Disconnect?

[Hoyt]

Quick question:

I have a 75kva xformer 480v prim with 120/208v sec. xformer is feeding a panelboard. Panelboard is located across the building not in the same room as the xformer.

Do I not need a disconnect on the secondary side of the xformer? NEC code referance?

I don't have an NEC where I currently am.

Thanks.

 

[iwire]

Your required to protect the secondary conductors. From your description I would say 240.21(C)(2) applies requiring overcurrent protection within 10' of secondary conductor.

 

[Hoyt]

Thats what was always my understanding...but the Owner is questioning it. Not sure why.

 

[dbnj49]

For this transformer you need an OCP on the secondary because you have a 3 wire primary and a 4 wire secondary. There are unbalanced loads. Here is how is looks mathematically. Had it been a 3 wire primary and 3 wire secondary then the primary OCP would also be able to protect the secondary.

power in = power out (transformer rule)
75000 VA = 480 X Ip, so the primary current is 156 amps,
From table 450.3(B) you would use a 400 amp CB (sized at 250%)
Ip X Ep = Is X Es, so 156 X 480 = Is X 120, so the secondary current is 625.
So if you didnt have secondary protection, then the secondary conductors would have to draw 625 amps in order for the primary breaker to open. So the primary OCP would not be sufficient to protect the secondary.

Ip X Ep = Is X Es Power in = Power out 75KVA Pri must have a 75KVA

 

[weressl]

For this transformer you need an OCP on the secondary because you have a 3 wire primary and a 4 wire secondary. There are unbalanced loads. Here is how is looks mathematically. Had it been a 3 wire primary and 3 wire secondary then the primary OCP would also be able to protect the secondary.

power in = power out (transformer rule)
75000 VA = 480 X Ip, so the primary current is 156 amps,
From table 450.3(B) you would use a 400 amp CB (sized at 250%)
Ip X Ep = Is X Es, so 156 X 480 = Is X 120, so the secondary current is 625.
So if you didnt have secondary protection, then the secondary conductors would have to draw 625 amps in order for the primary breaker to open. So the primary OCP would not be sufficient to protect the secondary.

Ip X Ep = Is X Es Power in = Power out 75KVA Pri must have a 75KVA

Incorrect reasoning. It is immaterial if it is a 3W or 4W secondary.

Based on table 450.3(B) if your protective device is rated 125% of the primary current - or the next highest standard protective device rating - no secondary protection is required. The conductors also would need to be sized to the same minimum current on the primary and the seciondary. The conductors are considered to be adequately protected this way.

 

[iwire]

The conductors also would need to be sized to the same minimum current on the primary and the secondary. The conductors are considered to be adequately protected this way.

I am not really following you.

Are you saying conductors on the secondary side of transformer described in the opening post

75kva xformer 480v prim with 120/208v sec

could be protected by the primary OCPD?

 

[weressl]

I am not really following you.

Are you saying conductors on the secondary side of transformer described in the opening post



could be protected by the primary OCPD?

Yes, if the protection and the wire is sized to 125%, it will protect the secondary conductors if they are sized appropriate to the ratio. As the previous poster had pointed out transformer is a device that does not change the power characteristics of the circuit. It changes the voltage and the current in direct and inverse realtionship to the winding ratio.

Power in = power out. (with slight ,3% losses ignored.)

Any overload that can occur over 125% happens due to the secondary connected loads misbehavior.

 

[iwire]

Yes, if the protection and the wire is sized to 125%, it will protect the secondary conductors if they are sized appropriate to the ratio.

From a electrical theroy standpoint I could never argue with you. :smile:

But from an NEC standpoint I can point out this.

240.21(C)(1) Protection by Primary Overcurrent Device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary to primary transformer voltage ratio.

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

(2002 wording, 2008 same or similar)

The OP has a delta-Wye transformer and the conductors on the secondary side must have overcurrent protection on the secondary side.

 

[weressl]

From a electrical theroy standpoint I could never argue with you. :smile:

But from an NEC standpoint I can point out this.

(2002 wording, 2008 same or similar)

The OP has a delta-Wye transformer and the conductors on the secondary side must have overcurrent protection on the secondary side.

True and you are correct.

However there are several exceptions to that rule too, isn't there?

 

[charlie b]

The notion of ?primary only protection? versus ?primary and secondary protection? has always confused me. The table?s Note 2 does not help, as it says ?if you need secondary protection, then you must . . . ,? but does not say when you need secondary protection. However, I have always understood the table as providing protection for the transformer windings themselves, not for the primary feeder or the secondary conductors.

In some cases, and this is not one of them, you can protect the secondary conductors (i.e., from the output of the transformer to the next load downstream) by using the same breaker or fuse that protects the primary feeder and that also protects the primary windings. We have a four wire secondary, so as Bob pointed out, you can?t protect the secondary windings with the primary breaker, even if you keep the setting below 125% of the rated current.

You must have a method of protecting the secondary conductors. Since you are beyond the scope of the 10 foot and 25 foot rules, you must have that protection at the point of origin of the conductors. That is, you must have an overcurrent device (not just a disconnecting means, but also overcurrent protection) within 10 feet of the transformer.

 

[don_resqcapt19]

True and you are correct.
However there are several exceptions to that rule too, isn't there?

There are no exceptions to 240.21(C)(1).

... Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

Note this has nothing to do with the protection of the secondary of the transformer itself...it only covers the protection of the secondary conductors.

 

[don_resqcapt19]

The notion of ?primary only protection? versus ?primary and secondary protection? has always confused me. The table?s Note 2 does not help, as it says ?if you need secondary protection, then you must . . . ,? but does not say when you need secondary protection. However, I have always understood the table as providing protection for the transformer windings themselves, not for the primary feeder or the secondary conductors.

Charlie,
The table is in Article 450 and only covers the protection of the transformer itself. Secondary protection for the transformer is required when ever the primary protection is greater than 125% of the rated primary current. Note 2 just tells you how you can provide the secondary protection when it is required by the table.