Tap Conductors and EGC Size

[infinity]

Simple question, have a feeder with #3 EGC and taps under the 25' rule with #4 EGC for the tap conductors which are larger than #4. Compliant or not? I've been dancing around with an engineer on this one. I said non-compliant, he said......

 

[petersonra]

Simple question, have a feeder with #3 EGC and taps under the 25' rule with #4 EGC for the tap conductors which are larger than #4. Compliant or not? I've been dancing around with an engineer on this one. I said non-compliant, he said......

Depends entirely on the OCPD on the feeder. Likely not compliant.

 

[iwire]

The EGC of the tap has to be sized by the OCPD supplying the tapped feeder but in no case does the EGC have to exceed the size of the circuit conductors.

Look at Table 250.122.

Rating or Setting of
Automatic Overcurrent
Device in Circuit Ahead
of Equipment, Conduit,
etc., Not Exceeding
(Amperes)

 

[petersonra]

The EGC of the tap has to be sized by the OCPD supplying the tapped feeder but in no case does the EGC have to exceed the size of the circuit conductors.

Look at Table 250.122.

I agree. In this case the OP said the EGC for the tap conductors is #4 and the tap conductors are larger than #4, so IMO the tap EGC would need to be #3 unless the EGC was oversized on the feeder.

is this in conduit?

 

[Smart $]

The EGC of the tap has to be sized by the OCPD supplying the tapped feeder but in no case does the EGC have to exceed the size of the circuit conductors.

Look at Table 250.122.

I "two" agree.

 

[infinity]

I agree. In this case the OP said the EGC for the tap conductors is #4 and the tap conductors are larger than #4, so IMO the tap EGC would need to be #3 unless the EGC was oversized on the feeder.

is this in conduit?

Yes in conduit. And the #3 in the feeder is not oversized nor are the feeder conductors.

Seemed simple enough to me, the #3 is sized based on the OCPD ahead of the feeder. The EGC with the tap conductors needs to be the same size unless the ungrounded tap condcutors are smaller than #3.

 

[ryan_618]

When I teach about feeder taps I always liken them to a motor circuit. The upstream device is proving ground fault and short-circuit protection for the tap conductors, the downstream device provides nothing but overload protection. Since you are discussing clearing a ground fault, you size it based on the ground fault protection device (the upstream breaker), not the overload device (the downstream breaker).

 

[infinity]

When I teach about feeder taps I always liken them to a motor circuit. The upstream device is proving ground fault and short-circuit protection for the tap conductors, the downstream device provides nothing but overload protection. Since you are discussing clearing a ground fault, you size it based on the ground fault protection device (the upstream breaker), not the overload device (the downstream breaker).

So in this case if the #3 EGC with the feeder had been increased in size you would still need to increase the size of the tap EGC to #3?

 

[ryan_618]

What is the rating of the overcurrent device (upstream)?

 

[charlie b]

Since you didn?t give the complete installation details, let me suggest something that would fit your description.

• Total calculated load for the feeder is 325 amps.
• Feeder size is 400 MCM (ampacity of 335 amps).
• Minimum EGC size is #3 copper.
• 25 foot rule requires tap conductors to have ampacity of at least one third the ampacity of the feeder. That would be 112 amps. Therefore, the smallest allowable tap conductor would be #2.
• For the sake of the argument, let us say that the calculated load for the tap is 220 amps. Let us then select a 4/0 for the tap conductors.

If you start with this set of conditions, and wish to select an EGC size for the tap portion of the circuit, you could reason in the following way. This reasoning is invalid, and its conclusion is wrong. But I am attempting to recreate the thought process that the engineer might be using.

• A 4/0 has an ampacity of 230 amps.
• Table 250.122 gives us an EGC of #4 for circuits between 200 amps and 300 amps.
• Therefore, a #4 is an acceptable size for the EGC.

Of course, I cannot know whether this was the engineer?s reasoning. But it would be a misinterpretation of the table. It is based on the OCPD rating, not the circuit ampacity.

That was a long story. The short story is that I agree that the original description is that of a non-compliant installation. The EGC for the tap conductors must be #3 or larger.

 

[petersonra]

Yes in conduit. And the #3 in the feeder is not oversized nor are the feeder conductors.

Seemed simple enough to me, the #3 is sized based on the OCPD ahead of the feeder. The EGC with the tap conductors needs to be the same size unless the ungrounded tap condcutors are smaller than #3.

if the tap is in conduit what difference does it make since the conduit serves as the EGC?

 

[charlie b]

So in this case if the #3 EGC with the feeder had been increased in size you would still need to increase the size of the tap EGC to #3?

Let?s look at an installation that would fit this new set of circumstances.

• Total calculated load for the feeder is 250 amps.
• Minimum feeder size is 250 MCM (ampacity of 255 amps).
• OCPD is set at 250 amps.
• Minimum EGC size is #4 copper.
• Rather than buy a new reel of 250 MCM, you decide to use the 300 MCM that you had left over from a previous job.
• Since this is an increase in size for the ungrounded conductors, you must proportionally increase the size of the EGC.
• Doing the math, you conclude that you need to use a #3 EGC.
• Let?s say that the load on the tap will be 110 amps, so the tap conductors will be #2.

Question: Can you run a #4 EGC with the tap conductors, since that value is associated with the OCPD and since you did not increase the size of the tap conductors? Or since you had increased the feeder conductors and their associated EGC, do you have to also increase the EGC associated with the tap conductors?

My answer is that you don?t increase the EGC associated with the tap conductors.

 

[iwire]

if the tap is in conduit what difference does it make since the conduit serves as the EGC?

Because it does for no other reason than how the code is written.

If we install a wire EGC it must be sized like there is no other EGC.

 

[charlie b]

if the tap is in conduit what difference does it make since the conduit serves as the EGC?

Metal conduit? We don't yet know that. :happyno:

 

[petersonra]

Because it does for no other reason than how the code is written.

If we install a wire EGC it must be sized like there is no other EGC.

Where does it say ALL EGCs associated with a particular circuit have to meet that criteria? If one were to snip off the installed green wire at both ends how is it an EGC at all anymore?

 

[don_resqcapt19]

Where does it say ALL EGCs associated with a particular circuit have to meet that criteria? If one were to snip off the installed green wire at both ends how is it an EGC at all anymore?

If you don't connect the green wire or if it does not exist there is no code problem, assuming a raceway that is suitable for use as an EGC. There are no exceptions for the sizing of an EGC of the wire type when used in parallel with other EGCs, so if you connect it is must meet the code rules of EGCs of the wire type.

 

[infinity]

I don't have much information since it was a one line diagram with some missing information. What I do know is that the feeder EGC is properly sized at #3 and the tap EGC is #4. IMO the tap EGC must be #3. Also the conduit is EMT.

 

[don_resqcapt19]

I don't have much information since it was a one line diagram with some missing information. What I do know is that the feeder EGC is properly sized at #3 and the tap EGC is #4. IMO the tap EGC must be #3. Also the conduit is EMT.

If the tap conductors are #3 or larger, than I agree the EGC run with the tap conductors has to be a #3.

 

[infinity]

If the tap conductors are #3 or larger, than I agree the EGC run with the tap conductors has to be a #3.

My simple response when asked the question mirrored this, if the tap conductors are as large or larger than the EGC in the feeder then the EGC with the tap conductors must be the same size as the feeder EGC.

 

[petersonra]

I don't have much information since it was a one line diagram with some missing information. What I do know is that the feeder EGC is properly sized at #3 and the tap EGC is #4. IMO the tap EGC must be #3. Also the conduit is EMT.

Since it is emt no wire egc is required at all. I would just disconenct it and leave it in place and call it good if the inspector was this picky.