NickParker
Electrical
How to use the derating factors table for cable grouping factors / number of trays from IEC?
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IEC Cables - Derating factor 1
- Thread starter NickParker
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First the table is referring to the same size of cables 100% loaded. IEC 60364-5-52 for low voltage cables and IEC 60502-2 for medium voltage cable present the same table. That means, if the are all same cable full loaded one tray medium voltage and one or two low voltage cable trays of full loaded cable this table is destined for.
There is a note on the down part of the table:
NOTE 3 Values are given for vertical spacings between trays of 300 mm and at least 20 mm between trays and wall. For closer spacing the factors should be reduced.
No indication is for the trays on the same level [horizontal distance]
In my humble impression the calculation error and load diversity of cable it is more than these factors.
There is a note on the down part of the table:
NOTE 3 Values are given for vertical spacings between trays of 300 mm and at least 20 mm between trays and wall. For closer spacing the factors should be reduced.
No indication is for the trays on the same level [horizontal distance]
In my humble impression the calculation error and load diversity of cable it is more than these factors.
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NickParker
Electrical
7anoter4 said:
Can you please elaborate this?
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From IEC 60364-5-52 A.52.4.2 (523.4.2) Installation types E and F in table A.52-1
NOTE 2 Group reduction factors have been calculated on the basis that the group consists of similar equally loaded insulated conductors or cables.
Since Table A.52-20 (52-E4) from IEC 60364-5-52 is identical with Table B.22 IEC 60502-2 that means the voltage is not important, but only the carrying current.
The indicated order of cable tray-from fire protection point of view-the higher voltage cables will run at higher level.
NOTE 2 Group reduction factors have been calculated on the basis that the group consists of similar equally loaded insulated conductors or cables.
Since Table A.52-20 (52-E4) from IEC 60364-5-52 is identical with Table B.22 IEC 60502-2 that means the voltage is not important, but only the carrying current.
The indicated order of cable tray-from fire protection point of view-the higher voltage cables will run at higher level.
NickParker:
From the table, "number of trays" means just that - cable trays located in close proximity IN THE VERTICAL DIRECTION. Think of this as not having much vertical clearance between trays - and therefore not a lot of air to carry away the resulting thermal load from current passing through the cable.
Once you know your "vertical stack" line, you then look up the "number of cables" factor for that number of trays. The cables must all be the same physical size (in terms of conducting cross-section) and be loaded to a reasonably close approximation of the same current in each cable. The "number of cables" refers to the number of adjacent conductors in a single tray, and assumes (as shown in the image accompanying the table) that the conductors within the tray do not have at least a one-cable-diameter dimension of free air before the next cable jacket is encountered.
As Zanoter mentioned, the voltage that is applied to each cable run can be different, as long as the conductor size is the same. When mounting trays for multiple voltage levels, put the higher voltage conductors further from the floor. As an example, the following three trays are correctly spaced per the table (300 mm between trays vertically, no tray closer than 20 mm to vertical (e.g. wall) surface):
Top tray has 6 cables of 100 mm2 conductor cross-section. Each cable is insulated for 11000 V.
Middle tray has 6 cables of 100 mm2 conductor cross-section. Each cable is insulated for 6000 V.
Bottom tray has 9 cables of 100 mm2 conductor cross-section. Each cable is insulated for 1000 V.
All 12 cables in the two top trays need to be limited to (nominal amp rating) * 0.71.
The 9 cables in the bottom tray need to be limited to (nominal amp rating) * 0.66.
If there are other derating factors that need to be considered (high altitude, high ambient, multiple layers within the tray, etc.) then these need to be accounted for as well - in addition to the "geometry" limits shown in the table you included in your post.
Converting energy to motion for more than half a century
From the table, "number of trays" means just that - cable trays located in close proximity IN THE VERTICAL DIRECTION. Think of this as not having much vertical clearance between trays - and therefore not a lot of air to carry away the resulting thermal load from current passing through the cable.
Once you know your "vertical stack" line, you then look up the "number of cables" factor for that number of trays. The cables must all be the same physical size (in terms of conducting cross-section) and be loaded to a reasonably close approximation of the same current in each cable. The "number of cables" refers to the number of adjacent conductors in a single tray, and assumes (as shown in the image accompanying the table) that the conductors within the tray do not have at least a one-cable-diameter dimension of free air before the next cable jacket is encountered.
As Zanoter mentioned, the voltage that is applied to each cable run can be different, as long as the conductor size is the same. When mounting trays for multiple voltage levels, put the higher voltage conductors further from the floor. As an example, the following three trays are correctly spaced per the table (300 mm between trays vertically, no tray closer than 20 mm to vertical (e.g. wall) surface):
Top tray has 6 cables of 100 mm2 conductor cross-section. Each cable is insulated for 11000 V.
Middle tray has 6 cables of 100 mm2 conductor cross-section. Each cable is insulated for 6000 V.
Bottom tray has 9 cables of 100 mm2 conductor cross-section. Each cable is insulated for 1000 V.
All 12 cables in the two top trays need to be limited to (nominal amp rating) * 0.71.
The 9 cables in the bottom tray need to be limited to (nominal amp rating) * 0.66.
If there are other derating factors that need to be considered (high altitude, high ambient, multiple layers within the tray, etc.) then these need to be accounted for as well - in addition to the "geometry" limits shown in the table you included in your post.
Converting energy to motion for more than half a century
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