I've only had to work with this a couple of times but, in each time, I got different answers from those that seemed "in the know". When installing a disconnect on the secondary side of a corner tapped delta transformer, does one need to fuse the grounded leg or install a dummy fuse in the jaws? I've been told to fuse it since it is one of the three legs going to the equipment but, it's also the grounded conductor which would mean NO FUSE. I can't find anything in the code about this. Any help?
Being an intentionally grounded conductor would make 240.22 (2002 NEC) apply "No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless"...(exceptions not relevant in this case)
...mind if I jump in and ask... if there are any problems with two-wire single phase loads? should I not use the grounded phase on a two pole breaker ? its been years since I worked on this type of service and I have a job moving a small shop with a single phase service to an old building with ths stuff ? I realize I need to check the motors that are running off of converters, I just ain't sure about the 2 pole gear
"430.36 Fuses — In Which Conductor. Where fuses are used for motor overload protection, a fuse shall be inserted in each ungrounded conductor and also in the grounded conductor if the supply system is 3-wire, 3-phase ac with one conductor grounded."
There is a difference between overload protection and short circuit and ground fault protection. Motors circuits break the overcurrent protection into these two parts: 1) Short circuit and ground fault protection 2) Overload protection
We usually provide #1 with the circuit breaker protecting the feeder, and #2 with heaters in the motor starter.
At a transformer providing power to a motor control center, all the conductors need to follow the rules in Article 240.
"II. Location 240.20 Ungrounded Conductors. (A) Overcurrent Device Required. A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor."
"240.21 Location in Circuit. Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (G)."
And most importantly:
"240.22 Grounded Conductor. No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following two conditions is met: (1) The overcurrent device opens all conductors of the circuit, including the grounded conductor, and is designed so that no pole can operate independently. (2) Where required by 430.36 or 430.37 for motor overload protection."
Fuses cannot simultaneously open all conductors of the circuit. That leaves us only (2).
The answer, then, is: unless the fuse is intended to provide the overload protection for a motor (instead of the heaters) then a fuse in the grounded conductor is not allowed. So, go ahead, install a two pole fused disconnect for your motor control center.
Does the listing on a three phase disconnect permit the installation of a six inch piece of copper pipe in place of a fuse? Or, does the manufacturer sell dummy fuses for this purpose? Or is it only proper to install a 2 pole fused disconect for this use?