amluto
Member
- Location
- San Mateo County, California
I'm thinking of proposing a change to 705.12(B)(2)(b), and I'm hoping for some input.
It seems to me that the current rule makes complete sense for basic panelboards
that have a main breaker and a bunch of identical branch breaker provisions on the
load side of the main breaker. It doesn't seem to anticipate panelboards that have
provisions for a sub-feed breaker and where the secondary power source is
connected using a branch breaker instead of a sub-feed breaker.
The current language is:
I propose to change this to:
Substantiation: I think the current language is somewhat dangerous and
unnecessarily restrictive.
Imagine a 200A top-fed main breaker panel that has an unused provision
for a sub-feed breaker at the bottom. Under the current language, it is
probably acceptable to install a 40A breaker (for a 32A power source) at
the bottom of the panel just above the sub-feed provision as long as
it's labeled with a warning sign saying "WARNING: ... DO NOT RELOCATE
THIS OVERCURRENT DEVICE". The installation would be safe as is.
However, a future electrician could see that sign, and, without thinking
about it hard enough, install a 200A subfeed breker at the bottom. The
result would still be safe (no part of the busbar could be overloaded),
but it would no longer comply. If sub-feed lugs were to be installed,
the result would be unsafe: the bus between the lugs and the 40A breaker
could be overloaded.
On the other hand, there are simple installations that would be safe but
are not permitted. Imagine the same 200A panelboard with a 40A breaker
for the secondary power supply at the lowest normal breaker position
and a 200A sub-feed breaker at the very botton. This would be entirely safe:
the bus between the 40A breaker and the sub-feed breaker would be protected by
the sub-feed breaker and the bus above the 40A breaker would be protected
as it would be if the breakers were in the other order.
It seems to me that the current rule makes complete sense for basic panelboards
that have a main breaker and a bunch of identical branch breaker provisions on the
load side of the main breaker. It doesn't seem to anticipate panelboards that have
provisions for a sub-feed breaker and where the secondary power source is
connected using a branch breaker instead of a sub-feed breaker.
The current language is:
Where two sources, one a primary power source and the other another
power source, are located at opposite ends of a busbar that contains
loads, the sum of 125 percent of the power source(s) output circuit
current and the rating of the overcurrent device protecting the busbar
shall not exceed 120% the ampacity of the busbar. The busbar shall be
sized for the loads connected in accordance with Article 220. A
permanent warning label shall be applied to the distribution equipment
adjacent to the backfed breaker from the power source that displays the
following or equivalent wording:
WARNING:
POWER SOURCE OUTPUT CONNECTION,
DO NOT RELOCATE THIS OVERCURRENT DEVICE
The warning sign(s) or label(s) shall comply with 110.21(B).
Exception: Equipment with multiple ampacity busbars or center fed
panelboards are not addressed by this provision.
power source, are located at opposite ends of a busbar that contains
loads, the sum of 125 percent of the power source(s) output circuit
current and the rating of the overcurrent device protecting the busbar
shall not exceed 120% the ampacity of the busbar. The busbar shall be
sized for the loads connected in accordance with Article 220. A
permanent warning label shall be applied to the distribution equipment
adjacent to the backfed breaker from the power source that displays the
following or equivalent wording:
WARNING:
POWER SOURCE OUTPUT CONNECTION,
DO NOT RELOCATE THIS OVERCURRENT DEVICE
The warning sign(s) or label(s) shall comply with 110.21(B).
Exception: Equipment with multiple ampacity busbars or center fed
panelboards are not addressed by this provision.
I propose to change this to:
Where two sources, one a primary power source and the other another
power source, are located on a busbar that contains loads and the
primary power source is at one end of the busbar, the sum of 125 percent
of the power source(s) output circuit current and the rating of the
overcurrent device protecting the busbar shall not exceed 120% the
ampacity of the busbar. The busbar shall be sized for the loads
connected in accordance with Article 220.
If the backfed breaker from the power source is at the opposite end of
the busbar from the primary power source and no devices or taps can be
installed beyond it, then a permanent warning label
shall be applied to the distribution equipment adjacent to the backfed
breaker from the power source that displays the following or equivalent
wording:
WARNING:
POWER SOURCE OUTPUT CONNECTION,
DO NOT RELOCATE THIS OVERCURRENT DEVICE
If the distribution equipment allows additional devices or taps to be
installed between the backfed breaker from the power source and the end
of the busbar opposite the primary power source, then all of the
following are required:
(i) No taps may be made between the backfed breaker from the power
source and the end of the busbar opposite the primary power source.
(ii) The sum of the ratings of any overcurrent devices between the
backfed breaker from the power source and the end of the busbar opposite
the primary power source must not exceed the rating of the busbar.
(iii) A permanent warning label shall be applied to the distribution
equipment adjacent to the backfed breaker from the power source that
displays the following or equivalent wording:
WARNING:
POWER SOURCE OUTPUT CONNECTION,
DO NOT RELOCATE THIS OVERCURRENT DEVICE.
NO TAPS MAY BE MADE ON THE BUS BEYOND THIS OVERCURRENT DEVICE,
AND THE SUM OF THE RATINGS OF ALL OVERCURRENT DEVICES
BEYOND THIS DEVICE MUST NOT EXCEED THE RATING OF THE BUS.
The warning sign(s) or label(s) shall comply with 110.21(B).
Exception: Equipment with multiple ampacity busbars or center fed
panelboards are not addressed by this provision.
power source, are located on a busbar that contains loads and the
primary power source is at one end of the busbar, the sum of 125 percent
of the power source(s) output circuit current and the rating of the
overcurrent device protecting the busbar shall not exceed 120% the
ampacity of the busbar. The busbar shall be sized for the loads
connected in accordance with Article 220.
If the backfed breaker from the power source is at the opposite end of
the busbar from the primary power source and no devices or taps can be
installed beyond it, then a permanent warning label
shall be applied to the distribution equipment adjacent to the backfed
breaker from the power source that displays the following or equivalent
wording:
WARNING:
POWER SOURCE OUTPUT CONNECTION,
DO NOT RELOCATE THIS OVERCURRENT DEVICE
If the distribution equipment allows additional devices or taps to be
installed between the backfed breaker from the power source and the end
of the busbar opposite the primary power source, then all of the
following are required:
(i) No taps may be made between the backfed breaker from the power
source and the end of the busbar opposite the primary power source.
(ii) The sum of the ratings of any overcurrent devices between the
backfed breaker from the power source and the end of the busbar opposite
the primary power source must not exceed the rating of the busbar.
(iii) A permanent warning label shall be applied to the distribution
equipment adjacent to the backfed breaker from the power source that
displays the following or equivalent wording:
WARNING:
POWER SOURCE OUTPUT CONNECTION,
DO NOT RELOCATE THIS OVERCURRENT DEVICE.
NO TAPS MAY BE MADE ON THE BUS BEYOND THIS OVERCURRENT DEVICE,
AND THE SUM OF THE RATINGS OF ALL OVERCURRENT DEVICES
BEYOND THIS DEVICE MUST NOT EXCEED THE RATING OF THE BUS.
The warning sign(s) or label(s) shall comply with 110.21(B).
Exception: Equipment with multiple ampacity busbars or center fed
panelboards are not addressed by this provision.
Substantiation: I think the current language is somewhat dangerous and
unnecessarily restrictive.
Imagine a 200A top-fed main breaker panel that has an unused provision
for a sub-feed breaker at the bottom. Under the current language, it is
probably acceptable to install a 40A breaker (for a 32A power source) at
the bottom of the panel just above the sub-feed provision as long as
it's labeled with a warning sign saying "WARNING: ... DO NOT RELOCATE
THIS OVERCURRENT DEVICE". The installation would be safe as is.
However, a future electrician could see that sign, and, without thinking
about it hard enough, install a 200A subfeed breker at the bottom. The
result would still be safe (no part of the busbar could be overloaded),
but it would no longer comply. If sub-feed lugs were to be installed,
the result would be unsafe: the bus between the lugs and the 40A breaker
could be overloaded.
On the other hand, there are simple installations that would be safe but
are not permitted. Imagine the same 200A panelboard with a 40A breaker
for the secondary power supply at the lowest normal breaker position
and a 200A sub-feed breaker at the very botton. This would be entirely safe:
the bus between the 40A breaker and the sub-feed breaker would be protected by
the sub-feed breaker and the bus above the 40A breaker would be protected
as it would be if the breakers were in the other order.