control transformer branch circuits

[opampguy]

Our customers often disagree about how 3 phase branch circuits should be protected when they serve control transformers. We interpret the electrical code(s) as required, but I am interested to see what others think about this relatively simple issue.

Specifically, imagine a simple enclosure with three motor starters (IEC combos, each with line disconnect/circuit breaker, the contactor, and output overload), all branched from a single main panel disconnect/circuit breaker. The 3 phase wiring from the main panel disconnect is daisy chained from starter to starter, using the same AWG wire, rated according to the main circuit breaker. An additional branch, connected identially as with the starters, serves a control transformer, which has the normal fuse on it's secondary. The issue that keeps coming up is: does the transformer PRIMARY need a fuse/circuit breaker? Customer opinion comes to about 50/50 on this, and all can support opinion with pages from the NEC. I am interested to hear what others, expert or otherwise, do in this (common!) situation.

 

[steve66]

Re: control transformer branch circuits

See table 450.3(B). The primary does need overcurrent protection. It is unlikely the panel main breaker would be sized small enough for it to be the required OCP.

OCP may not be required for the transformer secondary, but the conductors from the transfromer secondary also need protection. See 240.21(C).

Steve

 

[richwaskowitz]

Re: control transformer branch circuits

I concur with steve66. At one time, it was not required, nor was it the general practice to provide an OCP for the primary of a control transformer in many cases. When purchasing off-the-shelf combination starters, such as an Allen-Bradley bulletin 512, they did not provide such protection, only a fuse for the secondary. Now however, it is quite clear that primary protection is required.

 

[rbalex]

Re: control transformer branch circuits

Most CPT's are exempt under from the rest of Article 450 by 450.1 Ex 2 or 4.

If the installation described is field fabricated then the rules of 450.3 apply.

[ June 08, 2004, 03:41 PM: Message edited by: rbalex ]

 

[hawkeyefan]

Re: control transformer branch circuits

This is how I design my panels. I always use a distribution block after the main disconnect breaker and wire individually to branch motor circuits sizing the wire, breaker & starter to the motor.

Anyway, when wiring 2 poles to the control transformer I again use breakers (as opposed to fuses) - I use the 1492 series from AB for both the primary & secondary of the transformer.

I can't quote the NEC on what is acceptable & what is not but I've never had a problem. Can't remember if UL 508 calls for primary protection of not on the transformer.