wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Physics suggests the following amendment.
Cheers, Wayne
705.12(B)(2) 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 the square of 125 percent of the power-source(s) output circuit current and the square of the rating of the overcurrent device protecting the busbar shall not exceed120 104 percent of the square of the busbar ampere rating. 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 back-fed breaker from the power source that displays the following or equivalent wording:
WARNING:
POWER SOURCE OUTPUT DO NOT RELOCATE THIS OVERCURRENT DEVICE.
Statement of Problem and Substantiation for Public Input:
The short explanation:
Physics dictates that heating effects from current vary with the square of the current, so rules based on limiting heating within an enclosure naturally depend on sums of current squared, not linear sums of current.
The longer explanation:
Where two sources are connected to opposite ends of the busbar, to ensure that the busbar is not overloaded at any cross section it suffices to limit each source to the busbar rating. That alone would compromise a "200%" rule, rather than the "120%" rule we have.
The reason for the lower limit we have, as I understand it, is that heating within an enclosure with a busbar does not come just from the busbar's I2 * R heating itself, but also from each connection to the busbar. The heating from the connection resistance, as well as dissipated within any connected OCPD, will also vary with I2.
For a busbar connected to only one source, the worst case connection heating will occur with one load connection at 100% of the busbar rating. That is because if we assign to each load busbar connection the percentage of the total busbar current that connection sees, then the sum of these percentages is 100%, which means the sum of their squares will always be at most 100%. Equality occurs when one connection sees 100% of the load. The source connection heating will match this.
Similarly for a busbar with two sources, where each connection is limited to 100% of the busbar rating, and the sum of the sources is limited to 120% of the busbar rating, the worst case connection heating will occur when one load connection is 100% of the busbar rating, and the other load connection is 20% of the busbar rating. That provides a load connection heating effect that is 100%2 + 20%2 = 104% of the case in the previous paragraph. Likewise, the source connection heating will match this.
Thus the justification for the 120% rule, as I understand it, is that the CMP accepts that a 4% increase in total busbar connection heating is allowable when two sources are connected to a product designed and listed with only one source connection in mind.
For the case that the primary source connection is rated at 100% of the busbar, the new wording of the rule will provide identical allowances to the current wording. However, when the primary source connection is reduced to less than 100% of the busbar rating, as is a fairly common necessary practice when retrofitting secondary sources on existing equipment, the new wording will allow a larger "power source(s) output current." But it does so only to the extent that the connection heating is no worse than what is allowed when the primary source connection is rated at 100% of the busbar.
Thus the revision provides improved installer flexibility without any reduction in safety, simply by matching the math of the rule to the math of the underlying physics.
Cheers, Wayne
705.12(B)(2) 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 the square of 125 percent of the power-source(s) output circuit current and the square of the rating of the overcurrent device protecting the busbar shall not exceed
WARNING:
POWER SOURCE OUTPUT DO NOT RELOCATE THIS OVERCURRENT DEVICE.
Statement of Problem and Substantiation for Public Input:
The short explanation:
Physics dictates that heating effects from current vary with the square of the current, so rules based on limiting heating within an enclosure naturally depend on sums of current squared, not linear sums of current.
The longer explanation:
Where two sources are connected to opposite ends of the busbar, to ensure that the busbar is not overloaded at any cross section it suffices to limit each source to the busbar rating. That alone would compromise a "200%" rule, rather than the "120%" rule we have.
The reason for the lower limit we have, as I understand it, is that heating within an enclosure with a busbar does not come just from the busbar's I2 * R heating itself, but also from each connection to the busbar. The heating from the connection resistance, as well as dissipated within any connected OCPD, will also vary with I2.
For a busbar connected to only one source, the worst case connection heating will occur with one load connection at 100% of the busbar rating. That is because if we assign to each load busbar connection the percentage of the total busbar current that connection sees, then the sum of these percentages is 100%, which means the sum of their squares will always be at most 100%. Equality occurs when one connection sees 100% of the load. The source connection heating will match this.
Similarly for a busbar with two sources, where each connection is limited to 100% of the busbar rating, and the sum of the sources is limited to 120% of the busbar rating, the worst case connection heating will occur when one load connection is 100% of the busbar rating, and the other load connection is 20% of the busbar rating. That provides a load connection heating effect that is 100%2 + 20%2 = 104% of the case in the previous paragraph. Likewise, the source connection heating will match this.
Thus the justification for the 120% rule, as I understand it, is that the CMP accepts that a 4% increase in total busbar connection heating is allowable when two sources are connected to a product designed and listed with only one source connection in mind.
For the case that the primary source connection is rated at 100% of the busbar, the new wording of the rule will provide identical allowances to the current wording. However, when the primary source connection is reduced to less than 100% of the busbar rating, as is a fairly common necessary practice when retrofitting secondary sources on existing equipment, the new wording will allow a larger "power source(s) output current." But it does so only to the extent that the connection heating is no worse than what is allowed when the primary source connection is rated at 100% of the busbar.
Thus the revision provides improved installer flexibility without any reduction in safety, simply by matching the math of the rule to the math of the underlying physics.
