Fused disconnect feeding sub-panel

[electrofelon]

This topic is headed to left field with opinions...

The combined OCPD rating of taps off the transformer cannot be more than 125% of 75000/(208*(Sqrt3))=260.A Table 450.3(A) note (2) 2002 NEC

If a single tap is terminated in a disconnect of less than or equal to the above you can come off the disconnect with what ever amperage you want to how ever many disconnects/panels other BS you want....

*provided all other rules are followed

I believe that everything that has been said has been fact not opinion. Lets recap: Primary protection for the transformer is always required. Depending on the size of that primary protection, secondary protection may or may not be required. Most/all of the installations I have done and most of the installations I have seen do not require secondary transformer protection because the primary protection is not greater than 125% plus next size up (perhaps my experience can be construed as opinion). I dont see any disadvantage or lack of flexibility in having multiple sets of transformer secondary conductors. For the case where secondary protection is not required, there is no restriction on the number of sets or the sum of the sizes of the OCPD's at the terminations. Where secondary protection is required, we are allowed 2-6 OCPD's and the sum cant exceed the size of a single OCPD - which is 125%+next size up. So for the OP, 208A*1.25=260A. That is not a standard size, so I can use a 300A single OCPD OR multiple devices that add up to that.

Note that we can use next size up for the 450 provisions, but not for the conductors themselves.

 

[iwire]

"......didnt feel like that was complete: So if the OP had primary protection over 125%, then the most he could have is 300 amps of OCPD's on the secondary, so clearly a 200 and 125 would not be compliant. Now if primary protection was 125% or less, then it would be fine."

Please show me the math?

There is little math to show.

If the transformer primary protection is 125% or less the transformer is protected and no further transformer protection on the secondary side is required.

Yes you still have to protect the secondary taps but that has nothing to do with transformer protection.

On the other hand if the transformer primary protection is greater than 125% we must provide transformer protection on the secondary and in that case what you said here:

The combined OCPD rating of taps off the transformer cannot be more than 125% of 75000/(208*(Sqrt3))=260.A Table 450.3(B) note (2) 2002 NEC

Is true.

Here is note 2 of Table 450.3(B)

Notice it starts off 'where required'

2. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit
breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not
exceed the allowed value of a single overcurrent device

Transformer secondary protection is not always required.


Now that said.

Welcome to the forum, it might be worth saying that this is not some Home Depot DIY forum, many of the posters here know there code very well electrofelon is an example of that. Don't let the 'felon' name let you think he is not on top of his game.


Also I noticed you are using the 2002 NEC, you may really want to consider updating, a train load of stuff has changed since the 2002 came out. If your work is not dwellings and not solar maybe the 2002 will get you by but if you work in homes or do solar you really need the 2014 or 2017 NEC.

 

[kwired]

I think the main intent of 240.21 is conductor protection, I also believe that they somehow incorporated secondary winding protection into 240.21(C)(1) though they never called it that.

If you have a 10 kVA single phase 120/240 secondary 480 primary then if you have a 10 kva 240 volt load connected everything is fine.

Now connect 10 kVA of 120 volt load, but only to one half of the secondary. The primary overcurrent device doesn't see any overload, but half of the secondary winding is carrying twice it's rated load.

 

[iwire]

I think the main intent of 240.21 is conductor protection, I also believe that they somehow incorporated secondary winding protection into 240.21(C)(1) though they never called it that.

There is nothing in 240.21 that applies to transformer protection.

Any, and all transformer protection requirements are in 450.

If you have a 10 kVA single phase 120/240 secondary 480 primary then if you have a 10 kva 240 volt load connected everything is fine.

Now connect 10 kVA of 120 volt load, but only to one half of the secondary. The primary overcurrent device doesn't see any overload, but half of the secondary winding is carrying twice it's rated load.

So if I am following you correctly if we ignore other code rules we could overload the transformer?

 

[kwired]

There is nothing in 240.21 that applies to transformer protection.

Any, and all transformer protection requirements are in 450.




So if I am following you correctly if we ignore other code rules we could overload the transformer?

What I am saying is the rule sort of offers protection to the secondary winding even though they never mention it.

I guess you could put 200 amp conductors on a secondary only rated 100 amps and land in a 200 amp overcurrent device and meet code. Might not be a great design decision but is code compliant.

 

[iwire]

I guess you could put 200 amp conductors on a secondary only rated 100 amps and land in a 200 amp overcurrent device and meet code.

Exactly which means 240.21 offers / requires no transformer protection.

Might not be a great design decision but is code compliant.

Code compliant as long as the rules in 450 are followed right?

 

[kwired]

Exactly which means 240.21 offers / requires no transformer protection.




Code compliant as long as the rules in 450 are followed right?

yes.

 

[GoldDigger]

I keep coming back to post #17, stating, correctly in my opinion, that primary protection of a delta-wye transformer simply cannot serve as secondary protection.
I am willing to look at different ways of providing that secondary protection, but not eliminating it completely in favor of OCPD that protects only the conductors attached to the secondary.

 

[iwire]

I keep coming back to post #17, stating, correctly in my opinion, that primary protection of a delta-wye transformer simply cannot serve as secondary protection.
I am willing to look at different ways of providing that secondary protection, but not eliminating it completely in favor of OCPD that protects only the conductors attached to the secondary.

I am not following you. :huh:

Are you saying you feel a delta wye transformer always needs transformer secondary protection?

The secondary side field installed conductors of a delta wye always require protection but not the secondary windings dependent on the primary protection.