Transformer secondary conductor OCP location

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MasterSparky

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I have a question regarding the interpretation of NEC 240.21 and associated sub-articles (A thru G)...specifically as they relate to transformer secondary conductor overcurrent protection location (240.21(C)).

Typical indoor commercial setting: a 3 phase, 277/480 panel, with a 3 pole / 100 amp circuit breaker feeding a 75 KVA xfmr (this is the primary). Secondary is to be fed to a 3 phase, 225 amp, 120 / 208 volt panelboard (call it a lighting / branch panelboard), with a 225 amp main circuit breaker in it. For sake of argument let's say the xfmr is located 50 feet away from the primary side circuit breaker. Assume all wiring is sized on both primary and secondary side to be NEC compliant (#3 THHN CU on primary and 4/0 THHN CU on secondary). All wires are assumed to be in an approved raceway.

The question being raised (argued?) by me and a co-worker is the LOCATION of the OCPD for the transformer's secondary conductors. The debate is this: can the OCPD be located ANYWHERE after the secondary conductor connection point, or must it be located within a certain distance of the connection point?

My belief is this: Based on 240.21, "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) thru (G)." This to me means that you must provide overcurrent protection at the point conductors receive their supply unless something in (A) thru (G) applies, then you can utilize one of those rules to determine how / where your OCPD is located.

My argument is that essentially this installation needs to have an OCPD within 10 feet from the transformer (using the overall conductor length). The argument against me is that the required OCPD can be in the same panel as mentioned in my example, but it could be located 100 feet, 150 feet, 200 feet, etc. away from the transformer. This person argues that since an OCPD IS being installed in the panel, the provisions of 240.21(C) don't apply...that they only apply if you're considering not putting an OCPD in. I feel that if you're considering running the secondary conductors to a branch panelboard 150 feet away, then you'll need to put some sort of OCPD for the secondary conductors within 10 feet of the transformer then feed to the branch panelboard from that OCPD (and the panelboard can now be a main lug type).

I know this is a complicated question, but I'm looking for any clarification I can get. Thanks!
 
It is really not complicated, your right 240.21(C) applies to all transformer secondaries.


This person argues that since an OCPD IS being installed in the panel, the provisions of 240.21(C) don't apply...that they only apply if you're considering not putting an OCPD in.

That person is wrong for that 3 phase Wye transformer.

The code always requires secondary overcurrent protection for that type transformer

Read 240.21(C)(1), that section outlines the few times that the primary can protect the secondaries.

The NEC requires overcurrent devices within 10' or 25' of this transformer depending on the location.
 
iwire said:
The code always requires secondary overcurrent protection for that type transformer.

Lets split hairs.:rolleyes:

The code always requires secondary CONDUCTOR overcurrent protection for that type transformer.

Many people seem to only look at article 450 when a transformer is involved. As Bob pointed out, the correct place to start is 240.21(C).
 
Thanks! We know it requires overcurrent protection for the conductors (via 240.21(C)1 thru 6), that wasn't ever an issue. What was being debated was WHERE the overcurrent protection could be placed. I've typically always placed the OCPD within 10', unless the circumstances and the NEC has allowed differently. But I've never heard of the NEC allowing any required protection to be placed, say, 150 feet away without any other form of OCPD much closer to the point of conductor connection.
 
But I've never heard of the NEC allowing any required protection to be placed, say, 150 feet away without any other form of OCPD much closer to the point of conductor connection.
It is permitted in some cases. One example is 240.21(C)(4).
Don
 
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