45kVA transformer protection

[anbm]

Saw a print has a 45kVA xfmr (480V Pri-208/120V Sec) with 70A primary CB in upstream 480V panel that powers the xfmr,
xfmr feeds 208Y/120V panel with 175A main CB. I think the xfmr is protected by its primary 70A CB, but is it something wrong
with 175A main CB in downstream panel? I typically saw 150A main CB panel fed out of a 45kVA xfmr.

80% x 175A = 140A which exceeds xfmr FLA at 45kVA @208/3 (i.e. 125A)

 

[xformer]

Saw a print has a 45kVA xfmr (480V Pri-208/120V Sec) with 70A primary CB in upstream 480V panel that powers the xfmr,
xfmr feeds 208Y/120V panel with 175A main CB. I think the xfmr is protected by its primary 70A CB, but is it something wrong
with 175A main CB in downstream panel? I typically saw 150A main CB panel fed out of a 45kVA xfmr.

80% x 175A = 140A which exceeds xfmr FLA at 45kVA @208/3 (i.e. 125A)

IMHO.... down streams are sized to the load. I would say that in this case, the secondary OCPD is too large.

 

[augie47]

The primary protection meets the 125% rule so the transformer is protected.
From that standpoint the secondary protection is irrelevant as long as the conductors are sized to the secondary OCP

 

[electrofelon]

The primary protection meets the 125% rule so the transformer is protected.
From that standpoint the secondary protection is irrelevant as long as the conductors are sized to the secondary OCP

Just a reminder to the OP, the secondary device may be doing three things: 1. Transformer secondary protection (450.3), 2. Transformer secondary CONDUCTOR protection (240.21(C)), 3. Panelboard protection (408.36). As Augie said, due to the primary protection, you do not need transformer secondary protection, but you can evaluate what that secondary device is doing for #'s two and three.

 

[anbm]

Just a reminder to the OP, the secondary device may be doing three things: 1. Transformer secondary protection (450.3), 2. Transformer secondary CONDUCTOR protection (240.21(C)), 3. Panelboard protection (408.36). As Augie said, due to the primary protection, you do not need transformer secondary protection, but you can evaluate what that secondary device is doing for #'s two and three.

Not issue with #2, for #3, brekaker is kind of large vs. transformer FLA, my two cents.

 

[electrofelon]

Not issue with #2, for #3, brekaker is kind of large vs. transformer FLA, my two cents.

Personally, I don't care about the secondary breaker being "too large". I'll use whatever is convenient. I would probably actually use a 200 cuz readily available in super common, where 175 is an oddball.

 

[augie47]

Your two-cents is on target but it is allowed by NEC. IF the primary breaker was any larger then it would be a violation.
Unsure why they would use a 175 unless it was "whats available"... it's too large for the 45 kva and too small for the next standard size (75)

 

[jap]

Looks like they're using 125% on both the primary and secondary.

Pretty common.

54 x1.25= 67.50 next size up 70 amps
124.65 x 1.25=155.81 nex size up 175 amps

JAP>