Transformer Double Tap At Secondary

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vg-kmb

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New Jersey
Installed a fused 300A, 480/277 volt, 3 phase feeder from a 400 amp switch. The feeder is 350s run in 3" conduit approx. 250'. The feeder terminates in a 400A, 600V, non-fused disconnect, the directly to a 225 kVa, 480/120/208 volt step down transformer. From the transformer, a 4" conduit with 500s and a 3" conduit with 3/0s are run into a 12"x12"x6' trough. Tapped from the trough are (2) 200A, MCB panels and (2) 100A, MCB panels. The inspector failed this installation stating it is a double tap at the transformer secondary and to either install a 600A, 120/208 3 phase disconnect after the transformer (no room) or feed all panels with 500s. Any input on this inspectors interpretation would be appreciated.
 
dkidd

dkidd

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PE
How long are the 3/0's in question? I take it they are in parallel. Need a sketch.
 
Smart $

Smart $

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Welcome to the forum. :thumbsup:

You cannot tap transformer secondary conductors as they have to terminate in a single overcurrent device (a single set of fuses count as a single OCPD). See the rules in 240,21(C).

You're looking at pulling two sets of 250's in the 4" conduit for the 200A panels and two sets of #1's in the 3" for the 100A panels. And this assumes you meet the requirements of 240.21(C).
 
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GoldDigger

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I would say that you are SOL on this one.
Unless you install secondary winding OCPD at the transformer, you cannot make any connections, even to individual downstream OCPDS, that use a wire size smaller than that protected by the transformer primary-side OCPD. That is, no tap conductors can originate at the transformer secondary output.

JMO
 
Smart $

Smart $

Esteemed Member
Location
Ohio
GoldDigger said:
I would say that you are SOL on this one.
Unless you install secondary winding OCPD at the transformer, you cannot make any connections, even to individual downstream OCPDS, that use a wire size smaller than that protected by the transformer primary-side OCPD. That is, no tap conductors can originate at the transformer secondary output.

JMO
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That is not correct (as I understand it ;)).

He can run one set of transformer secondary conductors to each panelboard direct. Code permits up to six. See Note 2 to Table 450.3(B)... though I did not check the OCPD sum is not more than the rated output current times the 125% max.

He'd have to derate the conductors to 80% because he'll have to run two sets (6 CCC's) in each conduit.
 
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vg-kmb

Member
Location
New Jersey
Smart $ said:
That is not correct (as I understand it ;)).

He can run one set of transformer secondary conductors to each panelboard direct. Code permits up to six. See Note 2 to Table 450.3(B)... though I did not check the OCPD sum is not more than the rated output current times the 125% max.

He'd have to derate the conductors to 80% because he'll have to run two sets (6 CCC's) in each conduit.
Click to expand...
So, I could install quad lugs on the secondary side of the transformer and feed each panel individually?
 
don_resqcapt19

don_resqcapt19

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ronmath said:
Why not? This does not seem to meet the requirements of primary only protection in 240.21(C)(1)?
Click to expand...
There are two totally different sets of rules for transformer application. The overcurrent protective device rules in 450.3 apply only to the protection of the transformer primary and secondary windings. The rules in 240.21(C) apply only to the protection of the conductors that are connected to the transformer secondary terminals.
 
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