Transformer Secondary PRotection

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ron

ron

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The need for it or not is always a problem for me.
This is an existing condition:
175A-3P CB serving a 112.5kVA 480 delta - 208/120V wye transformer.
The secondary of the transformer has 1 set of 4-500kCMIL + GND running up a high rise building (no secondary protection for the transformer).
There is (4) 100A fused switches tapped off the 500kCMIL riser in the upper portion of the building each serving 100A panelboards.
Is this okay?
What if a 5th 100A fused switch was added?

The primary protection is the next size higher than 125% of the primary 135 FLA, so is primary protection only okay? Since it is a delta wye, the NEC says that it can't be protected by primary only.
 
Secondary protection would not be required, see 450.3(B).

However, the secondary of the transformer is a seperatly derived system and shall have an OCD since you have exceded the tap rules in 240.21(C)(1). The reason you want an OCD on the secondary is there is no direct connection from the secondary to the primary OCD and a short on the secondary with a long distance will not open the primary OCD. Check the last sentence of 240.21(C)(1).

I would install an OCD at the transformers secondary, within 10' and be within 250% [max allowed by 450.3(B)] of the ampacity of the secondary and you would be OK.
 
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Kevin,
For the same reason that you first sentence says no secondary protection and then the last says you need it, is why the code rules for this are confusing.
I am familiar with the reason why an SDS may have an overload or fault on the secondary that would "look" different on the primary OCPD, so that is why I normally put secondary protection. This is an existing installation in which they want to replace one of the switches and I want to be sure it is still code compliant.
 
I'm with you on the confusion, I had to read the articles a couple of times.

However, IMO you need to install an OCD on the secondary to protect the 500's headed north up the building. Then adding the last 100A switch is no problem, as long as the taps on the 100A's meet the tap rules in 240.21:D.

I think that would be the code compliant install. It's a shame the original installer didn't have the secondary OCD, you'd be done with the job already:grin:
 
Ron
Secondary of the windings of the transformer may not need additional protection. That is based on 450.3(B).


Secondary protection of the conductors is generally a different story.
Here you need to reference 240.21(C), and also 408.36(B).

using 240.21(C)((1) is very restrictive and not much use for our standard industry installations, including the transformer you posted.
What you will most likely need to reference is 240.21(C)(2) or (C)(6).


"The secondary of the transformer has 1 set of 4-500kCMIL + GND running up a high rise building (no secondary protection for the transformer)."

If this means the conductors run up the building further than 10 or 25 feet without a disconnecting mean, there is an issue.
 
Pierre C Belarge said:
Ron
Secondary of the windings of the transformer may not need additional protection. That is based on 450.3(B).


Secondary protection of the conductors is generally a different story.
Here you need to reference 240.21(C), and also 408.36(B).

using 240.21(C)((1) is very restrictive and not much use for our standard industry installations, including the transformer you posted.
What you will most likely need to reference is 240.21(C)(2) or (C)(6).


"The secondary of the transformer has 1 set of 4-500kCMIL + GND running up a high rise building (no secondary protection for the transformer)."

If this means the conductors run up the building further than 10 or 25 feet without a disconnecting mean, there is an issue.
Click to expand...


Thanks Pierre, I realized after reading your post that 240.21(C)(2) is the section I should have quoted:confused:, not sure what I was looking at:roll:
 
I neglected to say, that 450.3(B) and 240.21(C) overlap sometimes, thus "killing two birds with one stone."

Meaning that the protection on the secondary side of the transformer for the conductors will generally provide secondary protection for the transformer secondary as well.

Ron
Depending on the load calculations, you may or may not be able to add the additional 100A panel.


I wonder, how far to the panels from the secondary of the transformer?
 
None of the taps to the 100 amp switches are code compliant and even the 500 kcmil requires overcurrent protection within 25' of the transformer. Many AHJs would require that you provide the required protection for the 500 kcmil conductors before you add the additional switch.
 
Pierre,
The first (2) 100A taps to fused switches occur at approx 30', then the 3rd is 50' from the XFMR and then the 4th is at 70'.

The calculated load is approx 25kVA, as the taps are for panels that do not have loads connected at this time.

I will force them to cut in a OCPD on the secondary.
 
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