How to size the feeder

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dahualin

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I have a project with a resistance welder that is 208v, 3 phase, 50A primary current. The circuit is fed by a local panel say PB1. The circuit supply conductor is sized not less that 70% of the rated primary current. 70%x50=35A, #8 wires. The branch circuit breaker is sized not more than 300% of primary current. 300%x50=150A circuit breaker.

The total connected load of panel PB1 is 80A. The main circuit breaker of the panel is 125%x80=100A. The feeder to the panel is #3 wires. Since the main circuit breaker is smaller than welder branch circuit breaker. In order to have good coordination, we have to increase the main circuit breaker and up stream circuit breaker to 150A. It results in that the feeder is not protected by any circuit breakers.

I am confused by the scenario. Could anybody explain it to me? Thanks.
 
Pierre C Belarge said:
Based on your explanation, I do not believe the panel you are trying to supply the welder from is adequate in size.
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Do you mean the 150A panel is not big enough to supply the welder? I am not sure if I read your sentence right.
 
I believe that you are reading this part of the code incorrectly:

dahualin said:
The branch circuit breaker is sized not more than 300% of primary current. 300%x50=150A circuit breaker.
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The branch circuit breaker is not _required_ to be 300% of the welder primary current rating. Rather it may be _up to_ 300% of the welder primary current rating.

Depending upon the characteristics of the welder (inrush current, etc.) the branch circuit breaker might be anything from 70A on up to 150A. You might need to spend some time figuring out the inrush current characteristics of the welder and examining trip curves before you get to a breaker that works with the welder and properly coordinates with the upstream devices.

I don't think that the panel is undersized for the load, but a larger panel might be the simplest (or cheapest) in terms of providing proper protection of all conductors, avoiding nuisance tripping, and coordinating the breakers.

-Jon
 
Winnie,

I understand the branch circuit breaker can be smaller than 150A. But 150A circuit breaker still meet the code requirement. I assume it is the worse scenario. What are you going to do with the feeder to the panel in this situation? Thanks.
 
Could you clarify what is confusing you? The 150A breaker on the branch circuit would meet the article 630 requirements for the circuit feeding the welder. But the article 630 permissions don't extend 'down the line' to a feeder supplying the panelboard. So if you want to increase the size of your upstream devices to properly coordinate with the welder's breaker, then you would need to increase the ratings of the feeder and panel to match the upstream OCPD.

-Jon
 
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