ten foot tap rule

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billymoore

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i had an issue at work the other day, we installed a 75kava transformer with no secondary over current protection. I informed my boss that I believed that we needed some kind of protection on the secondary and we had a little debate. my understanding is that within ten feet of the secondary side you need an over current protective device. is this the ten foot tap rule ? thanks
 
You may certainly be right, take a look at 240.21(C)(1) through (6) and see if the installation meets one of those sections.
 
I usually use 240(B)(3) for transformer secondary conductors. That lets you run 25' before hitting overcurrent protection for the secondary (provided the primary wire is protected at its ampacity.)

One caution, you can't "upsize" the overcurrent protection. If you use a 200A breaker, the secondary wire must be rated for the entire 200 amps.

Steve
 
Steve,
If the primary conductors are protected at their ampacity, the there is no real difference between (B)(3) and (C)(6).
One caution, you can't "upsize" the overcurrent protection. If you use a 200A breaker, the secondary wire must be rated for the entire 200 amps.
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That applies to all tap conductors. You are not permitted to use the provisions of 240.4(B) for tap conductors.
Don
 
If the primary conductors are protected at their ampacity, the there is no real difference between (B)(3) and (C)(6).
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Don:

I do realize that. But the point I wanted to make is that it is probably a 25' tap rule, not the 10' rule. I don't think I have ever used 240.21(c)(3). I have never had occasion to have the secondary conductors at less than 1/3 the ampacity of the primary OCP when multiplied by the voltage ratio.


That applies to all tap conductors. You are not permitted to use the provisions of 240.4(B) for tap conductors.
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Yes, I meant that to apply to whatever tap rule to poster winds up using. I think "Can't use 240.4(B)" should be in the reply to anyone who asks about the tap rules. If they aren't familiar with the tap rules, they won't realize you can't upsize the OCP.

Steve
 
thank you for your replies. I want to know is it mandatory by the code to have secondary overcurrent protection at all? Or are you only required to have primary?
 
billymoore said:
thank you for your replies. I want to know is it mandatory by the code to have secondary overcurrent protection at all? Or are you only required to have primary?
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Secondary protection is required unless it is a delta-delta configuration or it is a single phase 2 wire secondary.
 
can someone tell me the article that requires secondary protection on transformers. my boss found some that say it is not required. i want to make sure we don't do any more illegel installations. thank you for all of your help..
 
Billy,
can someone tell me the article that requires secondary protection on transformers. my boss found some that say it is not required.
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Your boss is correct, in most cases the transformer does not require secondary overcurrent protection. See 450.3. However, also in most cases the transformer secondary conductors do require overcurrent protection. See 240.21(C). Many do as your boss does, just look at 450.3 and forget about 240.21. You have to apply the rules in both sections.
Don
 
240.21 (C) Transformer Secondary Conductors. Conductors
shall be permitted to be connected to a transformer secondary,
without overcurrent protection at the secondary, as
specified in 240.21(C)(1) through (6).
FPN: For overcurrent protection requirements for transformers,
see 450.3.
(1) Protection by Primary Overcurrent Device.
Conductors supplied by the secondary side of a singlephase
transformer having a 2-wire (single-voltage) secondary,
or a three-phase, delta-delta connected transformer
having a 3-wire (single-voltage) secondary, shall be permitted
to be protected by overcurrent protection provided on
the primary (supply) side of the transformer, provided this
protection is in accordance with 450.3 and does not exceed
the value determined by multiplying the secondary conductor
ampacity by the secondary to primary transformer voltage
ratio.
Single-phase (other than 2-wire) and multiphase (other
than delta-delta, 3-wire) transformer secondary conductors
are not considered to be protected by the primary overcurrent
protective device.

As Don said you may not have to protect the transformer secondary, but the secondary conductors often do require protection. It is perhaps a subtle difference.
 
i am confused. i assumed the transformer secondary was refering to the conductors as well as the transformer. what i should have asked is does the code require either a fused disconect or a main breaker after the secondary? is it legal to come from the secondary and land on a main lug fed panel/load center?
 
billymoore said:
i am confused. i assumed the transformer secondary was refering to the conductors as well as the transformer. what i should have asked is does the code require either a fused disconect or a main breaker after the secondary? is it legal to come from the secondary and land on a main lug fed panel/load center?
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There is no general requirement at all for a disconnecting means after the transformer.

A MLO panel is OK if:
it is a delta-delta or 2 wire secondary transformer, and
the MLO panel had fewer than 6 breakers, and
the sum of setting of the breakers is equal to or less than the ampacity of the wire feeding the MLO panel.

(did I miss anything?)

I usually do this with a panel that has a main. simplifies things a lot. I tend to have a lot more breakers in my panels than power because it is a convenient place to disconnect things for servicing. I might have 10 breakers that total maybe 20 amps of actual current.
 
if it is a three phase wye secondary it needs to terminate at a overcurrent device (the feeders). the only systems not required are delta delta and two wire sigle phase. true or false?
 
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