# Thread: 208 Step up 400 fusing/cable question

1. Junior Member
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## 208 Step up 400 fusing/cable question

Hello, I am hoping to get some engineering calculations/general input on a step up X-former installation, my background is by and large with electronics and control systems but occasionally my job requires installation support for new machines. My Q: I need to specify the correct fuse's for 25' run (max) from the 250 amp service buss/100 amp Buss plug and cable feed down to a 208 3 phase 15KW delta transformer primary. On the secondary/load side the machine spec calls for 63 amp fuses and 16mm sq cable with the total power spec. at 23KW. An apparently identical machine model is fused at 100 amps on the buss plug to an identical 15KW transformer primary with 4/4 AWG SOOW and then looks like 6 or 8 AWG SOOW on the secondary feed to that existing older model machine. My calculations for fuse and cable size are somewhat different based on my math, I hope someone here can render the correct answers based on the info. provided. Thanks for your help.

2. I assme you are required to the use the NEC? The metric sizes made me doubt.
15 kw @ 208 volts = 42 amps x1.25 = 52 amps 15 kw @ 480 volts = 11 amps

If you size the OC device at 250% then the 100 amp is correct. If you do that, then the secondary
OC device can not exceed 125% of the secondary FLA. That would be 1.25 x 11 A = 14 A .

My info says that #4 SOOW is good for only 60 amps. Can not use 100 amp OC device. #6 SOOW rated at 45 amps. Can not use 63 amp OC device.

We will get some additional inputs.
Last edited by bob; 09-21-12 at 04:12 PM.

3. Bob, I may be mistaken, but I show the secondary at 22 amps or max OC)P at 22 X 1.25 = 27.5 amps

As far as the primary, I agree. If the buss plug is fused at 100 amps (legal at 250%) then a #4 SOOW with 60 amps max ampacity would not be protected.

4. Originally Posted by bob
I assme you are required to the use the NEC? The metric sizes made me doubt.
15 kw @ 208 volts = 42 amps x1.25 = 52 amps 15 kw @ 480 volts = 11 amps should be 18 amps

If you size the OC device at 250% then the 100 amp is correct. If you do that, then the secondary
OC device can not exceed 125% of the secondary FLA. That would be 1.25 x 11 A = 14 A . should be 18 x 1.25 = 23 amps

My info says that #4 SOOW is good for only 60 amps. Can not use 100 amp OC device. #6 SOOW rated at 45 amps. Can not use 63 amp OC device.

We will get some additional inputs.
Augie I did it 10 times a got 15 kw/.48x1.73 = 18 amps

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## New input

Thanks alot for looking into this Bob and Augie, I went ahead and brought in an EC, he is apparently OK with 100 amp Buss plug fused at 100amp with 2 AWG SOOW for the primary feed and doesn't think according to NEC that we need to provide any fuse or circuit breaker on the secondary side because there is an interlock on the machine. He is also OK with 6 AWG for the secondary machine feed except that it is wye w/neutral so we are now using a 6 AWG 5 conductor with a ground (I would have made sure the machine was grounded in any case) Also this european machine is 380-420 so the transformer secondary is 400Y/231 VAC not 480. This is great experience and working knowledge for me, I look forward to your insights in light of this new information.

Machine spec.
23KW 16MM sq. cable fused at 63 amp 380-420 VAC at 50 Hz.

Transformer: Pri: 3P 208 Delta
Sec: 3P 400/231 Wye w/neutral

100 amp I-line Buss plug fused at 100 amps.
2 AWG 4 conductor SOOW @ less than 25' total to pri.
6 AWG 5 condcutor SOOW @ less than 8' total to load (no CB/disconnect)

We should definitely meet NEC requirements for this install.

6. Originally Posted by bob
Augie I did it 10 times a got 15 kw/.48x1.73 = 18 amps
Actually I did it 10 times wrong. 15kw/.400x1.73 = 22 amps x 1.25 = 27.5