Failed inspection because of tap rule

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zappy

Senior Member
Location
CA.
Tapped off of a 480v. 800 amp breaker to a disconnect. That disconnect was 250amp feeding a 200amp panel. This was a solar install, so the 200amp panel was the main. So we ran 4/0 from the main to the disconnect, then from the disconnect to the 800amp breaker. That distance was thirty feet, and was 4/0 also. Inspector said the 4/0 from the disco. to the 800amp breaker was to small because of the tap rule. He said it had to be sized 1/3 of 800amps. So the wire needs to be 250MCM. He said if it was only ten feet, it would have been fine. So why is ten feet of wire ok, but thirty feet isn't? Ten feet can handle the amps, but thirty feet can't? I don't understand the safety aspect. Is it because of less ohms in ten feet then thirty feet? Where not getting power from the 800amp breaker, where feeding it power. Can someone also explain what is a tap and what isn't. I thought you size the wire to the load not the breaker. Thank you very much for your help.
 

chris kennedy

Senior Member
Location
Miami Fla.
Can't help you with the first part but the definition of tap is found in 240.2

Tap Conductors. As used in this article, a tap conductor is defined as a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.
 

iwire

Moderator
Staff member
Location
Massachusetts
Regardless of the load the tap conductors have to be large enough to trip the feeders overcurrent device if there was a line to line or line to ground short in the tap condutors between the feeder and the new overcurrent device.

As the distance gets longer the size has to increase to keep the impeadance down.


Don't forget your EGC that runs with the tap conductors must be sized based on the 800 amp breaker not the 200 amp disconnect.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
I don't think that is the reason, Bob. If you have a ground fault, there will be enough current to trip the breaker, and the difference in impedance between 10 feet of 4/0 and 30 feet of 4/0 will make very little difference in the amount of fault current. I think it is all about fault currents, but the basis is something else.

What I believe to be the basis starts with recognizing that for the entire length of the tap conductor, the conductor is not protected against overcurrent at its own ampacity value. Start with a 250 MCM (ampacity 255), and tap a 4/0 (ampacity 230), and then consider a high impedance fault within the 4/0, such that the fault current is 245 amps. Since the overcurrent protection for the 4/0 is at the end of the run, and since the upstream breaker is set at 250, nothing will trip to terminate this event, and the 4/0 will eventually burn up.

So the very nature of a tap situation is risky. There is no protection against a certain kind of short circuit within the conductor itself. Since we can't do anything about the consequences of such an event, we reduce the risk by lowering the probability that the event can ever happen in the first place. We do that in two ways. One is to limit the length of the tap conductors, so that there will be only a short run that is susceptable to a fault. The other is to require physical protection of the tap conductors, so that the chances of it being damaged are small.
 

SEO

Senior Member
Location
Michigan
Regardless of the load the tap conductors have to be large enough to trip the feeders overcurrent device if there was a line to line or line to ground short in the tap condutors between the feeder and the new overcurrent device.

As the distance gets longer the size has to increase to keep the impeadance down.
If this were true what about section 240.21 (5) for outside taps of unlimited length?
 

Cold Fusion

Senior Member
Location
way north
...what about section 240.21 (5) for outside taps of unlimited length?
That is a really good question. I have a real problem with unprotected (OCPD)feeders.
To my thinking, outside, unlimited length taps work fine - as safe as most anything else. My reasoning follows charlie's and adds a bit.

The OCPD at the begining does not particularly protect the conductors. It is mostly there to protect the structure. Any overload will be caught by the OCPD at the termination. The front end OCPD function is to put the fire out if the tap faults.

Any overload coming from the middle of the tap is a forklift or backhoe attack. A circuit breaker won't protect against this, concrete and steel will.

Supposing the tap faults as in charlie's example - an arcing ground fault, but not quite enough to trip the CB. First there is no structure to burn - it's outside. Second, arcing ground faults will eventually go phase to phase - and that will trip.

Outside taps appear to be at least as safe or safer as a service drop or lateral.

cf
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Who is to say that an "outside tap" is outside the third little pig's brick house, and not outside one of its brother's straw or stick houses? The outside tap rule is not limited to buildings constructed of non-combustible materials. If the tap conductor melts, the building can burn down. So I don't buy the notion that "an outside tap can be longer because a conductor can safely melt outside."

I think the rule about outside taps of unlimited length is consistent with my theory about the reason taps are allowed in the first place: There is less of a risk of damage, since there is less nearby human activity, and the risk is made even lower by requiring physical protection of the conductors.

 

charlie b

Moderator
Staff member
Location
Seattle, WA
How is that different from the SE cable running down the side of my straw home?
What protects the SE from overcurrent, and where is the OCPD located (i.e., at the begining or end of the SE)?


We are discussing a tap conductor for which the only OCPD capable of providing protection at the conductor's ampacity level is at the load end of the conductor. Such a device would not protect the conductor from a failure in the middle of the run. That's a real risk. We accept that risk because we are making the probability of the failure very low.
 
Regardless of the load the tap conductors have to be large enough to trip the feeders overcurrent device if there was a line to line or line to ground short in the tap condutors between the feeder and the new overcurrent device.

As the distance gets longer the size has to increase to keep the impeadance down.


Don't forget your EGC that runs with the tap conductors must be sized based on the 800 amp breaker not the 200 amp disconnect.
We will remain in disagreement here.



For outside taps I just do not think they care if the conductors melt, very much the same situation as service conductors.
How is that different from the SE cable running down the side of my straw home?

I have been taught basically the same reasoning as what Bob has stated.

Not to argue with you Charlie, where have you received your information to come to your conclusions?
 

erickench

Senior Member
Location
Brooklyn, NY
You know what? I think 250 MCM is too small! The rule is very clear about 1/3 OCPD rating for the conductor ampacity of 25 ft or larger taps. In this case the ampacity at 75' C is 255A and not 800/3=266.67A.
 
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don_resqcapt19

Moderator
Staff member
Location
Illinois
What protects the SE from overcurrent, and where is the OCPD located (i.e., at the begining or end of the SE)?

We are discussing a tap conductor for which the only OCPD capable of providing protection at the conductor's ampacity level is at the load end of the conductor. Such a device would not protect the conductor from a failure in the middle of the run. That's a real risk. We accept that risk because we are making the probability of the failure very low.
In most cases there will be even less protection of the service conductor than the tap conductor. The only line side protection of the service conductor is on the line side of the utility transformer and that protection is not sized to protect either the transformer or the service conductors on the secondary side of the transformer.
 

dana1028

Senior Member
You know what? I think 250 MCM is too small! The rule is very clear about 1/3 OCPD rating for the conductor ampacity of 25 ft or larger taps. In this case the ampacity at 75' C is 255A and not 800/3=266.67A.
OK I'm confused.

Where are you coming up with this '1/3' rule? The 1/3 is good for taps not over 25' long, the OP indicated the distance was 30'; so, unless this installation falls under 240.21(B)(5) - 'Outside Taps', then the whole job is wrong.

That said, this being a PV install, is it fair to think this is an outside install? In that case the only limitation is the OCPD at the termination [I believe the OP indicated 250A]....so his 4/0 is too small, but not because of a '1/3' rule.
 

erickench

Senior Member
Location
Brooklyn, NY
Well in that case there is no specific rule for sizing this tap. The only requirement is that the OCPD, the 250A CB, limits the load to the ampacity of the conductor as stated in 240.21(B)(5)(2).
 

zappy

Senior Member
Location
CA.
OK I'm confused.

Where are you coming up with this '1/3' rule? The 1/3 is good for taps not over 25' long, the OP indicated the distance was 30'; so, unless this installation falls under 240.21(B)(5) - 'Outside Taps', then the whole job is wrong.

That said, this being a PV install, is it fair to think this is an outside install? In that case the only limitation is the OCPD at the termination [I believe the OP indicated 250A]....so his 4/0 is too small, but not because of a '1/3' rule.
The inspector just eyeballed it, he figured it was 25' or less.
 
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