120% rule

[eds]

How does the 120% rule come into play if there is no bussing, i.e meter
To breaker, breaker to loadcenter.

Do you need to do the 120% on all bussing involved in the solar
Configuration. Sub panel has been increased to allow for 15 kw of solar,
Does the feed panel bussing also need to be up sized?

 

[PVfarmer]

I'm not quite sure what you're describing, but I'll try.
Are the feed panel and load center the same thing?
If you have an 80A breaker for the 15kW of PV...and a 200A main breaker....and 225A bus in the load center?

Then...225 * 1.2 = 270, while 200 + 80 is 280, so...

yes, since 280 > 270, the 225A bus is not enough.

Like this?

METER
|
|
BREAKER (200A?)
|
<---conductors?
|
MLO panel (225A bus not enough?)
|
|
<---- 80A breaker for 15kW PV

 

[eds]

320 meter socket to top side of breaker, bottom side of breaker to 200 main lug panel with a 100 amp breaker and hold down. 15 kw of solar landed on 80 amp breaker. How do you rate the service disconnecting breaker, since there is no bussing? Utilizing the outside tap rule for the main lug panel.
There is another 200 disconnect for the house

 

[pv_n00b]

Still not clear eds. Maybe it looks like this:

METER
|
|
Some breaker (need rating)
|
<---conductors
|
MLO panel through 100A breaker in panel
|
<—Panel bus (200A)
|
|
<---- 80A breaker for 15kW PV

What code version?

 

[eds]

200 amp breaker as service disconnect

 

[PVfarmer]

bottom side of breaker to 200 main lug panel with a 100 amp breaker and hold down. 15 kw of solar landed on 80 amp breaker.

How do you rate the service disconnecting breaker, since there is no bussing?

You aren't allowed to have a 200A main breaker and an 80A PV breaker connected to the 200A rated main lug panel.

200A bus * 120% = 240A max for PV+main breaker, and 280>240, even a 225A bus * 120% =270A max.

If you had a 150A main breaker, 150 + 80 = 230A, so the 200A main lug panel would be ok.

METER
|
|
MAIN BREAKER 150A ok, 200A not ok
|
|
200A main lug panel
|
|
--->100A (of loads?)
|
|
<--- 80A PV

If you had a 225A rated lug panel, so same 270A max of main breaker+PV, a 175A main breaker (+80A PV =255A) would work.

 

[eds]

Technicality I am using the outside tap rule from the 200 service disconnect breaker to feed 100 to the barn main lug panel, rated at 200 amps, utilizing a 100 amp back fed breaker with hold down and a 80 amp breaker for the solar.
How does the service disconnect breaker get calculated in regards to the 120% rule since there is no bussing, just wire into the breaker and wire out of the breaker?

 

[PVfarmer]

How does the service disconnect breaker get calculated in regards to the 120% rule

The 120% doesn't apply to the 200A service breaker, if you are talking about the 100A breaker being in the 200A rated main service panel *between* the 80A and the 200A breakers- in that case you have 200A (panel rating) * 120% = 240A, and 100A+80A is <240A. Like I said, 150A+80A=230A would be ok.

If the 100A breaker is NOT between the 200A main and 80A PV, then would potentially be 280A going to that 200A rated/240A max barn panel, and that is not ok.


METER
|
|
200A service breaker
|
|<-- conductors between 200A and 100A breakers
|
100A "barn" breaker
|
|
<-- 80A PV breaker

Since you said the 100A breaker is backfed, it sounds like you are describing the above, which is ok.
It's a 200A rated panel, and you've downsized the main breaker in that panel to 100A.

---

If we exceed the rating of the busbar without regard to the 120% rule, we are creating a scenario where we are feeding a panel board with too much energy and have nothing in place to prevent the sum of the loads in that panel from drawing more current than the busbar can handle. Let’s say we have a 200A service with a 200A main breaker and have installed a PV system with an output of 60A. In this case we have exceeded the bus bar rating of the panel board and could potentially be applying 20A of excess energy to it. This could be hazardous and lead to a fire. This may sound like a no-brainer and a something that couldn’t possiblly slip by an inspection, but it really does happen and hopefully a lot less frequently now with knowledgeable building inspectors.
What do we do in this case? Above all else, we need to have a PV system that is safe and installed to code. What can we do to remedy this and still retain the system size we have targeted? There are a few things that can be done that do not force a load or a line side tap to be done.

The simplest solution would be to downsize the main breaker in that service panel. So in our case above we could remove the 200A main breaker and replace it with a 175A main breaker to allow us to install up to 65A of PV. Call it good, right? Well, this approach is effective when taking careful consideration of all the existing loads in that panel board. There could be a potential for these loads to draw close to or more than the ampacity of that 175A breaker. We do not want nuisance tripping, so this approach must be done carefully. Refer to NEC 220 when considering this approach.
https://www.civicsolar.com/support/installer/articles/120-rule-what-it-and-why-it-important-0

 

[jaggedben]

Going back to the beginning...

How does the 120% rule come into play if there is no bussing, i.e meter
To breaker, breaker to loadcenter.

Do you need to do the 120% on all bussing involved in the solar
Configuration. Sub panel has been increased to allow for 15 kw of solar,
Does the feed panel bussing also need to be up sized?

Nothing on the supply side of the service disconnect is subject to the 120% rule. Also the solar does not count towards calculated load. So the answer to your last question is: No.

I think we fully discussed your load side questions in the other thread you started.

 

[Smart $]

Technicality I am using the outside tap rule from the 200 service disconnect breaker to feed 100 to the barn main lug panel, rated at 200 amps, utilizing a 100 amp back fed breaker with hold down and a 80 amp breaker for the solar. ...

That's perplexing if you are under pre-2014. Definitely debatable. AHJ's should use 2014 (below) or 2017 as guidance IMO if under earlier cycle purview.

(2) Bus or Conductor Ampere Rating. One hundred
twenty-five percent of the inverter output circuit current
shall be used in ampacity calculations for the following:

(1) Feeders. Where the inverter output connection is made
to a feeder at a location other than the opposite end of
the feeder from the primary source overcurrent device,
that portion of the feeder on the load side of the inverter
output connection shall be protected by one of
the following:

(a) The feeder ampacity shall be not less than the sum
of the primary source overcurrent device and
125 percent of the inverter output circuit current.

(b) An overcurrent device on the load side of the inverter
connection shall be rated not greater than the
ampacity of the feeder.

(2) Taps. In systems where inverter output connections are
made at feeders, any taps shall be sized based on the
sum of 125 percent of the inverter(s) output circuit
current and the rating of the overcurrent device protecting
the feeder conductors as calculated in 240.21(B).

(3) Busbars. ...

 

[jaggedben]

That's perplexing if you are under pre-2014. Definitely debatable. AHJ's should use 2014 (below) or 2017 as guidance IMO if under earlier cycle purview.

That part you highlighted in blue is a terribly worded section that I tried - and failed - to get fixed this last code cycle. But the great thing about an outside tap is that 240.21(B) contains no minimum ampacity requirement. So even under the most wronghead interpretation, the sum of both requirements is still no more than (..wait for it...) 125 percent of the inverter output. :lol:

 

[Smart $]

That part you highlighted in blue is a terribly worded section that I tried - and failed - to get fixed this last code cycle. But the great thing about an outside tap is that 240.21(B) contains no minimum ampacity requirement. So even under the most wronghead interpretation, the sum of both requirements is still no more than (..wait for it...) 125 percent of the inverter output. :lol:

I agree it is terribly worded, but I don't read it that way. It says the sum of 125% inverter output and the OCPD rating protecting the feeder conductors... not the tap.

 

[PVfarmer]

1 How does the 120% rule come into play if there is no bussing,

2 Do you need to do the 120% on all bussing involved in the solar configuration.

3 Does the feed panel bussing also need to be up sized?

If the 100A breaker in the barn is where the conductors are terminated...

(1) It doesn't, so (2) no and (3)no.
It only applies to the barn panel, and there you have 200A rating *120% = 240A.
240A - 80A of PV = 160A limit on barn breaker.

Outside feeder tap of unlimited length rule [240.21(B)(5)] — You don't have to install an OCPD at the tap point of a feeder tap of unlimited length as long as you meet the following requirements (Fig. 2 below):

• The tap conductors are suitably protected from physical damage.
• The tap conductors are terminated at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors. This single OCPD may supply any number of additional overcurrent devices on its load side.
• The OCPD for the tap conductors is an integral part of a disconnecting means or located immediately adjacent to the device.
• The disconnect is located at a readily accessible location outside the building or structure, or nearest the point of entry of the service conductors.

http://ecmweb.com/code-basics/understanding-rules-feeder-taps

 

[Smart $]

.

 

[Smart $]

If the 100A breaker in the barn is where the conductors are terminated...

(1) It doesn't, so (2) no and (3)no.
It only applies to the barn panel, and there you have 200A rating *120% = 240A.
240A - 80A of PV = 160A limit on barn breaker.

...

The graphic and link only consider a feeder tap. It does not incorporate interconnected sources (705 requirements) into the discussion.

IMO the 120% rule, or the "sum of" rule(s) apply between PV system disconnect and the service disconnecting means... the part in red below.

METER|
|
200A service breaker
|
|<-- conductors between 200A and 100A breakers
|
100A "barn" breaker
|
|
<-- 80A PV breaker

 

[PVfarmer]

The graphic and link only consider a feeder tap. It does not incorporate interconnected sources (705 requirements) into the discussion.

IMO the 120% rule, or the "sum of" rule(s) apply between PV system disconnect and the service disconnecting means... the part in red below.

I'm simply unclear as to where the 80A PV breaker is relative to the 200A and 100A breakers.
If we're talking 2(1) or 2(2) in bold, then a 200A rated barn panel won't do.
If we're talking 3(b), then the 200A panel is ok.
I think?
The 100A breaker is "in the red" because it is "before" the 80A breaker, if it was "after"(under) the 80A, the 200A would be "in the red".

200A service breaker
|
|<-- conductors between 200A and 100A breakers
|
100A "barn" breaker
|
|
<-- 80A PV breaker

705.12
(D) Utility-Interactive Inverters.
The output of a utilityinteractive inverter shall be permitted to be connected to
the load side of the service disconnecting means of the
other source(s) at any distribution equipment on the premises. (the barn panel, BP)

Where distribution equipment, including switchgear,
switchboards, or panelboards, or BP is fed simultaneously by a
primary source(s) of electricity and one or more utility interactive
inverters, and where this distribution equipment
is capable of supplying multiple branch circuits or feeders,
or both, the interconnecting provisions for the utility interactive
inverter(s) shall comply with 705.12(D)(l) through (D)(6).

(1) Dedicated Overcurrent and Disconnect. The source
interconnection of one or more inverters installed in one
system shall be made at a dedicated circuit breaker or fusible
disconnecting means. (the 80A PV breaker)

(2) Bus or Conductor Ampere Rating. One hundred
twenty-five percent of the inverter output circuit current (80A)
shall be used in ampacity calculations for the folIowing:
(1) Feeders. Where the inverter output connection is made
to a feeder at a location other than the opposite end of
the feeder from the primary source overcurrent device,
(would be: 200A --> 80A --> 100A)
that portion of the feeder on the load side of the inverter
output connection shall be protected by one of
the following:
(a) The feeder ampacity shall be not less than the sum
of the primary source overcurrent device and
125 percent of the inverter output circuit current.

(b) An overcurrent device on the load side of the inverter
connection shall be rated not greater than the
ampacity of the feeder.???

(2) Taps.ln systems where inverter output connections are
made at feeders, any taps shall be sized based on the
sum of 125 percent of the inverter(s) output circuit
current and the rating of the overcurrent device protecting
the feeder conductors as calculated in 240.21 (B).
(would be same as above)

(3) Busbars. One of the methods that follows shall be used
to determine the ratings of busbars in panelboards.
(a) The sum of 125 percent of the inverter(s) output
circuit current and the rating of the overcurrent device protecting
the busbar shall not exceed the ampacity of the
busbar.

Informational Note: This general rule assumes no limitation
in the number of the loads or sources applied to busbars
or their locations.

(b) Where two sources, one a utility and the other an
inverter. are located at opposite ends of a busbar that contains loads,
the sum of 125 percent of the inverter(s) output
circuit current and the rating of the overcurrent device protecting
the busbar
(would be: 200A --> 100A --> 80A)
shall not exceed 120 percent of the ampacity
of the bus bar. The busbar shal1 be sized for the loads
connected in accordance with Article 220.

 

[Smart $]

I'm simply unclear as to where the 80A PV breaker is relative to the 200A and 100A breakers.
If we're talking 2(1) or 2(2) in bold, then a 200A rated barn panel won't do.
If we're talking 3(b), then the 200A panel is ok.
I think?
The 100A breaker is "in the red" because it is "before" the 80A breaker, if it was "after"(under) the 80A, the 200A would be "in the red".
...

Let's say, until we learn otherwise, the 100A breaker is at one end, the 80A PV breaker at the other. The panel would be okay per 3(B), green bolded below.

200A service breaker
|
|<-- conductors between 200A and 100A breakers
|
100A "barn" breaker (200A-rated MLO panelboard)
|
|
<-- 80A PV breaker.


What is still in red is what is in question, and that part falls under 2(1) or 2(2). IMO, 100A conductors are not compliant.

 

[jaggedben]

I agree it is terribly worded, but I don't read it that way. It says the sum of 125% inverter output and the OCPD rating protecting the feeder conductors... not the tap.

Yeah, but then it says 'as calculated in 240.21(B)'. Which has to mean, well... something that is a calculation according to 240.21(B), which the OCPD rating is not. I suppose you're correct that there could be an even more wrongheaded interpretation than what I suggested. :lol:

 

[jaggedben]

...

IMO the 120% rule, or the "sum of" rule(s) apply between PV system disconnect and the service disconnecting means... the part in red below.

To be clear, that's only true under the 2011 code or earlier. In 2014 and 2017 the rule only applies to a panelboard.

 

[PVfarmer]

Let's say, until we learn otherwise, the 100A breaker is at one end, the 80A PV breaker at the other. The panel would be okay per 3(B), green bolded below.

200A service breaker
|
|<-- conductors between 200A and 100A breakers
|
100A "barn" breaker (200A-rated MLO panelboard)
|
|
<-- 80A PV breaker.

What is still in red is what is in question, and that part falls under 2(1) or 2(2). IMO, 100A conductors are not compliant.

Right, I agree- the conductors in red, even though there will only be 100A of load, are still "exposed" to the 200A breaker is what you're saying?
If it goes 200A breaker --> main lugs, OR if it goes 200A --> 100A breaker, 100A is not enough for those conductors.

BUT, if they go 200A breaker to main lugs, the 200A rated panel is not enough, period, because those conductors and the main lug panel bus are exposed to 280A.
The confusing part is- it can't be a "MLO" panel as in "Only", as in the lugs are connected to the 200A breaker(by the conductors of course)... main lug OUT for the 100A, ok. There has to be a 100A breaker (or up to 160A with the 80A of PV) between the 200A breaker and the 200A rated bus for that bus to be ok.

there is no bussing, i.e meter to breaker, breaker to loadcenter.
bottom side of breaker to 200 main lug panel with a 100 amp breaker and hold down.
using the outside tap rule from the 200 service disconnect breaker to feed 100 to the barn main lug panel,rated at 200 amps, utilizing a 100 amp back fed breaker

The 1st line sounds like 200A breaker/conductors to lugs, but the 2nd and even more so the 3rd sound like:
200A breaker --> 100A breaker (not lugs), then below the 100A are the 80A PV and some lugs to a 100A breaker in sub panel?.