Tap Rule Question

[Larry_S]

We have a 5.2A servo drive panel of unknown design to be installed on the load side of a 30A non fused disconnect. The line side feed is a 50A circuit from a distribution panel with #8AWG 3 phase wiring. On the load side of the disconnect, the wire size is dropped to #12 AWG. Is it correct to use a 30A disconnect and drop the wire size to #12 AWG on the load side by applying the tap rule? This is without knowing the circuit protection inside the servo drive panel or the distance between it and the disconnect. I appreciate your help.

 

[Dennis Alwon]

For one #12 is only good for 25 amps at 75C assuming the load fits the description in 240.4(E) or (G).

 

[Dennis Alwon]

There is also this

408.36 Overcurrent Protection

In addition to the requirement
of 408.30, a panelboard shall be protected by an overcurrent
protective device having a rating not greater than that of
the panelboard. This overcurrent protective device shall be
located within or at any point on the supply side of the panelboard.

Exception No. 1: Individual protection shall not be required for a
panelboard used as service equipment with multiple disconnecting
means in accordance with 230.71. In panelboards protected by three or
more main circuit breakers or sets of fuses, the circuit breakers or sets of
fuses shall not supply a second bus structure within the same panelboard
assembly.

Exception No. 2: Individual protection shall not be required for a
panelboard protected on its supply side by two main circuit breakers or
two sets of fuses having a combined rating not greater than that of the
panelboard. A panelboard constructed or wired under this exception
shall not contain more than 42 overcurrent devices. For the purposes of
determining the maximum of 42 overcurrent devices, a 2-pole or a
3-pole circuit breaker shall be considered as two or three overcurrent
devices, respectively.

Exception No. 3: For existing panelboards, individual protection shall
not be required for a panelboard used as service equipment for an individual
residential occupancy.

 

[jaggedben]

...Is it correct to use a 30A disconnect and drop the wire size to #12 AWG on the load side by applying the tap rule?

No. The tap rule requires that tap conductors terminate in a overcurrent device rated for their ampacity, and you haven't mentioned such a device. You could make it correct by either:
- downsizing the breaker from 50A to 20A,
- replacing the 30A disco with a fused version and installing the appropriate size fuses once you know the right size,
Or,
- changing the servo panel from 'unknown' design' to one where it's known the #12 terminates on an overcurrent device of 20A or less

 

[wwhitney]

If the servo drive panel is subject to Article 430 and has, in effect, an MCA and an MOCP, then it is common to have #12 Cu on a 30A OCPD.

Cheers, Wayne

 

[Larry_S]

Thanks so much for the replies. This piece of equipment with the servo drive is being provided by a European supplier into a U.S. facility. If the drive panel complies to NEC, and I assume it will, then it sounds like this probably isn't an issue. My concern was about publishing drawings without knowledge of the actual design.
Thanks again, I appreciate the comments.