Todays graphic of the day

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Jpflex

Jpflex

Electrician big leagues
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Victorville
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Electrician commercial and residential
I don’t understand todays graphic of the day. Apparently a panel board (interior board “guts” of such as a service enclosure) if fed from a transformer must be protected by an overcurrent device on the secondary of a transformer.

However this has not been the case since utility transformers and wiring ARE NOT protected on the system secondary windings of their transformers. Instead an EGC system for equipment is made up to creat a return path for ground fault current with a current draw large enough to effect PRIMARY system and trip the primary side fuse or breaker, cutout.

In todays graphic question it is stated otherwise that the secondary of a transformer must be protected by an overcurrent device.

The only overcurrent device is the main disconnect breaker and branch circuit breakers but the main breaker is on the load side of the feeder? Unless this is what is meant?

Also code does allow both primary and secondary protection to my knowledge
 
It is a bit tricky since this requirement is in 408 and not 240.

First, remember that the utility is not bound by the NEC and often follow different rules.
Second, all panels ARE protected on the secondary of the utility transformer by the main(service) disconnect OCPD--either ahead of the panel or as the main breaker in the panel.
 
I remember reading hereabouts that the OCPD is to protect the -conductors.- To protect the conductors, the OCPD would have to be at the xformer. I guessed that to protect the conductors, it would have to be on the secondary.
I guessed correctly... but have I missed anything??
 
IIMHO, the question could be worded a bit better but, as roger notes, the question points out the requirement in 408.36 for a panelboard to be protected by a main overcurrent device.
On non-utility (SDS) transformers you need to refer to 450.3b and, yes, not only does the Code allow primary and secondary, in some cases it requires it (250% primary)
 
There are several things which need protection. It is possible for one device to protect more than one area.

Panelboards per 408.36. This was not always the case, so old timers may not have taught this.
Transformer secondary conductors per 240.21(C). Over the past decades, these rules have been modified slightly to correspond with 408.36.
Transformer primary per 450.3
 
Electromatic said:
It is a bit tricky since this requirement is in 408 and not 240.

First, remember that the utility is not bound by the NEC and often follow different rules.
Second, all panels ARE protected on the secondary of the utility transformer by the main(service) disconnect OCPD--either ahead of the panel or as the main breaker in the panel.
Click to expand...
This is what I was referring to since the main breaker service disconnect or first point of disconnect is on the load side of phase feeders.

Therefore, this does not protect the feeders cables and the secondary system windings due to a short to ground and feeders upstream of main breaker.

However you are right about utility transformer being exempt from NEC code - which answers the majority of the question
 
Jpflex said:
This is what I was referring to since the main breaker service disconnect or first point of disconnect is on the load side of phase feeders.

Therefore, this does not protect the feeders cables and the secondary system windings due to a short to ground and feeders upstream of main breaker.

However you are right about utility transformer being exempt from NEC code - which answers the majority of the question
Click to expand...
This question was only aimed at an SDS installation and not a service from an utility.

Just like with service conductors that have the OCPD at the load end of the conductors, transformer secondary conductors only have overload protection. In both cases the primary OCPD may provide some short circuit and ground fault protection, but that is rare in the case of service conductors.
 
Also note that the question left out some verbiage of the parent article. The article referenced in the answer, 408.36, starts off with, "...a panelboard shall be protected by an overcurrent protective device..." This is really about what is protecting the panelboard, not the conductors.
 
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