Disconnect switch sizing

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HI,
I have a single phase 480-120/240 transformer (10KVA). I have fused disconnect on the primary side of the transformer. I can size the fuse but cannot find information on sizing the disconnect switch.

So i guess my general question is: how do you size disconnect switches?

Help would be appreciated!

Thanks
 

LEO2854

Esteemed Member
Location
Ma
HI,
I have a single phase 480-120/240 transformer (10KVA). I have fused disconnect on the primary side of the transformer. I can size the fuse but cannot find information on sizing the disconnect switch.

So i guess my general question is: how do you size disconnect switches?

Help would be appreciated!

Thanks

That is about 50 amps, so you would need a 60 amp fused disconnect, The next size up is 100.
 
I actually got:

10000/(480*1.73)=12.04 --> Which I multiplied by 1.25 (since the amps are more than 9 amp), to get 15.05, so I used the next size up which is 20 amp fuse.

But I am confused about the disconnect sizing. I know you size it using Horsepower, but in this case all my loads are not motors but things like PLC and lights.

Sorry, for not clearly mentioning but its 3 phase in -> 2 legs of single phase out.
 

Smart $

Esteemed Member
Location
Ohio
You are required to protect the transformer secondary conductors per 240.4(F) and 240.21(C)... so you have the option to use either a primary only or a primary and secondary transformer protection scheme per 450.3(B). It amounts to OCP required on the secondary either way, but it affects how you can size your transformer primary OCP.
 

Smart $

Esteemed Member
Location
Ohio
PS: If you do not use the high leg of the secondary, it actually increase the transformer rated current. At full-loading of the A to C winding, the A to B and B to C windings will be handling half the current through the A to C winding. That is: A to C 2/3 the current, A to B to C 1/3 the current. Rated kVA is reduced by 1/3, effecting 6.7kVA to be the transformer's rating in use... but at the 1? output.

6.7kVA ? 240V = 27.9A

10kVA ? 240V ? sqrt(3) = 24.1A

The primary's rated current is also affected. Technically, the primary leg corresponding to the secondary high leg (B) is not required to be run (it will handle no current).

Ultimately, the safe approach is to provide OCP as if was wired for 3? in and out and run all three legs to the primary... just in case someone decides to use all three legs of the secondary in the future. Just providing this info if you have to s-t-r-e-t-c-h the output current for the 1? scenario.
 
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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I actually got:

10000/(480*1.73)=12.04 --> Which I multiplied by 1.25 (since the amps are more than 9 amp), to get 15.05, so I used the next size up which is 20 amp fuse.

But I am confused about the disconnect sizing. I know you size it using Horsepower, but in this case all my loads are not motors but things like PLC and lights.

Sorry, for not clearly mentioning but its 3 phase in -> 2 legs of single phase out.
Once you have picked the fuse size, you just use the disconnect that the fuse will fit in. If you are using a 20 amp fuse, you would use a 30 amp disconnect. The common disconnect sizes are 30, 60, 100, 200, & 400.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The NEC just says the disconnect has to be minimum of 115% of the load, but when it is a fused disconnect and you size the fuses for 125%, the minimum size of the disconnect is met by default. So that rule basically only counts for non fused disconnects or mains where there are branch circuit protection devices below it.

The issue about it being HP rated is for when it is for ONLY a motor branch circuit.
 
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