Panelboard Size

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Alwayslearningelec

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Trying to determine size/calculation of 277/480 v panel board. Panel with have the following loads.

  • (4) 30A disconnect switches for welders. Not sure of breaker size yet.
  • (1) 20A breaker for lighting
  • (1) breaker for 30kva step down xfrmr to feed a 120/208v panel
 
A panelboard can use bolt on breakers, so get a main lug panel, and backfeed a main breaker of what ever size you want.
Don't go with the smallest panel as loads always get added.
 
horsegoer said:
So to figure main breaker size I think I need to calculate total volt amps or watts? How would I do that? TY
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Art 220.

Welders are probably what can make a huge difference in your application. More info is needed on them.

30kVA transformer - can be significant also, depends on load it is feeding. you can factor in it's rating of 30 kVA but maybe you only have a real load of 19 kVA and a 30 is next standard sized unit.
 
Calculate load in KVA. Welders have a duty cycle and that impacts load, IE cheap buzz boxes you can only weld 2 minutes out of 10.
 
kwired said:
Art 220.

Welders are probably what can make a huge difference in your application. More info is needed on them.

30kVA transformer - can be significant also, depends on load it is feeding. you can factor in it's rating of 30 kVA but maybe you only have a real load of 19 kVA and a 30 is next standard sized unit.
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This is the welder cut sheet. They are saying to feed them with 30A disconnect.
 

Attachments

  • 08_Exhibit E - Welding Power Source.pdf
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horsegoer said:
So to figure main breaker size I think I need to calculate total volt amps or watts? How would I do that? TY
Click to expand...
Calculate the VA in lieu of watts. You do that by adding all the VA of all the loads. If they are all 3-phase loads then it's the sum of all the loads. If you have single phase loads then you need to pay attention to how the single phase loads are installed on the phases.
 
infinity said:
According to the cut that sounds about right for 480 volts.
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Agree.
At 280A at 35% duty, the average secondary current is about 166A. Translated to the 480V primary, that would be 10.8A. For welder circuit breaker, use 200% of the current = 10.8 X 2 = 21.6A~ 30A!
 
horsegoer said:
This is the welder cut sheet. They are saying to feed them with 30A disconnect.
Click to expand...
30 amp disconnect is what is needed for individual circuit. You are trying to figure out the feeder capacity. It draws 17.5 amps at max output, but also has a duty cycle of 35% at this level. Highest current at 100% duty cycle is 12.7 amps.

630.11(B) gives us a method to determine minimum ampacity needed for a group of welders.

Exception and informational note that follow still indicate it is not one size fits all method. If you know how those welders will be used you might have some idea on what load levels are realistic for your application.

If a general maintenance/repair setting may be rare for all of them to be going at same time, if a production shop that chance goes up, but even then they may never run at full output, and duty cycle still can vary quite a bit depending on nature of the production.

Method given in 630.11 likely gives you a lot of capacity that in many cases will never be fully used.
 
Personally, I just like to run 4/0 with a 225 amp breaker. Plenty of future power available. I like to have plenty of extra power because some load is going to pop up in the near future and you will be glad you put lots of power out there. But back to reality:

What size breakers do you have available to feed this 480V, 225 amp three phase panel? How long will the feed be?
Your 480V panel can be main lug only, it doesn't have to have a main breaker. Although a main breaker in 480V panel can be convenient.
However, if you shut down the main breaker in panel, keep in mind that the line side of that breaker still has arc flash potential.
 
oldsparky52 said:
or the last line 4.8/7.6 and you get .63 :)

I bet the PoCo hates these.
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Depends on duration of loading. High production environment might even be forced to correct power factor. General repair shop welding usage, normally don't get enough use that it would be a problem. Those may only get a few seconds to a minute or two of loading at at time, and probably not at full rating of welder all that often if ever.

By no means a welding expert, but done a little of it, you find out pretty fast if you select too high of setting for the thickness of what you are welding that it is too easy to just burn holes in it instead of fusing it together. So unless you are welding thick material pretty often you maybe only rarely use more than half to three fourths the capacity of a unit like OP has here.
 
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