an "entire load" on it's subpanel and I believe each of the two feeders could be sized at 83%. Tell me if I'm wrong.
 
You're right if each feeder supplies one entire dwelling.

I was addressing adding a circuit to the same dwelling.
 
I have never understood this rule. As I understand it say I add a branch breaker to the Meter Main to feed a well pump or an A/C outdoor unit or even a outside receptacle. Now I must size the feeder at 100% even though it is caring less load than it would if the well pump or AC were supplied from the subpanel fed by the Meter Main. This makes sense how?
 
In theory it's based on the load diversity in a dwelling unit. The 83% takes into account that the various loads cycle and the 125 amp feeder would not see a full load for any extended duration.
As you remove loads you loose that "whole house" diversity. In many cases it might not make sense but making the determination as to what or how much load could be fed directly would be complicated and diverse hence make it "whole house".
 
electricman2 said:
I have never understood this rule. As I understand it say I add a branch breaker to the Meter Main to feed a well pump or an A/C outdoor unit or even a outside receptacle. Now I must size the feeder at 100% even though it is caring less load than it would if the well pump or AC were supplied from the subpanel fed by the Meter Main. This makes sense how?
Click to expand...

I'm confused as well. IMHO:

If the dwelling unit is operating with a whole house load allowing an 83% feeder and at a later date another breaker is added to the meter main, the load for the dwelling unit is exactly as diverse as it was before. From the feeder point of view nothing has changed.

Previously, the service conductors to the meter main were carrying the whole house load and could also be sized to 83%. Now that an additional breaker has been added to the meter main, it is still carrying the entire load and can still be sized at 83%. The only thing that has changed is that the load has increased. As long as the service conductors are sized to handle the entire load, nothing is wrong.

I should clarify that the meter main is 200 amps and will have 200 amp service conductors, so it will not be overloaded by an additional breaker.
 
I think that the the 83% rule could be extended to the case where "the total calculated dwelling unit load does not exceed the feeder size." Because if you move a load from the subpanel to the meter main, you'd be strictly reducing the load on the subpanel. Versus adding a load to the meter main, which might push the total dwelling unit load above the feeder size, so you'd be non-compliant adding that load to the subpanel anyway.

For what it's worth, if your service were 125A, then you'd still be able to use the 83% rule for the 125A subpanel feeder after you move loads to the meter main. So this possibility is sort of an extension of that idea, if the total dwelling unit load doesn't exceed 125A, you don't need a 200A service, and there's no reason not to extend the same flexibilities you'd have if did you have a 125A service.

Cheers, Wayne
 
The service is a 200A service, supplying the entire load of a dwelling. So you get to take the 83% sizing.

The feed to the house is 125A, and is supplying the entire load of the dwelling. So you get to take the 83% sizing.

Now you add an AC _for the dwelling_, tapped at the service. So now the 125A is no longer carrying the entire load of the dwelling, and are no longer permitted to take the 83% sizing. I see the argument that AC has not changed the load on the 125A feeder, but to this I respond with two answers: 1) by changing the temperature of the dwelling it is possible that the electrical load would change and 2) code is a simplified, conservative approach to conductor load calculations, and many completely reasonable and safe installations will be prohibited simply to avoid unsafe installations as well.

If the service were 125A to the meter main, and you had a 125A feeder to the dwelling along with an AC connected to the meter main, you _would_ still be able to take the 83% sizing for the feeder. In this case the feeder would not be carrying the entire load of the dwelling, but the feeder is not required to be larger than the service conductors.

-Jon
 
Just for further information:

The meter main is 200 amps because that is what is readily available. 125 amp meter mains have to be ordered and are only 10 bucks cheaper. I also have a large spool of 4/0 AL in my inventory which I bought at a 50% off sale so I'd rather use this 200 amp rated wire than purchase some 125 amp rated wire.

The 125 amp house subpanel is being replaced with one of the same size in order to avoid triggering a local rule that requires the dryer and range circuits to upgraded to four wire if the panel is upsized. The dryer isn't a big deal, but replacing the range circuit would make the job significantly more expensive. Otherwise I would put in a 200 amp panel.
 
If the original main is being turned into a subpanel you will need to upgrade the range circuit anyway. You can't bond the frame of an appliance using a neutral from a subpanel.

Jon
 
Great time to settle this question, as I get such good and diverse answers here. I have a similar case in question and want the advice of those here.

As a solar installer, we often run panels, breakers and wire right to the limit. Under 2014, we were allowed to downsize any subfeed in the house, but some PV and ESS backfeed panels may operate near the limit for their whole lives. Sizing these feeders at 83% seems like a poor design choice as there is literally no load diversity. Likewise a 100A panel with (2) 50A car charger outlets is another good example of a feeder you don't want to apply an 83% factor to.

I was having this discussion yesterday with @wwhitney on another forum and I came here to get more points of view.
View attachment 2554318
Feeder diagram1.jpg

The core of my confusion comes back to the language in 310.15(B)(7)(3). I am told that the 3/0 doesn't make sense, since its larger then the feeder to the whole dwelling per 310.15(B)(7)(3)

If this is true, then the concern I have is that wires sized for a very large PV or ESS system would be allowed to take the same 83% reduction, when perhaps inappropriate. If I had a 200A service, and I connected a 400A subpanel with a 200A main breaker to it, I could backfeed 200A(160A continuous) of PV+PW through a wire rated at 166A because of the section (3) exemption, since my service conductors were allowed this exemption.

Additionally, the Powerwall ESS system has a Power Control System function, so if a home started to pull more power from the grid than the PCS setting, the system deploys battery energy to serve loads and prevent the main breaker from tripping. It is actually reasonable to pull near the limit of the service for longer than its otherwise likely, as the Tesla GW2 is allowing the system to theoretically run very near the 160A continuous limit for hours if that is the "Site Control" setting.

Perhaps I am being too conservative, but please give your thoughts. I'd be happy to use smaller wire
 
If it's a true meter/main and has a single overcurrent protection device in it rated (100-400amp) with distribution buss bars below it for the installation of branch breakers, aren't the buss bars themselves on the load side of the single main actually feeder conductors even if they so happen to be buss bars?

If that's the case then the main feeder buss bars on the load side of the single overcurrent protection device in the meter/main "are" serving the entire load, and, 83% feeder capacity is already taken care of.

If that's taken care of, then, the only concern for adding branch breakers to the distribution portion of the meter/main would only be of overloading the already reduced service conductors.

I'm keeping my head ducked down cause I may be all wet on this, and, I'm sure shots will be fired. :)

JAP>
 
electricman2 said:
I have never understood this rule. As I understand it say I add a branch breaker to the Meter Main to feed a well pump or an A/C outdoor unit or even a outside receptacle. Now I must size the feeder at 100% even though it is caring less load than it would if the well pump or AC were supplied from the subpanel fed by the Meter Main. This makes sense how?
Click to expand...
Assuming that the feeder has the same rating as the service disconnect, then you can still use the 83% rule even where main panel has additional loads connected. See 310.15(B)(7)(3).
(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).
Click to expand...
 
Vines41 said:
If I had a 200A service, and I connected a 400A subpanel with a 200A main breaker to it, I could backfeed 200A(160A continuous) of PV+PW through a wire rated at 166A because of the section (3) exemption, since my service conductors were allowed this exemption.
Click to expand...
I think it's worth reiterating that wires with an ampacity of 166A can carry 166A continuously. So your example does not represent a problem.

Cheers, Wayne
 
Vines41 said:
Great time to settle this question, as I get such good and diverse answers here. I have a similar case in question and want the advice of those here.

As a solar installer, we often run panels, breakers and wire right to the limit. Under 2014, we were allowed to downsize any subfeed in the house, but some PV and ESS backfeed panels may operate near the limit for their whole lives. Sizing these feeders at 83% seems like a poor design choice as there is literally no load diversity. Likewise a 100A panel with (2) 50A car charger outlets is another good example of a feeder you don't want to apply an 83% factor to.

I was having this discussion yesterday with @wwhitney on another forum and I came here to get more points of view.
View attachment 2554318
View attachment 2554319

The core of my confusion comes back to the language in 310.15(B)(7)(3). I am told that the 3/0 doesn't make sense, since its larger then the feeder to the whole dwelling per 310.15(B)(7)(3)

If this is true, then the concern I have is that wires sized for a very large PV or ESS system would be allowed to take the same 83% reduction, when perhaps inappropriate. If I had a 200A service, and I connected a 400A subpanel with a 200A main breaker to it, I could backfeed 200A(160A continuous) of PV+PW through a wire rated at 166A because of the section (3) exemption, since my service conductors were allowed this exemption.

Additionally, the Powerwall ESS system has a Power Control System function, so if a home started to pull more power from the grid than the PCS setting, the system deploys battery energy to serve loads and prevent the main breaker from tripping. It is actually reasonable to pull near the limit of the service for longer than its otherwise likely, as the Tesla GW2 is allowing the system to theoretically run very near the 160A continuous limit for hours if that is the "Site Control" setting.

Perhaps I am being too conservative, but please give your thoughts. I'd be happy to use smaller wire
Click to expand...

Your feeder with the question mark appears to be protected from overload at its load end with a 200A breaker. Since this is the case I would say that 310.15(B)(7) still applies.

Arguably the 200A breaker should be at the supply end of the feeder (at the gateway), and what you've got feeder the backedup subpanel is a tap. However 2/0 would still meet the 10 or 25ft tap rules.

It is also arguable whether you can apply the 83% allowance to the feeder and service panel you are backfeeding. I would say probably not. But in your example 2/0 is sized high enough for your backfeed without trying to invoke that rule.
 
don_resqcapt19 said:
Assuming that the feeder has the same rating as the service disconnect, then you can still use the 83% rule even where main panel has additional loads connected. See 310.15(B)(7)(3).
Click to expand...
This part of the code makes sense, and is logical. The narrative in the 2017 handbook, in the Exhibit 310.5 seems to say that all of section (7) only applies to the feeders to the first distribution equipment. Even my worst case still ends up with code compliant wire.

Going deeper, when applied to a 2x200 service panel if this system was tied into the 200A Main breaker only portion of the 400A meter main panel, then all the 200A wires are 3/0 Cu, unless that 200A main breaker fed a single dwelling entirely?
 
jaggedben said:
Arguably the 200A breaker should be at the supply end of the feeder (at the gateway), and what you've got feeder the backedup subpanel is a tap. However 2/0 would still meet the 10 or 25ft tap rules.
Click to expand...
I don't follow this purported argument at all. The #1 to the 125A non-backed up panel is obviously a tap (and subject to 705.12(B)(2)(2)). But the 200A feeder from the Gateway to the Backup Panel is just a continuation of the 200A feeder from the meter main to the Gateway. 705.12(B)(2)(1)(b) allows the power source interconnection in the Gateway like that because of the 200A main breaker in the backed up loads panel.

Now if the Backuped Panel had no main breaker (not sure if that's possible because of 408 requirements, I didn't check), then 705.12(B)(2)(1)(a) would govern, and the required ampacity would be ~325A. This is definitely a situation where 705.12(B)(2)(1)(a) should overrule 310.15(B)(7)(3), and perhaps 310.15(B)(7)(3) should have some language in it to make that explicit.

jaggedben said:
It is also arguable whether you can apply the 83% allowance to the feeder and service panel you are backfeeding. I would say probably not.
Click to expand...
Are you referring to the feeder from the 200A meter main to the Backup Gateway? Why wouldn't 310.15(B)(7) apply to that?

Cheers, Wayne
 
Vines41 said:
...

Going deeper, when applied to a 2x200 service panel if this system was tied into the 200A Main breaker only portion of the 400A meter main panel, then all the 200A wires are 3/0 Cu, unless that 200A main breaker fed a single dwelling entirely?
Click to expand...

It actually depends on the calculated load. Believe it or not, if the calculated load were, say, 190A, then the code allows you to have 2/0 service conductors on the line side of that 400A panel, as well as a 2/0 feeder coming out of it on a 200A breaker, and the other 200A breaker in the panel. However if the calculated load were, say, 220A , then 3/0 conductors would be required throughout.

Of course, if the calculated load were less than 200A, you could use a 200A service panel instead of a 400A 2x200. On the other hand, it's a good bet that a few 400A residential services have been put despite the calculated load being under 200A anyway, because nobody did a real load calculation. And if you want a '400A service' then you put in a 400A service.
 
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