leon2012
Member
- Location
- Marietta Ga 30067
I have a 20 MVA transformer 20KV to 600 V. protected by a 1200 Amp breaker. This transformer feeds a rectifier that converts this AC to 750 V DC for the electrical trains in my city.
I need to do a tap from the secondary bars to feed a 100 KVA transformer 600V to 480V.
The distance to the small transformer is going to be longer than 10 ft. but not over 25 ft. The tap wires leave the enclosure and will be protected by rigid pipe.
So far I am thinking to apply 240.21(B) (3). “Taps supplying a transformer (Primary Plus secondary not over 25 ft. long.)”
If I apply condition (1): “The conductors supplying the primary of a transformer have an ampacity at least one third the rating of the overcurrent device protecting the feeder conductors.”
Then: 1200amps / 3 = 400amps.
Condition (2) is OK as well as (3) (4) & (5).
My point in question is that in the small transformer the primary current is 83.4 amps protected by a fused disconnect with 110 amps fuses.
1. Do I need to tap with a 400 Amps. wire to the 110 amps disconnect and then continue to the small transformer primary with 3 AWG?
2. Do I need to use 400 amps. disconnect with 110 amps fuses?
There are other taps done in the past in other different stations (by other contractors) and the tapping wires are not that big in similar situations (similar loads). They just used 3 AWG all the way from the tap to the small transformers and by the way with wires over 25 ft. long.
Is this the correct article to apply or is there other that fits more properly?
I need to do a tap from the secondary bars to feed a 100 KVA transformer 600V to 480V.
The distance to the small transformer is going to be longer than 10 ft. but not over 25 ft. The tap wires leave the enclosure and will be protected by rigid pipe.
So far I am thinking to apply 240.21(B) (3). “Taps supplying a transformer (Primary Plus secondary not over 25 ft. long.)”
If I apply condition (1): “The conductors supplying the primary of a transformer have an ampacity at least one third the rating of the overcurrent device protecting the feeder conductors.”
Then: 1200amps / 3 = 400amps.
Condition (2) is OK as well as (3) (4) & (5).
My point in question is that in the small transformer the primary current is 83.4 amps protected by a fused disconnect with 110 amps fuses.
1. Do I need to tap with a 400 Amps. wire to the 110 amps disconnect and then continue to the small transformer primary with 3 AWG?
2. Do I need to use 400 amps. disconnect with 110 amps fuses?
There are other taps done in the past in other different stations (by other contractors) and the tapping wires are not that big in similar situations (similar loads). They just used 3 AWG all the way from the tap to the small transformers and by the way with wires over 25 ft. long.
Is this the correct article to apply or is there other that fits more properly?
