Secondary conductor rating for primary only fused transformer

bzzt

Member
Location
mn
Occupation
wirer
Hello all-
Have a 30kva 480-208/120 transformer and hoping to get 100a secondary out of it. 450.3 states fusing w/o secondary OCPD shall be 125% of primary. If the secondaries feed an MLO panel what could the panel ampacity be loaded to? transformer nameplate states primary @ 36.08a and secondary @83.27a

what is the advantage/disadvantage of having both protected?
 

bzzt

Member
Location
mn
Occupation
wirer
what can i size the panel load to? 125% of 83.27a? I'm looking at 450.3(B)

Also, i dont see where secondary protection is required. Article?
 
Last edited:

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
what can i size the panel load to? 125% of 83.27a?
With your choice of primary protection not greater than 125%, the secondary panel can be sized to any rating you want, even 1200A or more.

The secondary panel must be provided with a suitably sized overcurrent protection device, which can be remote from the panel.
The conductors feeding the panel must be sized to be protected by the protective device.

However, if you increase the primary side protection, upto the maximum 250%, then secondary protection not be larger than 125% must be provided.
The panel protection can be the same device as the transformer secondary protection.
 

bzzt

Member
Location
mn
Occupation
wirer
The secondary panel must be provided with a suitably sized overcurrent protection device, which can be remote from the panel.
The conductors feeding the panel must be sized to be protected by the protective device.

Is this b/c its a delta to wye setup?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
The panel requires some type of 'main' per 408.36, regardless of the transformer winding configuration.
The wye secondary conductors require protection according to 240.4(F), which is sized per 240.21(C).
 

david luchini

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Staff member
Location
Connecticut
Occupation
Engineer
Is this b/c its a delta to wye setup?
On a single phase, 2 wire secondary and a three phase, 3 wire secondary the panelboard protection and secondary conductor protection can be on the transformer primary.

For your wye secondary (or 4 wire delta secondary, or single phase, 3 wire secondary) the panelboard and secondary conductor protection must be on the secondary side.
 
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