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#218765 09/23/17 06:41 PM
Joined: Oct 2002
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Been a while since I've had to figure a 3 phase load. I've got a customer that already has a 175 amp 3 phase panel installed. They're dividing their building up to put a tanning bed business in. The lights, etc. for the tanning bed side are already coming out of the existing panel. They are planning on putting (5) 3 phase tanning beds in and each one has an actual amperage of 37 amps. I'm not sure, but I think I would have to figure each one at 100% or more (continuous duty, 3 hrs or more). If I just figure 100%, that's 185 amps. They have a 75 kva transformer feeding the building. On 3 phase, how do I figure the load it can handle? Do I just divide 240 into 75,000 ? I'm thinking there is something else I'm missing.
Thanks for the help.

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Joined: Mar 2003
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208 x 1.73 = 359.84 round up use 360

75000 divided by 360 = 208.33 amps

yoopersup

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Had someone else figure it: 75000 / (208 x 1.73)= 208.3333 × 1.25 = 260.416 amps

Which way is right? And wonder where the 1.25 came from?
Thanks

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You can fuse the secondary 125% . transformers still only good for 208 amps.

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Originally Posted by sparkync
And wonder where the 1.25 came from?


You said that it was a continuous load and continuous loads are figured at 125%

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If the load is continuous you multiply the LOAD by 1.25...not the source.
So if your 37 Amp tanning beds are continuous you need to treat them as (37x1.25)=46.25 Amp loads.

If you'd rather move the calculation to the transformer you would multiply the transformer rating by the inverse of 1.25 (which is 0.8).
That means that you would treat your 208Amp transformer output as a source that is capable of supplying (208x0.8)=166.4 Amps.
Since it is rare that a transformer feeds ONLY continuous loads pretty much nobody uses the 0.8 method...it's just a thumbnail shortcut.

Last edited by ghost307; 09/29/17 09:43 AM. Reason: typo

Ghost307
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450.3(B) ((Primary & secondary Protection.)) Secondary 125% see note 1

Three Phase Transformer
KVA 208V 240V
45 125 108
50 139 120
60 167 145
75 208 181



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sparkync:

First thing before you jump into this....

Are the tanning beds 208 volt or 240 volt?? Careful!

Check the nameplates an each unit. If the existing transformer is 120/208 and you need 240 you will need buck/boost transformer(s).

BTW, common practice for 75 KVA 480-120/208 is 200 amp MOCP, or some engineers push to 225 amp MOCP.




John
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Unless the transformer is only feeding a single load I'd use a 225A main breaker to feed a panel.
Most of the panelboard manufacturers have standardized their bussing as 125A or 225A so there shouldn't be a cost impact.
The only time that I would choke a 75kva transformer to 96% of its rating is if I'm using fuses and there's not enough room to mount a 400A switch fused at 225A.


Ghost307

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