About transformer

[Toros]

Hi
installed a 112.5 kva xfrmer to feed some loads (240v, 3w to 240V,4w conversion)
I have the primary 100A-fused disconn. sw. adjacent to xfrmer, thru (3) #2 thhn,cu ;
the xfrmer is 40 feet away from source (ocpd)

what is minimum and maximum size ocpd at primary/ source??????

thank you

 

[topgone]

What's your primary voltage?

 

[JoeStillman]

What's your primary voltage?

I think he says its 240 3W to 240 4W, presumably 4-Wire Delta. If so, the primary current will be 270A.

Toros, the primary protection depends on what you have on the secondary. The secondary conductors need an OCPD at the point where they receive their supply (Transf Sec). They will not be protected by the primary no matter what size OCPD is on the primary.

If your goal is to re-use existing #2 primary wires, your maximum primary OCPD is 125A (assuming 75°C wire and lugs). You will have trouble with tripping a 125A CB or blowing a 125A fuse because the transformer will have an inrush current of about 10X for 0.1 seconds.

I would use a 400A fuse or breaker on the primary with 500 kCM CU or 750 kCM AL, same size primary and secondary.

Or, if you just want to convert a #2 3W circuit to a #2 4W circuit, use a 30 kVA transformer instead.

 

[augie47]

I think he says its 240 3W to 240 4W, presumably 4-Wire Delta. If so, the primary current will be 270A.

Toros, the primary protection depends on what you have on the secondary. The secondary conductors need an OCPD at the point where they receive their supply (Transf Sec). They will not be protected by the primary no matter what size OCPD is on the primary.

If your goal is to re-use existing #2 primary wires, your maximum primary OCPD is 125A (assuming 75°C wire and lugs). You will have trouble with tripping a 125A CB or blowing a 125A fuse because the transformer will have an inrush current of about 10X for 0.1 seconds.

I would use a 400A fuse or breaker on the primary with 500 kCM CU or 750 kCM AL, same size primary and secondary.

Or, if you just want to convert a #2 3W circuit to a #2 4W circuit, use a 30 kVA transformer instead.

Agree with the info but:::
Two points of concern: 240.6 shows a 350 amp breaker as a standard size and if I did my math correctly to provide 125% protection that would be the correct size. Secondly, with 240.21(C) transformer secondary conductors you can NOT go "next size up" {240./4(B)}


(edited to add "Not"... typo or brain fart thankfully caught by Infinity)

 

[infinity]

Agree with the info but:::
Two points of concern: 240.6 shows a 350 amp breaker as a standard size and if I did my math correctly to provide 125% protection that would be the correct size. Secondly, with 240.21(C) transformer secondary conductors you can go "next size up" {240./4(B)}

Short answer no. 500 kcmil copper or 750 kcmil Al are too small for 400 amp OCPD.

 

[augie47]

Short answer no. 500 kcmil copper or 750 kcmil Al are too small for 400 amp OCPD.

correct & opps.. My post should have stated "not".. typo or old age
(I changed it but noted it was a change)

 

[JoeStillman]

Short answer no. 500 kcmil copper or 750 kcmil Al are too small for 400 amp OCPD.

I forgot to size a secondary breaker. So do we agree my suggestion wouldn't be complete without a 350A fuse or breaker within 25' of the transformer secondary?

 

[kwired]

Hi
installed a 112.5 kva xfrmer to feed some loads (240v, 3w to 240V,4w conversion)
I have the primary 100A-fused disconn. sw. adjacent to xfrmer, thru (3) #2 thhn,cu ;
the xfrmer is 40 feet away from source (ocpd)

what is minimum and maximum size ocpd at primary/ source??????

thank you

112.5 kVA is 270 amps. Nothing prohibits using 100 amp OCPD, but could be subject to tripping on inrush current when first energizing. So there basically is no minimum per NEC.

 

[JoeStillman]

112.5 kVA is 270 amps. Nothing prohibits using 100 amp OCPD, but could be subject to tripping on inrush current when first energizing. So there basically is no minimum per NEC.

It will trip. I plugged it into SKM. The inrush point is way to the right of a fuse and a breaker. Might not trip every time - depends on if you luck out and hit a zero-crossing on magnetizing.

 

[kwired]

It will trip. I plugged it into SKM. The inrush point is way to the right of a fuse and a breaker. Might not trip every time - depends on if you luck out and hit a zero-crossing on magnetizing.

OK, nothing in NEC prohibits using 100 amp OCPD, good design practice very well does.

 

[Sahib]

NEC is not a design manual.