Maximum Length of Xfmr Secondary Conductors

[VTer]

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

 

[7anoter4]

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.

 

[Mbrooke]

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.

 

[dpc]

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

 

[waross]

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

 

[VTer]

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