Tap Rule Violation

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mbrooke

mbrooke

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Is it a violation to take 500MCM protected at 250amps, tap it with 4/0 and then land it on a 225 amp OCPD. The catch is it over 25 feet. This is in an old building. Was it ever complaint?
 
mbrooke said:
Is it a violation to take 500MCM protected at 250amps, tap it with 4/0 and then land it on a 225 amp OCPD. The catch is it over 25 feet. This is in an old building. Was it ever complaint?
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If 240.4(B) can apply, it would comply, since the 230A rating of #4/0 Cu can "round up" to 250A.
 
Carultch said:
If 240.4(B) can apply, it would comply, since the 230A rating of #4/0 Cu can "round up" to 250A.
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Good point, I had not though about it that way.

What about a riser with multiple taps off it?


(B) Overcurrent Devices Rated 800 Amperes or Less. The
next higher standard overcurrent device rating (above the
ampacity of the conductors being protected) shall be permitted
to be used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cord-andplug-
connected portable loads.

(2) The ampacity of the conductors does not correspond with
the standard ampere rating of a fuse or a circuit breaker
without overload trip adjustments above its rating (but
that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed
800 amperes.
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But then what would stop me from running 4/0 straight out of the switchboard?
 
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Depends on the load. This sounds like the (odd?) rule of being able to size a feeder conductor to the calculated load, regardless of the number or size of panels it feeds, and then use next size up for the OCPD it the conductor ampacity doesn’t match a size.
 
bwat said:
Depends on the load. This sounds like the (odd?) rule of being able to size a feeder conductor to the calculated load, regardless of the number or size of panels it feeds, and then use next size up for the OCPD it the conductor ampacity doesn’t match a size.
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Even without calculated load the riser is protected, and the tap feeds a main breaker panel. This practice is common in NY state- however in old buildings I see single lines where it goes past 25 feet.
 
mbrooke said:
Would splice still hold for a 250 amp riser with several 225amp MB panels coming off it?
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Splice still applies. You don't have a tap unless you have conductors that are protected from overload by a down stream OCPD instead of an upstream one. 4/0 could have been run out of the main panel.

It seems likely that either the 'several' 225A panels are oversized or that the 250A feeder is undersized for the calculated load, but it is possible with demand factors to have the sum of the requirements for the panels add up to more than the requirement for the feeder.
 
jaggedben said:
Splice still applies. You don't have a tap unless you have conductors that are protected from overload by a down stream OCPD instead of an upstream one. 4/0 could have been run out of the main panel.

It seems likely that either the 'several' 225A panels are oversized or that the 250A feeder is undersized for the calculated load, but it is possible with demand factors to have the sum of the requirements for the panels add up to more than the requirement for the feeder.
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Over sized + load diversity.

There is an OCPD down stream, as I said its a Main breaker panel. I will look at the single line diagram some more- there are other examples.
 
mbrooke said:
Over sized + load diversity.

There is an OCPD down stream, as I said its a Main breaker panel. I will look at the single line diagram some more- there are other examples.
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I was a little unclear. The downstream OCPD does not make it a tap if the upstream OCPD is rated to protect all the conductors.
 
mbrooke said:
Good point, I had not though about it that way.

What about a riser with multiple taps off it?




But then what would stop me from running 4/0 straight out of the switchboard?
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As long as calculated load is 230 or less 4/0 is fine. You have a feeder, it just happens to supply multiple panels.

You could have a single 225 amp feeder supplying a 60 circuit panel, or you could supply all the same loads from a 225 amp feeder and three 20 circuit panels or five 12 circuit panels for that matter. Total load on the feeder along with overcurrent protection rules is the key for sizing feeder conductors. If you went with five 12 circuit panels instead of the single 60 circuit panel, chances are they are not all in same location and it maybe even has chance that you reduce the overcurrent protection and conductor size in some situations as it probably does cost less to install that way.
 
The tap rules allow for a conductor to be protected only downstream.
The longer the run of unprotected wire, the more likely you are to have a fault along that run of unprotected wire which could be a fire hazard.
The reduced EGC is not a greater hazard in proportion to its length. If the run is long enough that voltage drop is an issue, the current carrying conductors will be upsized and the EGC must be upsized proportionally.

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GoldDigger said:
The tap rules allow for a conductor to be protected only downstream.
The longer the run of unprotected wire, the more likely you are to have a fault along that run of unprotected wire which could be a fire hazard.
The reduced EGC is not a greater hazard in proportion to its length. If the run is long enough that voltage drop is an issue, the current carrying conductors will be upsized and the EGC must be upsized proportionally.

Sent from my XT1585 using Tapatalk
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Well... Remember that voltage drop is not enforced by code.

Pushing VD aside, in theory you could have 3/0 protected at 200amps, #6 tap going 45 feet into a 60amp MB panel. The #6 would be protected from overload, and if a short circuit presented itself it would be no different than that which passes through the #6 EGC.

Just a thought :)
 
mbrooke said:
Well... Remember that voltage drop is not enforced by code.

Pushing VD aside, in theory you could have 3/0 protected at 200amps, #6 tap going 45 feet into a 60amp MB panel. The #6 would be protected from overload, and if a short circuit presented itself it would be no different than that which passes through the #6 EGC.

Just a thought :)
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If the tap is outdoors there is no limitation on length.

Also keep in mind the EGC run with the #6 tap conductor you used as an example still needs to be 6 AWG because of the 200 amp feeder OCPD.

The overall circuit impedance at the load end of those #6 taps is more than if 3/0 would have been run the entire distance, so it will reduce the amount of fault current that flows and probably results in longer fault duration before OCPD opens.
 
This particular example falls into an interesting corner case.

You have a conductor rated at 230A, with a load (presumably) of less than 225A, protected at the supply end by a 250A breaker and at the load end by a 225A breaker. This would be a tap except for the round up rule that permits a 4/0 to be considered fully protected by a 250A breaker as long as the load is less than 230A.

If the over-all load on the 500kcmil conductors is less than 380A, then the entire thing could be protected by a 400A breaker, and then the same mix of conductors (500kcmil and 4/0AWG) _would_ have to follow tap rules, and the >25 foot length would be a violation.

-Jon
 
kwired said:
If the tap is outdoors there is no limitation on length.

Also keep in mind the EGC run with the #6 tap conductor you used as an example still needs to be 6 AWG because of the 200 amp feeder OCPD.

The overall circuit impedance at the load end of those #6 taps is more than if 3/0 would have been run the entire distance, so it will reduce the amount of fault current that flows and probably results in longer fault duration before OCPD opens.
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Yes, yup, and agree :)
 
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