Transformer primary protection and wire

Status
Not open for further replies.

rey-man

Senior Member
Location
New york
there's a 10kVA XFMR 480-240/120V. The engineer provided a 40A 3P CB and 3#8 wire +G.

Is the XFMR protected in this scenario? just wondering because the 125% of the FLA will be at 15 Amps, which both the 40A and the #8 wire can still carry. Do in need to install a disconnecting means local to the XFMR. The upstream breaker is not readily accesible.

Please shed some light brothers!

TNX
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
No, the transformer is not properly protected. Per T450.3(B), the max primary OCPD would be not greater than 250% of the primary rated current. This would be a 30A c/b (assuming a properly sized secondary OCPD.)

There is no requirement for a disconnecting means at the transformer.
 
No, the transformer is not properly protected. Per T450.3(B), the max primary OCPD would be not greater than 250% of the primary rated current. This would be a 30A c/b (assuming a properly sized secondary OCPD.)

There is no requirement for a disconnecting means at the transformer.


David
Would you mind showing your calculations for the above?
I agree with your answer, both of them.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Thank you, I appreciate the quick reply.

No problem...but, I just gave myself a DUH!

I read in the OP that:

the 125% of the FLA will be at 15 Amps

and immediately translated that to a 3 phase transformer. But the first sentence in the OP says a 10kVA, 480-240/120V xfmr.

So, at 10kVA/480 = 20.833 primary amps, and 20.833 x 250% = 52.083A, a 40A primary c/b is acceptable per T450.3(B) (again assuming a properly sized secondary OCPD.)
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
David,
I agree with your post #6, assuming single phase, which I would imagine is the case. I have not seen a 10 kva "standard" as a three phase
As you noted, the 40 amp would only be acceptable if the secondary is protected at 125%.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
No problem...but, I just gave myself a DUH!

translated that to a 3 phase transformer. But the first sentence in the OP says a 10kVA, 480-240/120V xfmr.

But the OP also said the CB is:
a 40A 3P CB and 3#8 wire +G.

But:
10kva is not a normal 3ph size, but is a normal single phase size, and

The OP said also said, '480-240/120V", which is the usual nomenclature for a 240D, center-tapped, wild-leg secondary.

I would not be certain which it was.

ice

edit to add: Still a slow poster. Adding to Auggie's answer ---

second edit to add: And the OP said the feed is 3 conductor also makes it sound 3ph.
 
Last edited:

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
iceworm, thanks for pointing that out. As I age, I read more poorly and depend on experience too much. Experience tells me 10kva is single phase, but everything posted does indicate 3 phase

help us, rey-man
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
. . . it is a single phase transformer. does everything change?
Your original post said that the primary was a three phase breaker, and that the primary feeder had three ungrounded conductors. Is that what the engineer said, or was that a typo?

 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
For transformer protection your guide is 450.3
A 10 kva single phase would have a primary current of 20.8 amps and would normally have OCP not greater than 30 amps unless your secondary protection is limited to 50 amps, then you primary OCP could be as large as
50 amps.
The conductors would need to be protected at their ampacity.

Three phase.obviously the numbers change
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I think a 480 /120-240 Xfmr would be single phase.
10,000/480=20.83 amps
20.83 x 250% = 52.08

A 480-240/120V transformer could be EITHER single phase or three phase.

The calculation I give in post #4 is for a three phase transformer.

The calculation I give in post #6 is for a single phase transformer.
 
Status
Not open for further replies.
Top