Sizing Busduct

[anitharts]

Dear All
I have transformer 1000kVA 6600/400V with FLA 1445A. I want to use bus duct for secondary side. is there any regulation inform the minimum size of the busduct. example like cable, the ampacity should be 1.25 rated load. how about bus duct? my vendor propose busduct with rated current 1500A. and i also know that the standard loaded to transformer not more than 80% which is means if i use the bus duct 1500A the bus duct will never be loaded 1500A or the maximum load only 1156A. is it already ok and safe? or any calculation reference to sizing rated busduct?

 

[resqcapt19]

As long as the busway (busduct) is protected at or below its ampacity, I don't see an issue. 368.17 in the NEC.

 

[che12345]

"... if i use the bus duct 1500A the bus duct will never be loaded 1500A or the maximum load only 1156A. is it already ok and safe? or any calculation reference to sizing rated busduct?"
I have the following opinion for your consideration.
1. " ...the rated current is based on the allowable temperature rise, which can only be determined in the field...".
1.1 Copper or aluminium with contact coated or bare has different allowable temperature rise.
2. Attention: Check the Type test data, which is usually tested in free air at ambient T_ambient. The current rating should be derated if any length passed through locations having higher than the test temperature, or exposed to indirect sun light...
Che Kuan Yau (Singapore)

 

[RRaghunath]

Oil cooled transformers can be overloaded for short time in accordance with IEC. Hence, the busduct needs to be sized higher than the rated current of the transformer.
Rating the busduct to 125% of the transformer full load current (in the same lines as Power cables) seems appropriate.
Vendor can be asked to confirm the busduct capability to meet the transformer overloads for the respective allowed durations (IEC/IEEE).

R Raghunath

 

[waross]

When in doubt, RTFI.
Or in this case;
Read The Flippin Code;
The code that I work to does not specify "bus duct".
It refers to the Ampacity of CONDUCTORS.

The secondary conductors connected to transformers shall have an ampacity rating
(a) not less than 125% of the rated secondary current of the transformer for a single transformer; or
(b) not less than 125% of the sum of the rated secondary currents of all the transformers operated in
parallel.
(3) Notwithstanding Subrules (1) and (2), primary and secondary conductors shall be permitted to have an
ampacity rating not less than that required by the demand load, provided that they are protected in
accordance with Rules 14-100 and 14-104.

Applicable Codes and AHJs overide all opinions, assumptions and suggestions.
And beware of making assumptions in regards to the codes.
The code may already be based on an assumption.
Example: Available Short Circuit Current ratings and Fault currents. The code assumes a reasonable X/R ratio that may not be valid in all cases.


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Ohm's law
Not just a good idea;
It's the LAW!