×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Maximum Length of Xfmr Secondary Conductors

Maximum Length of Xfmr Secondary Conductors

Maximum Length of Xfmr Secondary Conductors

(OP)
The application is in US and the question is regarding the NEC requirements.

Scenario: 480V Panel with feeder breaker and primary conductors feeding transformer: 45kVA, 480V Delta - 208/120V solidly grounded wye. Transformer then feeds a 208V panel which is 42ckt tub with 150A MCB.

Question: Is there a distance limitation on secondary conductors between the transformer secondary terminals and the 150A MCB terminals in the panel?

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

RE: Maximum Length of Xfmr Secondary Conductors

In my opinion it is not a cable length limiting but a voltage drop limiting.
However it is only an information note: NEC /2014 art. 215.2 Minimum Rating and Size. A(b) Informational Note no.2: maximum voltage drop [recommended] 5%.
Per Tb.310.15.(B)(16) for 1/0 copper cable the ampacity is 150 A.
Following Chapter 9 Tables/Table 9 Alternating-Current Resistance and Reactance for 600-Volt Cables, 3-Phase, 60 Hz, 75°C (167°F) — Three Single Conductors in Conduit.
for 1/0 copper conductor we'll get X=0.144 ohm/km R=0.39 ohm/km and using formula Vd=sqrt(3)*Iamp*(R*cosfi+X*sinfi)*length and if cosfi=0.85 for 0.1 km[100 m] we'll get 5% Vdrop.
If cable reactance does not change much with the conductor cross section [but only with the length] then for 4/0 X=0.135 and R=0.203 we’ll get 5% for 160 m.

RE: Maximum Length of Xfmr Secondary Conductors

FWIW, if the transformer is delta delta or single phase 2 wire, the primary fuse being 125% within the full load primary current rating means the secondary conductors are considered protected and may travel any distance without requiring OCPD at the secondary voltage panel board.

However, I could be wrong, but in the case of a wye seoncdary an OCPD is required at the panel board, however I cant seem to find anything that dictates conductor distance between the transformer and secondary OCPD, say the way tap rules do.

RE: Maximum Length of Xfmr Secondary Conductors

It's considered a tap if the conductors are running indoors. See 240.21 (C).

RE: Maximum Length of Xfmr Secondary Conductors

The Canadian code calls for a maximum voltage drop of 5% to the point of utilization, not the panel or breaker.
"(b) not exceed 5% from the supply side of the consumer’s service (or equivalent) to the point of
utilization; and
(c) not exceed 3% in a feeder or branch circuit."

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Maximum Length of Xfmr Secondary Conductors

(OP)
Thanks for the replies! I looked into 240.21(C)(3) because the application falls under Industrial installations and we are limited to 25ft. Thanks again!

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources