Control Panels: Control Xfmr protection

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juicethief

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I have been reviewing a design for an electric duct heater and the vendor schematic for the SCR controller shows two control transformers (480/120V - 200VA & 480/24V - 50VA, Class 2?). Both control transformers receive their supply from the load side of a manual disconnect supplied with the controller. The controller is being fed from an MCC cubicle with 35A fuses. However, there is no fusing in the primary of either control xfmr. There is a 3A fuse in the secondary of the 480/120V control xfmr and no fusing in the secondary of the 480/24V control xfmr. The control xfmr primary & secondary wiring is #14 & #18 for the 480/120V. The primary and secondary wiring is #18 for 480/24V.
My questions are: do control panels have to follow NEC rules and, if not, how can these be installed seemingly unprotected?

I would think this would need to conform with 450.3(B).

The panel is UL listed and none of the control wiring leaves the enclosure if that makes a difference.
 
If the panel had a UL (or other NRTL) listing the wiring may well be designed to a standard other than the NEC.
IMHO, you question is valid and I will be glad to give you a UL contact by PM if you desire and you can follow-up with them.
 
If the primary is a L-L voltage such as 480v we always used (2) ktkr 600v fuses. The secondary commonly has one ungrpunded line with the other grounded. We used an fnm fuse to fuse the ungrounded line.
The primary fuses do not prove protection for the transformer. Their job is to protect the upstream distribution system should the transformer itself fail. It is the fnm fuse the protects the transformer from overload.
 
juicethief said:
I have been reviewing a design for an electric duct heater and the vendor schematic for the SCR controller shows two control transformers (480/120V - 200VA & 480/24V - 50VA, Class 2?). Both control transformers receive their supply from the load side of a manual disconnect supplied with the controller. The controller is being fed from an MCC cubicle with 35A fuses. However, there is no fusing in the primary of either control xfmr. There is a 3A fuse in the secondary of the 480/120V control xfmr and no fusing in the secondary of the 480/24V control xfmr. The control xfmr primary & secondary wiring is #14 & #18 for the 480/120V. The primary and secondary wiring is #18 for 480/24V.
My questions are: do control panels have to follow NEC rules and, if not, how can these be installed seemingly unprotected?

I would think this would need to conform with 450.3(B).

The panel is UL listed and none of the control wiring leaves the enclosure if that makes a difference.
Click to expand...

UL508a allows the primary protection on this size of a class 1 xfmr to be 500%. I don't see how you get that with 35A fuses.
 
A little bit more detail:

The SCR panel is a vendor supplied unit. Our design guys are simply supplying power from an MCC. The 35A fuses in the MCC are for a 22 kW, 480V, 3? heater (which seems OK). Surely, with the UL listing someone has deemed this configuration as acceptable?
 
juicethief said:
A little bit more detail:

The SCR panel is a vendor supplied unit. Our design guys are simply supplying power from an MCC. The 35A fuses in the MCC are for a 22 kW, 480V, 3? heater (which seems OK). Surely, with the UL listing someone has deemed this configuration as acceptable?
Click to expand...

It is entirely possible that the instructions or drawings that come with the unit offer some guidance. it may say something like a maximum allowed OC device size protecting the incoming feed.

We often put such things on our drawings. Just like we put an SCCR rating on the cabinet. It is up to the installer to make sure that they follow the instructions provided.
 
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