Tap rules confuse me!

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L

lady sparks lover

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Could someone explain to me gist of the tap rules for 10ft, 25ft, and for secondary side of transformers??

I get so confused with this rule, and it results in a change order, which drives me crazy! :)

[ September 30, 2004, 03:37 PM: Message edited by: lady sparks lover ]
 
Re: Tap rules confuse me!

The easiest way for me to understand it, is by breaking down "overcurrent" into two classes of "overload" and "fault" rather than the three that the official definition uses: "overload," "short circuit" and "ground fault."

In reality, "overload" protection of conductors can come anywhere in a circuit between the source and the load. In fact, it may even be the inherent nature of the load itself to not "overload;" e.g., a heater on a dedicated circuit.

"Fault" protection however must come between the source and the protected conductors since a fault can occur anywhere in the circuit.

The 10' rule essentially says overloads are prevented by either the nature of the load, by overcurrent (overload in this case) protection of the load, or a combination of both. "Fault" protection is provided by an upstream device ahead of the transformer.

The 25' rule says essentially the same thing except that the "overload" protection must specifically be grouped "overcurrent" devices.

The "outside secondary rule" extends the concept but overload protection must be supplied by a single overcurrent device.

In each case, mechanical protection of the conductors becomes progressively relaxed.
 
Re: Tap rules confuse me!

You would still have to have a disconect for the second transformer.
110.58 Disconnecting Means.
A switch or circuit breaker that simultaneously opens all ungrounded conductors of the circuit shall be installed within sight of each transformer or motor location for disconnecting the transformer or motor.
And for the life of me I cannot picture why you would have a transformer feeding a transformer without wanting Overcurrent devices between the 2
 
Re: Tap rules confuse me!

Jeff,
Please note that 110.58 is a very limited section. It only applies to systems having a voltage over 600 volts and that are installed in tunnels.
Don
 
Re: Tap rules confuse me!

For instance, what rule applies if you have a transformer feeding a tranformer, and you want to not supply a disconnect at the tap? Do you need a disconnect if it's over 25 ft? Yikes!! ;)

"Disconnect" and "Overcurrent Protection" aren't necessarily the same thing. You may still need to provide a "disconnect" under 490.22.

You will also need to carefully consider whether the secondary conductors of the "downstream" transformer have adequate "fault" protection.

[ September 30, 2004, 04:33 PM: Message edited by: rbalex ]
Click to expand...
 
Re: Tap rules confuse me!

Originally posted by rbalex:

The 10' rule essentially says overloads are prevented by either the nature of the load, by overcurrent (overload in this case) protection of the load, or a combination of both. "Fault" protection is provided by an upstream device ahead of the transformer.

The 25' rule says essentially the same thing except that the "overload" protection must specifically be grouped "overcurrent" devices.
Click to expand...
I hope this doesn't annoy you too much, rbalex, but...here's how I take it.

10' or less, you can run tap conductors without OCP at the tap if the tap conductors are sized to suite the load it's supplying.

25' or less, you can run tape conductors without OCP at the tap if the the conductor terminates into a OCP device and the tap conductors are rated higher that 1/3 the ampacity of the OCP which supplies the tap.

over 25', supply OCP at the tap.

For a transformer the same basically applies except the ration of .433 time the tap conductors on the secondary must be equal to or more than the ampacity of the OCP on the primary side...

Is this correct?? :)

[ September 30, 2004, 04:41 PM: Message edited by: lady sparks lover ]
 
Re: Tap rules confuse me!

I think lady sparks has it. The tap doesn't distinguish between loads wether it be a panel,fused disconnect or xfmr.You must follow the tap rules for protection of the tap conductors and follow the rules for the load devices as they apply to a specific load.
 
Re: Tap rules confuse me!

Originally posted by lady sparks lover:
...I hope this doesn't annoy you too much, rbalex, but...here's how I take it.

10' or less, you can run tap conductors without OCP at the tap if the tap conductors are sized to suite the load it's supplying.

25' or less, you can run tape conductors without OCP at the tap if the the?conductor terminates into a OCP device and the tap conductors are rated higher that 1/3 the ampacity of the OCP which supplies the tap.

over 25', supply OCP at the tap.

For a transformer the same basically applies except the ration of .433 time the tap conductors on the secondary must be equal to or more than the ampacity of the OCP on the primary side...

Is this correct?? :)
Click to expand...
First, it doesn't annoy me at all. I was annoyed with myself. You have consistently demonstrated a desire to be a "professional" as well as a "competent" engineer, i.e., you want to know "why" as well as "how."

Second, it reminded me that when I pontificate, I'm prone to overlooking important things like "when the upstream OCPD only provides SC/GF protection for the tap, it does actually have to provide it." I glossed over the fact in each of the tap rules that the ampacity of the conductors must reflect the rating of the SC/GF OCPD in some way; e.g.,for a 25' tap the conductor's ampacity needs to be a minimum of 1/3 the rating of the OCPD. You pointed that out. :eek:

Finally, some taps over 25' may or may not need an "overload" OCPD where it receives it's supply. If the circuit is over 600V it will need some form of "isolating means" per 490.22. For transformers that would usually be a "disconnect" but it may not necessarily be an OCPD. It may only be a switch. The upstream SC/GF OCPD must be able to "clear the fault" per 110.9 & 10.

[ October 01, 2004, 07:58 AM: Message edited by: rbalex ]
 
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