The Meter-Main Combo Panel
They are widely used in most parts of the
country. They have a meter socket enclosed in the same case with the main
circuit breaker. Some of them consist of a space that can be used to attach
load breakers. You are not allowed to use conductors that are exposed from the
main breaker present between the socket and the main breaker to supply the PV
connection. By doing so you are offending and breaking terms and conditions
listed on the manual of the equipment. Those terms and conditions stipulate
that all associated equipment should be used according to the instruction and
limitations are given.
Sometimes, the producer of the meter main-combo
may enclose explanations and hardware to construct a PV connection. An accredited inspector, from the National,
Recognized Testing Laboratory (NRTL) or other authority certified by OSHA and
may be needed by the meter producer or the Authority Having Jurisdiction (AHJ).
In some cases, the usages take into
consideration the circuit amidst the main breaker and meter socket and no
connection is allowed under no condition.
If you have a meter that consists of load
breaker space in the case, then a back-fed PV breaker can be installed.
Something to note is that the breaker and PV connections are on the load side
of the main breaker, making them be load-side connection, thus allowing section
705.12(B) to be used.
conventional meter main combo panel, there are similar ratings between the
conductor and the main breaker. If you have load breakers and PV breakers
connected to such conductor, then section 705.12(B)(2)(3)(b) will come into
effect and the back-fed PV breaker should be positioned at the furthest point
from the main breaker. Once you have a back-fed PV breaker with 20 amps for a
100-amp conductor, 25 amps for a 125-amp and 40 amps for a
200-amp conductor, then a rule called 120 percent rule will be used.
A general principle here
is that, if you take 120% of the conductor’s value then you subtract the main
breaker value you will get the maximum value of back-fed breaker. Take an
example of a conductor with 200-amp and the main breaker having 200-amp. You
will have to compute the following mathematics 1.2 x 200 – 200= 40 amps which
are the maximum value of the back-fed breaker.
It is required by the law
that for a PV inverter, 125% of the output current to be used for calculations.
But this gives a translation of the breaker ratings because the breaker should
be at a minimum of 125% of the PV inverter output.
Some producers of
meter-main combo panel design these meters with interchangeable breakers. Even
if there is a 200-amp conductor or 150-amp conductor a small main breaker may
be used as usual. For successful safety, careful inspection and analysis should
be done, this should involve the whole building to make sure that the
established main breaker is oversize and the main breaker with low ratings may
be used for substituting it. Under all these circumstances the manufacturer’s
manuals should indicate this. If you use a main breaker with low ratings, you
may allow a huger back-fed PV breaker to be used.
Look at the following
mathematics. A 300-amp meter main combo panel has allowed 250-amp. 1.2x
300-250= 110 amps. This is a maximum rating to be used for the back-fed PV
to do if there is no breaker position.
If that happens then
connect it at remote subpanel. If you find out that your meter-main combo panel
does not have a position to attach an open load breaker, then two options are
solutions. The first solution is to meet all specifications of the meter-main
combo panel and subpanel, as stipulated under section 705.12, (B)(2)(3) by
making a connection between the load side PV connection and the meter-main
A breaker sending to the
subpanel should be located at the furthest point from the main breaker on the
conductor. It is necessary to make sure that, this condition also applies to
the subpanel. Also, a back-fed PV connector should be located on the furthest
point of the conductor as much as possible.
This distance could be either from the main lug or the main breaker of
The 120% rule is used too
to the conductor and main breaker of the subpanel to determine the maximum
allowed ratings for the back-fed PV breaker.
Consider an example below.
You have a meter-main
combo panel which has 300-amp, the main breaker which has 300 and a breaker
with 200-amp offering protection to a 200-amp subpanel. As explained above, the
200-amp panel will be shifted to the furthest point away from the main breaker.
This will be done by exchanging positions on the breaker of the meter-main
combo panel. The 200-amp subpanel is the only main lug in the subpanel. Once the 120% rule is used as per the
above-quoted code section, it would permit up to 40-amp from the back-fed PV
breaker provided that the breaker is placed on the furthest position from the
There is no need to
change the size of the feeder due to the fact that the generation of currents
by the PV will counteract the utility current thus lowering the total current
in the feeder and the meter-main conductor.
Feeder Tap with the new panel board.
This is a second method
whereby a connection to a PV system is by tapping a position of a feeder on the
present meter-main combo panel. This should be at a precise position which is
found almost to the remote load. There should be strong protection of the
feeder running from the connection area to the remote panel. This protection is
against overcurrent which may come from the utility or PV system. The code
gives the options on how to deal with the problem, this is under section
705.12(B)(2)(1)(a) and (b). It is widely recommended to use, option (b) rather
than option (a) because it is relatively difficult to substitute both the whole
feeder to the remote position.
The simplest method is to
increase a small lug on the panel board. This addition should be at a minimum
of two positions at the junction between the PV output circuit and the breaker
feeder. A function of a first breaker is to defend the remaining feeder and the
remote panel while a second breaker functions to act as the PV inverter
alternative current output breaker.
Consider the following
One hundred and
twenty-five-ampere breaker supplying to the one hundred and a
twenty-five-ampere main lug sub panel in the two hundred ampere main-meter
combo panel. The breaker is placed in the right location far away on the
main-meter conductor. The load center will be placed next to the main-meter
combo. The feeder will be channeled via the load center and the circuit from
the breaker will pass via the new load center to the main lugs. The elongation
of the channel from the feeder to the remote subpanel will be achieved through
joining this circuit to the breaker with one hundred and twenty-five amperes in
the load center.
The inverter for the PV is
rated at fifteen amperes output, thus a twenty-ampere breaker can be put in the
new load center to join the PV system output. As stated in section 690.9, the
calculations here will be as follows. 15 amp x 1.25 = 18.75 amp thus the
maximum allowed current would be to nearly 20 amps. If one hundred and twenty
percent rule is applied a maximum of twenty-five back-fed PV systems can be
allowed to be put. Under these conditions, overcurrent is prevented and thus
there is no need to replace or upgrade. If you are using this panel, there is
no problem if you add a new two hundred panel ampere panel board and you may
use up to one hundred and fifteen amperes in a back-fed PV system. At this
condition you will be implementing a one hundred and twenty percent rule thus
feeder overloading will be avoided.
This rated breaker will
be able to offer protection from a remote sub panel to the downstream feeder.
Also, if the upstream feeder is rated the same as the downward stream then, the
passing utility and PV currents would not be high to exceed the rated amount.
Thus, the utility currents in the feeder will be lowered by PV currents coming
back to the main-meter panel since they will cancel each other and hence
reducing any current which is less than one hundred and twenty-five amperes.
You should bear into your
mind that, if you are computing breaker values, then when you add breakers that
have proper ratings, you are offering protection to various conductors against
overload and short-circuits.
Main-Lug-Only Service Panels having Six Main Breakers
positions that are not filled.
These panels are widely
available all over the country and bring some problems when connecting a PV
system. These are used only when meter conductors are joined to the main lugs.
In most situations, the manufacturer’s instructions on the panel board provide
that if you want to use your appliance as a piece of service equipment, then
you should select one position to use it as the main breaker. Under this
situation, remaining positions become load-side connections, if connected to
the PV back-fed breaker then section 705.12(B) comes into effect. The simplest
and unusual condition occurs if one of the six positions is not used. If you
use such a position in a back-fed PV breaker then, this position will be a
supply-side connection. You may use any ratings for the back-fed breaker, panel
conductor or service provided that the panel board producer allows.
A 300-amp service panel
consists of a single empty breaker position. Both the panel and service panel
are having ratings of 300 amps. Under this situation, a 300-amp back-fed
breaker can be put in the empty position provided that, panel board
manufacturer allows. A 300-amp breaker allows an inverter outputs rating to be
240-amps which is equal to 80% of the 300-amps. Note that most panels do not
permit a breaker which is full conductor rating.
A factor behind this is
that the total load on a panel conductor must be less than the conductor rating
as per section 230 and 240 of the NEC handbook. It is limited to have a total
rating of power productions to be greater the ratings of the service
devices. This note is per section
Though it is not a code
prerequisite, it is recommended that those back-fed PV breakers having ratings
that are almost the same as the conductor ratings should be placed far away
from the input of the main lug. The advantage of this act is to avoid addition
of PV currents to utility currents. If the process is not avoided and thus
occurs, accompanied by high PV production then these currents will overload the
conductor. The PV currents will resist the utility currents in the conductor
leading to the reduction of the conductor current.
In circumstances where
all breaker positions are full, fixing the feeder may be done through the
installment of the new panel board close to the existing panelboard as
explained before. When you have a feeder with a short distance, in the middle
of the service panel and new panel, then you can increase the distance to
permit a larger quantity of back-fed PV. The ratings for the newly installed
panel board may equate with those of the existing original feeder breaker. This
has one advantage that it can be a protective measure to the downward parts of
The NEC does not
recommend the usage of PV connections when installing a service panel. Thus it
is very important to make a close examination of other requirements stipulated
in the law that may help to install connection safely. All installers should be
careful when dealing with these situations and how are they dealt with by the
PV installers. Amendments of the NEC will address these allowances and