I'm looking at a potential project that is calling for (4) 2500kVA 13.8-480V Transformers feeding (4) 4000A 480V Switchgear Lineups. The current conceptual design has 2500kVA transformers feeding the 4000A lineups with 4000A main breakers. This will likely require (11) sets of 500MCM cables in underground conduit for some distance which can be quite costly and difficult to install.
The transformer secondary current rating is 3007A so using 125% of that per table 450.3(A) gives secondary main breaker size of 3758A which rounds up to 4000A. I suppose you could also round down to 3500A which would also in turn reduce cables to (10) sets of 500MCM. I guess the choice of 3500A vs 4000A is depended on actual load requirements. (Load requirements dictate both primary and secondary cable ratings and can be less than transformer FLA values if load permits)
I've always understood the 125% multiplier of transformer secondary to size main breaker to be due to the fact that breakers are 80% rated and thus need to increase breaker to account for this and allow full capacity of transformer (common practice). Its my understanding that this does not in any way compromise the overload rating of the transformer?
So my question is......Is there a possible cost savings in this case for using a 3000A 100% rated main breaker as opposed to 80% rated breaker. Obviously breaker and switchgear can be reduced to 3000A which would present a cost savings but I'm not sure how the increased cost of 100% rated breaker may offset these two reductions? This would also require only (8) sets of 500MCM cable as opposed to the oriognal 11 which would also present a cost savings. In my opinion if the cost savings from reduced breaker, switchgear, and cable significantly outweighed the added cost for 100% rated breaker than this seems like a good option to me.
Does anyone agree with me or am I missing something?
The transformer secondary current rating is 3007A so using 125% of that per table 450.3(A) gives secondary main breaker size of 3758A which rounds up to 4000A. I suppose you could also round down to 3500A which would also in turn reduce cables to (10) sets of 500MCM. I guess the choice of 3500A vs 4000A is depended on actual load requirements. (Load requirements dictate both primary and secondary cable ratings and can be less than transformer FLA values if load permits)
I've always understood the 125% multiplier of transformer secondary to size main breaker to be due to the fact that breakers are 80% rated and thus need to increase breaker to account for this and allow full capacity of transformer (common practice). Its my understanding that this does not in any way compromise the overload rating of the transformer?
So my question is......Is there a possible cost savings in this case for using a 3000A 100% rated main breaker as opposed to 80% rated breaker. Obviously breaker and switchgear can be reduced to 3000A which would present a cost savings but I'm not sure how the increased cost of 100% rated breaker may offset these two reductions? This would also require only (8) sets of 500MCM cable as opposed to the oriognal 11 which would also present a cost savings. In my opinion if the cost savings from reduced breaker, switchgear, and cable significantly outweighed the added cost for 100% rated breaker than this seems like a good option to me.
Does anyone agree with me or am I missing something?
