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Joined: Jun 2007
Posts: 20
A
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I've Installed a transformer that on the secondary has nearly 36' of wire parallel feeding a 120v/208v panel. The engineer over seeing the job has quoted the reference in the code on a particular 10' rule. The problem I have with this rule is there are additional rules dictating that 25' is acceptable ,how does one go about determining whether the 10' or 25' apply. Either way I believe I'm going to be required to put a disconnect at the transformer.

2017 / 2014 NEC & Related Books and Study Guides
Joined: Jan 2005
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Cat Servant
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Ever wonder why the tap rules are all referred to according to the length of the tap? Did you also notice that the distances, and other conditions, are more limited the longer the tap?

If you can't reach it with a ten foot tap, you can't use the ten foot rule.

Joined: Dec 2000
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The tap rules are in Article 240.21, and include the 25' tap rule, among others.


Joined: Jul 2001
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J
JBD Offline
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[quote=Angel...how does one go about determining whether the 10' or 25' apply.[/quote]

The simple differences are:
10' tap rule conductors can be any size.
25' tap rule requires the conductors to be a minimum of 1/3 the transformer capacity

Joined: Jan 2004
Posts: 1,507
G
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Is this an inside or outdoor tap? Looks like we are limited to a max of 25' for all secondary conductors unless it is an outdoor installation.


240.21(C)(1) through (C)(6)


George Little
Joined: Jan 2004
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G
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More on Transformer Secondary taps:

Just read the original post and the issue is what is the length of the tap.

The tap measurement is to my understanding the length of wire, not the raceway.

For a 10' tap to a 100a. "device" requires 100a conductors, even though the load may only be 75a and it is fused at 80a. If it's a CB than the tap can sized at 80a.

240.21(C)(2)(1)b.
See definition of Device in Article 100


George Little
Joined: Jun 2007
Posts: 20
A
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The problem originated when we had to place a transformer into the nearest gap nearly 28' from where it was shown on the print . The engineering shown placed the transformer suspended into the air ,supported from the building steal. When we went to place this 112kva trans into the air the GC. proclaimed that the structural engineer will not allow us to suspend the trans from the steal because its to heavy. Now the electrical engineer has said we don't comply with the 10' tap rule. I feel
that even though the length of the secondary taps are 26' that we may comply with the 25' tap rule. Basically I have to size the wires according to the percentage ampacity given for the conductors for the 25' rule verses the 10' rule.
Next realization being is that we sized the wires according to the line diagram the electrical engineer specified, and he is probably aware that the wires may only comply with the ampacity of the 10' rule. Solutions ? Move transformer back to where it is shown on print? Build a shelf? Comments appreciated

Joined: Dec 2000
Posts: 4,287
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Originally Posted by Angel_Electric
I feel that even though the length of the secondary taps are 26' that we may comply with the 25' tap rule. Basically I have to size the wires according to the percentage ampacity given for the conductors for the 25' rule verses the 10' rule.


Sure....26' is not 25' or less.

Solution = Let the GC worry about it, and between the EE and the Structural Engineer not doing their jobs properly, let them come up with a fix.

Don't try to "save the day" yourself, you'll be rewarded with only headaches.

Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
This situation is what change orders were created for!

The solution is simple: You need to add over-current protection at some point that will let you meet the 25 ft rule. Or, even, the 10 ft rule. A disconnect with fuses would accomplish this.

Joined: Mar 2005
Posts: 1,213
S
Member
Or just larger cabling- if the cable meets the full ampacity of the main OCP, it's not a bus tap, it's merely an extension of the bus.

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