Sizing parallel conductors

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kdutter40

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I'm an electrical apprentice and have been working on a job the past few weeks and was given the duty of trying to size the conductors for the secondary side of our 112.5Kva transformer. We would like to do it in parallel but its my first time sizing these conductors in any given job. I was wondering if there is a calculation or any which way to size them. Any help would be appreciated



Kevin
 
don_resqcapt19

don_resqcapt19

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retired electrician
Of if you don't know the load you can size the conductors to match the secondary current of the transformer.
 
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dana1028

Senior Member
Location
San Francisco Bay area
kdutter40 said:
I'm an electrical apprentice and have been working on a job the past few weeks and was given the duty of trying to size the conductors for the secondary side of our 112.5Kva transformer. We would like to do it in parallel but its my first time sizing these conductors in any given job. I was wondering if there is a calculation or any which way to size them. Any help would be appreciated
Kevin
Click to expand...

As others have mentioned, before you can size your secondary conductors you must know what size panelboard you will be installing.

The size of the panelboard is determined by one or more of several factors:

1. What is my need? [i.e. what is the calculated load going to be?]

2. What is my 'want' factor? [i.e. you may have a calculated load of 200A, but 'want' to have an extra 200A available for future use.]

3. What can my transformer deliver? [Obviously, if it is not big enough for 1 & 2 above you've got a problem].

Given a 112.5 kVa transformer, you must calculate how much this will provide. If I use a secondary voltage of 208 -

112,500 VA/ (208V x 1.73) = 312.6A

Table 450.3(B) indicates 2 things: 1) you can size your secondary OCP at 125%, and 2) if this ampacity does not correspond to a standard rating of an OCPD, you can go to the 'next higher' rated OCPD.

312.6A x 125% = 390.75A....next higher breaker = 400A

I calculate you can have a panelboard on the secondary rated as high as 400A

Forgive me if I made a mistake in information or calculations....but I feel confident fellow members will quickly chastise me for any errors I have commited.
 
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Leestarkweather

Member
Location
Los Angeles CA
The calculation shown is correct, and so it continues on that (2) 3/0 75 degree C conductors per phase would be fine with a 3P 400A OCPD within 25 feet of the connection to the secondary lugs. Of course, if you put all 8 phase and neutral wires in 1 conduit, you'd have to derate to 80% (or 70% if there are harmonics or other current causing factors on the load side). If you use a wireway from transformer to 1st OCPD you don't have any deration problems, or a wireway and less than 24" of flex conduit if sound transmission is a necessary consideration. Every single job site and configuration has a different net outcome, based on many factors. Good luck, and I hope you enjoy being in the electrical field. We need new, inquisitive incoming people in this field.
 
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gmtt

Member
Table 450.3(B) indicates 2 things: 1) you can size your secondary OCP at 125%, and 2) if this ampacity does not correspond to a standard rating of an OCPD, you can go to the 'next higher' rated OCPD.

Table 450.3(B) is for "Maximum" rating, not minimum. In that context, shouldn't the OCP be at "Next Lower", rather than "Next Higher" rated OCPD,if need be? Could someone please comment on this?
 
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dana1028

Senior Member
Location
San Francisco Bay area
gmtt said:
Table 450.3(B) is for "Maximum" rating, not minimum. In that context, shouldn't the OCP be at "Next Lower", rather than "Next Higher" rated OCPD,if need be? Could someone please comment on this?
Click to expand...

I'll answer 1/2 of it - yes, T440.3(B) is for the 'maximum', and that maximum permits the next higher breaker.

In my example, the permitted next higher breaker was 400A [at 100% we were at 319A]. If you remember the panelboard can only be loaded to 80% continuously [that's 320]...which is the rating of the XO. XOs can run at 100%.

I'll leave the inner workings of the XO to others to better explain the dynamics of a working XO.
 
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kdutter40

Member
we are planning on a 400a secondary at 120-208. basically what im looking for is how exactly to size the conductors. Were coming out of the transformer and into a 400a fuseable disconnect then continuing to a 400a panel 100 feet away.
 
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