10 foot taps

[electrofelon]

I was looking at 240.21(B)(1) ("10 foot tap rule"). Say I have a 1000 amp feeder (1000 amp ocpd) supplying a 1000 amp mlo panel board. Say I have a 500 amp tap off the buss lugs to the buss lugs of another 1000 amp mlo panel board. the tap conductors are under 10 feet. Compliant or not? What is the purpose of the statement "not less than the rating of the device supplied by the tap conductors.." ? What could this device be?

 

[GoldDigger]

I was looking at 240.21(B)(1) ("10 foot tap rule"). Say I have a 1000 amp feeder (1000 amp ocpd) supplying a 1000 amp mlo panel board. Say I have a 500 amp tap off the buss lugs to the buss lugs of another 1000 amp mlo panel board. the tap conductors are under 10 feet. Compliant or not? What is the purpose of the statement "not less than the rating of the device supplied by the tap conductors.." ? What could this device be?

That would be the 1000 amp mlo panel board unless you are in a position to add up all of the loads (based on OCPD) that would be fed from that panelboard.
It might be easier/cheaper/more practical to add a 500A main breaker on the second panelboard or just use 1000A rated tap conductors. (Which would not be a tap anymore, because they would be full current rated compared to to the 1000A feeder and its OCPD.)

 

[electrofelon]

So a "device" could be a panel board, or piece of utilization equipment such as a widget stretcher? Could I feed the widget stretcher directly from a tap if it didnt specify a max ocpd size or I was under that max size? Mike Holt says that basically a tap always has to terminate on an OCPD but I am not seeing that wording in the 10 foot tap rule.

Can anyone think of a situation where a tap can feed a MLO panel?

 

[infinity]

What about a dimming board?

http://forums.mikeholt.com/showthread.php?t=128524&highlight=dimming+board

 

[electrofelon]

What about a dimming board?

http://forums.mikeholt.com/showthread.php?t=128524&highlight=dimming+board

Thanks Infinity. It seems you were contemplating exactly what I was: essentially having the tap be a branch circuit and not a feeder. This begs a larger question, that was brought up in your thread, of protecting the equipment. However I dont see any blanket clause in the NEC that generally says utilization equipment must be protected at its rating. Sure we have transformers, panel boards, motors....but if the instructions in my widget stretcher are silent on overcurrent protection?

I find the "ampacity not less than the rating of the device supplied by the tap conductors" to be a strange statement. It makes sense if the "device" is a OCPD because that would protect the tap conductors from overload, but if its not..?

 

[Glennsparky]

...Say I have a 500 amp tap off the buss lugs to the buss lugs of another 1000 amp mlo panel board. the tap conductors are under 10 feet. Compliant or not?

Not. 240.21(B)(1)(4) If you're describing a field installation and the tap has left the enclosure or vault in which the tap was made. Then there must be an OCPD on the line side of the tap conductors.

 

[GoldDigger]

Thanks Infinity. It seems you were contemplating exactly what I was: essentially having the tap be a branch circuit and not a feeder. This begs a larger question, that was brought up in your thread, of protecting the equipment. However I dont see any blanket clause in the NEC that generally says utilization equipment must be protected at its rating. Sure we have transformers, panel boards, motors....but if the instructions in my widget stretcher are silent on overcurrent protection?

I find the "ampacity not less than the rating of the device supplied by the tap conductors" to be a strange statement. It makes sense if the "device" is a OCPD because that would protect the tap conductors from overload, but if its not..?

If the device is a motor with integral overload protection, that will limit the current through the branch/tap wires except in the case of a short circuit.
If a short circuit occurs, up in the instantaneous trip range of the 1000A breaker, then that breaker will also protect the 500A wiring just fine.

But what if the device is a widget stretcher which contains a widget heater as well, since hot widgets stretch better, as every body knows?
Now the motor overloads will keep the motor from pulling more than 300A for any significant period of time. And the heater's normal draw is just 200A. So far so good. But a fault inside the heater could cause it to pull 700A instead. Under those conditions the total load will be 1000A. The 1000A breaker will not open, but the 500A tap wires will overheat.

The only reason that you can have the motor branch wires protected by a higher current breaker is that the motor itself contains an overload. If it does not, then you will have to size the conductors by the OCPD amps instead, which will require more copper.

If iwire's lighting board does not have internal OCPD or at least current limiting, then you would have to have a separate OCPD protecting the tap conductors IF it is a branch circuit tap and it is not one of the kinds of loads enumerated in the 210.19(A)(4) exception.

Remember that the branch tap conditions are set in 210.19(A), including the tap exception in (A)(4), as referenced in 240.21(A). That is one of your hypothetical conditions.
Feeder taps, which fit the scenario you originally described, are governed by 240.21(B) and following instead, and pretty much require that there be downstream overcurrent protection of some sort.

 

[GoldDigger]

If the device is a motor with integral overload protection, that will limit the current through the branch/tap wires except in the case of a short circuit.
If a short circuit occurs, up in the instantaneous trip range of the 1000A breaker, then that breaker will also protect the 500A wiring just fine.

But what if the device is a widget stretcher which contains a widget heater as well, since hot widgets stretch better, as every body knows?
Now the motor overloads will keep the motor from pulling more than 300A for any significant period of time. And the heater's normal draw is just 200A. So far so good. But a fault inside the heater could cause it to pull 700A instead. Under those conditions the total load will be 1000A. The 1000A breaker will not open, but the 500A tap wires will overheat.

The only reason that you can have the motor branch wires protected by a higher current breaker is that the motor itself contains an overload. If it does not, then you will have to size the conductors by the OCPD amps instead, which will require more copper.

If iwire's lighting board does not have internal OCPD or at least current limiting, then you would have to have a separate OCPD protecting the tap conductors IF it is a branch circuit tap and it is not one of the kinds of loads enumerated in the 210.19(A)(4) exception.

Remember that the branch tap conditions are set in 210.19(A), including the tap exception in (A)(4), as referenced in 240.21(A). That is one of your hypothetical conditions.
Feeder taps, which fit the scenario you originally described, are governed by 240.21(B) and following instead, and pretty much require that there be downstream overcurrent protection of some sort.

Try this on for size:
If you are tapping from a feeder, and there is no OCPD downstream of the tap, the tap wires are a branch circuit since they are downstream of the last OCPD, whether there are also other loads with OCPD on the conductor from which the tap was made.
So that cannot happen. The world will now come to an end, as a logical inconsistency has been found.

 

[david luchini]

I find the "ampacity not less than the rating of the device supplied by the tap conductors" to be a strange statement. It makes sense if the "device" is a OCPD because that would protect the tap conductors from overload, but if its not..?

240.4 requires that conductors be protected at their ampacity. 240.21 requires the overcurrent protection to be located at the point where the conductors receive their supply, except as specified in 240.21(A) through (H). (Note that 240.21(B) says that the provisions of 240.4(B) shall not be permitted for tap conductors.) So, applying 240.21(B)(1)(1)(b) to a "device" does not remove the requirement to protect the tap conductors at their ampacity.

Your 500A tap can terminate at a MLO panel, but the sum of the OCPD's in the panel cannot exceed 500A, or the conductors would not be properly protected at their ampacity.

 

[electrofelon]

Not. 240.21(B)(1)(4) If you're describing a field installation and the tap has left the enclosure or vault in which the tap was made. Then there must be an OCPD on the line side of the tap conductors.

There is an OCPD on the line side of the tap conductors and the ampacity of the tap is only half where the allowance is 1/10th. I agree as stated its not compliant but it does not violate the section you quoted. Think maybe you misread that section as "load side"?

So, applying 240.21(B)(1)(1)(b) to a "device" does not remove the requirement to protect the tap conductors at their ampacity.

Your 500A tap can terminate at a MLO panel, but the sum of the OCPD's in the panel cannot exceed 500A, or the conductors would not be properly protected at their ampacity.

I am not sure I buy that. Sure it makes logical sense that you could sum the OCPD's and use that as your protected value, but I dont really see that wording....?

 

[david luchini]

I am not sure I buy that. Sure it makes logical sense that you could sum the OCPD's and use that as your protected value, but I dont really see that wording....?

How else would you protect the tap conductors at their ampacity when they don't terminate directly at an OCPD?

 

[infinity]

By having a connected load less than or equal to the ampacity of the tap conductors similar to 230.90(A)Ex#3.

 

[david luchini]

By having a connected load less than or equal to the ampacity of the tap conductors similar to 230.90(A)Ex#3.

These are not service conductors, 230.90(A) does not apply. (And 230.90(A) Ex 3 doesn't protect the conductors at their ampacity, it allows them to be protected at ABOVE their ampacity. There is nothing in the 240.21(B) that allows tap conductors to be protected at ABOVE their ampacity.)

 

[infinity]

These are not service conductors, 230.90(A) does not apply. (And 230.90(A) Ex 3 doesn't protect the conductors at their ampacity, it allows them to be protected at ABOVE their ampacity. There is nothing in the 240.21(B) that allows tap conductors to be protected at ABOVE their ampacity.)

I did say similar not implying that they're the same. What part of the 10' tap rule would require a singular OCPD?

 

[electrofelon]

What part of the 10' tap rule would require a singular OCPD?

Am I understanding correctly that you are saying you believe you can sum the ocpd's and size you taps equal or greater to that value? Its an interesting question. 240.21(B)(1)(1)(b) uses the singular for device and ocpd. However the 25' tap rule is worded differently and states "....single circuit breaker or single set of fuses" so they definitely went out of their way to specify "one" for the 25' case

 

[david luchini]

I did say similar not implying that they're the same. What part of the 10' tap rule would require a singular OCPD?

Nothing in the 10' tap rule requires a singular OCPD (nor did I say that it did.) And nothing permits the tap conductors to be protected at greater than their ampacity.

Your 500A tap can terminate at a MLO panel, but the sum of the OCPD's in the panel cannot exceed 500A, or the conductors would not be properly protected at their ampacity.

I noted previously that the 10' tap rule permits you to terminate on a device rather than a single OCPD. But the conductors must still be protected at their ampacity. For the 500A tap, you can land on a mlo panel or switchboard, but the sum of the OCPDs in the panel cannot exceed 500A.

 

[George Stolz]

What part of the 10' tap rule would require a singular OCPD?

240.21, last sentence. Bussing of a panelboard is a conductor.