Seperately Derived System

[jimberg1]

How many 225 amp 42 circuit panelboards can I supply from a single 75KVA transformer. If more than one, do the second and third set of derived secondary conductors origionate at X1, X2, X3 and X0 and subsequently terminate on three individual 225 amp main breaker in each of three panelboards??? The transformer has 100 amp primary protection and the 4/0secondaries meet the 25 foot rule.

Our three-phase computer whips contain #12 black, red, light red, blue, white, white, green and green. I assume phases 1-2-3, green isolated ground, equipment grounding conductor and white grounded conductor. What is the additional white and light red for ???

To best "Quiet the Fields", we will isolate one green from all but the X0 via an isolated terminal bar in the panelboard. We will bond down the building steel with a #2. We will upsize the white to #10. We will then hope for trouble free, gauss-free, linear consumption, and minimal odd triplen harmonic overloading and circulation in our non-K rated transformer.
Any other suggestions???

How do I do a spellcheck???

Thank You

 

[jtester]

How many 225 amp 42 circuit panelboards can I supply from a single 75KVA transformer. If more than one, do the second and third set of derived secondary conductors origionate at X1, X2, X3 and X0 and subsequently terminate on three individual 225 amp main breaker in each of three panelboards??? The transformer has 100 amp primary protection and the 4/0secondaries meet the 25 foot rule.

Read Note 2 to Table 450.3(B)

Jim T

 

[bob]

I am assuming the transformer is 480/208 3 phase. Since the transformer has primary protection that does not exceed 125% of the rating, it appears that
note 1 applies as far as the transformer protection. The secondary conductors are covered in 240.21.C.

 

[jimberg1]

Read Note 2 to Table 450.3(B)

Jim T

Thanks for your reply.

Note 2 speaks to secondary overcurrent protection. Secondary overcurrent protection begins immediately as the derived conductors exit the transformer and before they enter the panelboard. The 225 Amp main breaker in the panelboard protects the panel bus bars, not the derived transformer secondaries.

My question is:

How many 225 amp 42 circuit panelboards can I supply from a single 75KVA transformer? If more than one, do the second and third set of derived secondary conductors originate at X1, X2, X3 and X0 and subsequently terminate on three individual 225 amp main breaker in each of three panelboards??? The transformer has 100 amp primary protection and the 4/0secondaries meet the 25 foot rule.

This installation relies heavily on the 100 Amp primary breaker to protect the transformer and derived secondaries from overload, short circuit and ground fault current. I am concerned that 675 Amps of load might be put upon this separately derived system in the future.

JBerg

 

[jimberg1]

I am assuming the transformer is 480/208 3 phase. Since the transformer has primary protection that does not exceed 125% of the rating, it appears that
note 1 applies as far as the transformer protection. The secondary conductors are covered in 240.21.C.

Thanks for your reply.

240.21-C-3-1 may or may not grant permission to impose a 675 Amp possible load on this separately derived system. That's my question!

Also is a commercial installation, by definition, an industrial application???

 

[infinity]

Our three-phase computer whips contain #12 black, red, light red, blue, white, white, green and green. I assume phases 1-2-3, green isolated ground, equipment grounding conductor and white grounded conductor. What is the additional white and light red for ???

Typically this arrangement would have one set of blk, red, blu, wht and grn the other set would be light red, wht, grn IG. The second set would used for so called computer circuits. This would not use a shared neutral.

 

[bob]

Thanks for your reply.
240.21-C-3-1 may or may not grant permission to impose a 675 Amp possible load on this separately derived system. That's my question!
Also is a commercial installation, by definition, an industrial application???

I assume someone did load calculations and chose a 75 kva transformer for the load. The primary breaker will not allow you to put 675 amps on the transformer. The fact that you installed 3 225 amps panels does not translate to 675 amps load at this time. Prior to that load being added, load measurements should reveal that the transformer is not large enough for the addition. If you did add the 675 amps the primary breaker would trip in about 40 sec. If you are concerned why not make a suggestion to install a 150 kva.

 

[W6SJK]

Iso Ground

Iso Ground

Not sure I understand your description. The second (isolated) ground can be isolated from the panelboard tub using an isolated ground bar but the iso gnd bar has to be connected to the transformer ground or neutral, which are bonded together, which are then grounded to a local electrode. (An iso ground is not really of much help when you are at the first panel downstream of the xfmr.) In other words don't connect this iso ground only to building steel. It has to go back to the ground at the source just like any other ground conductor. It's just isolated from all panelboard tubs along the way.

You might want to use an ohmmeter to see if either of these grounds in your whips are truly isolated from the receptacle strap or steel office partition and avoid the assumptions of which ground is which.

 

[jimberg1]

Not sure I understand your description. The second (isolated) ground can be isolated from the panelboard tub using an isolated ground bar but the iso gnd bar has to be connected to the transformer ground or neutral, which are bonded together, which are then grounded to a local electrode. (An iso ground is not really of much help when you are at the first panel downstream of the xfmr.) In other words don't connect this iso ground only to building steel. It has to go back to the ground at the source just like any other ground conductor. It's just isolated from all panelboard tubs along the way.

You might want to use an ohmmeter to see if either of these grounds in your whips are truly isolated from the receptacle strap or steel office partition and avoid the assumptions of which ground is which.

Sparkie,

Thanks for a helpful reply. You are right about the building steel and XO; I wish I could convince the "wet behind the ears" engineer of your line of thinking. Also the ohmeter is a great idea, but the equipment is not in place to take readings and we may never see it. If the future presents an opportunity for analysis, I'll let you know.

JimBerg

 

[jimberg1]

I assume someone did load calculations and chose a 75 kva transformer for the load. The primary breaker will not allow you to put 675 amps on the transformer. The fact that you installed 3 225 amps panels does not translate to 675 amps load at this time. Prior to that load being added, load measurements should reveal that the transformer is not large enough for the addition. If you did add the 675 amps the primary breaker would trip in about 40 sec. If you are concerned why not make a suggestion to install a 150 kva.

Bob,

Thanks for your reply,

This is a rewire, old tenant out, new tenant (Publix) in. Our third pnl supplies office computer equipment and much of the load in the two existing pnls has been eliminated. However, I cannot speak to future use, hence my concern. Actually the origional installation utilizes a 225 backfeed bkr to the first pnl and bus tap for the second. Have no idea why such a frail install.


JimBerg