About transformer

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Toros

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Tujunga, CA
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 said:
What's your primary voltage?
Click to expand...
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.
 
JoeStillman said:
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.
Click to expand...


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)
 
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augie47 said:
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)}
Click to expand...

Short answer no. 500 kcmil copper or 750 kcmil Al are too small for 400 amp OCPD.
 
infinity said:
Short answer no. 500 kcmil copper or 750 kcmil Al are too small for 400 amp OCPD.
Click to expand...

correct & opps.. My post should have stated "not".. typo or old age
(I changed it but noted it was a change)
 
Toros said:
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
Click to expand...
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.
 
kwired said:
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.
Click to expand...

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.
 
JoeStillman said:
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.
Click to expand...
OK, nothing in NEC prohibits using 100 amp OCPD, good design practice very well does.:p
 
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