Multiple secondary feeds from Xfmr

[Mino1313]

My drawings are indicating a feeder 180 feet from a 400amp bus plug fused at 350amps in elec room-A to a 150 kva transformer in elec room -B. Then shows (4) separate secondary feeds to 4 separate sub- panels with (4) separate 100 amp mains in each panel. My question is there any code violations tapping transformer 4 times on the secondary X0,X1,X2,X3.

 

[Joe Villani]

No issues. 240.21(C), but depending on the length of the secondaries you might need to upsize the secondary conductors.

 

[augie47]

Allowed by 240.21(C) with a limit of 6 sets per 450.3

 

[packersparky]

Allowed by 240.21(C) with a limit of 6 sets per 450.3

That is only true if secondary protection is required, the way it read it. If the primary overcurrent protection is 125% or less (current is over 9 amperes), secondary overcurrent protection not is required. So Note 2 to the table, which limits the secondary protection to not more that 6 circuit breakers or sets of fuses when secondary protection is required, would not apply, correct?

 

[augie47]

That is only true if secondary protection is required, the way it read it. If the primary overcurrent protection is 125% or less (current is over 9 amperes), secondary overcurrent protection not is required. So Note 2 to the table, which limits the secondary protection to not more that 6 circuit breakers or sets of fuses when secondary protection is required, would not apply, correct?

Good point. (I have never seen more than 6 in either case so I never questioned it)
Since 408.36 requires panel protection would that come into play ?

 

[infinity]

That is only true if secondary protection is required, the way it read it. If the primary overcurrent protection is 125% or less (current is over 9 amperes), secondary overcurrent protection not is required. So Note 2 to the table, which limits the secondary protection to not more that 6 circuit breakers or sets of fuses when secondary protection is required, would not apply, correct?

In what scenario would secondary protection not be required for a 3 phase, 150 kva transformer?

 

[david luchini]

In what scenario would secondary protection not be required for a 3 phase, 150 kva transformer?

A 150kVA, 480-208/120V transformer with a 225A primary OCPD.

 

[don_resqcapt19]

One of the issues with multiple sets of transformer conductors becomes the minimum conductor size required by 240.21(C)(6)(1).
With the 125% primary protection shown in the previous post, the minimum secondary conductor ampacity would be 173 amps. If you are running multiple sets, you may be supplying loads substantially less than 173 amp.

 

[wwhitney]

One of the issues with multiple sets of transformer conductors becomes the minimum conductor size required by 240.21(C)(6)(1).
With the 125% primary protection shown in the previous post, the minimum secondary conductor ampacity would be 173 amps. If you are running multiple sets, you may be supplying loads substantially less than 173 amp.

There's always 240.21(C)(2) if you can locate secondary conductor OCPD within 10' of the transformer. That would reduce your minimum secondary conductor ampacity by a factor of 3/10.

Cheers, Wayne

P.S. I sill don't see why the common interpretation of 240.21(C)(6) for these 25' secondary conductors permits a 5.2x overload factor, when 240.21(B)(2) for 25' feeder taps limits the overload factor to 3.0x

 

[OldBroadcastTech]

One of the issues with multiple sets of transformer conductors becomes the minimum conductor size required by 240.21(C)(6)(1).
With the 125% primary protection shown in the previous post, the minimum secondary conductor ampacity would be 173 amps. If you are running multiple sets, you may be supplying loads substantially less than 173 amp.

Pardon me for butting in here...I'm not a 'real' electrician, nor do I play on on TV, BUT.........

Why conductor ampacity of 173 amps if each sub-panel has a 100 amp main breaker ?

Or is this one of those 'because the NEC says so' things ?

Again, my apologies for posting what may be a stupid question

Thank you all for what you do, I learn a lot that I will no doubt never use...........

 

[don_resqcapt19]

Pardon me for butting in here...I'm not a 'real' electrician, nor do I play on on TV, BUT.........

Why conductor ampacity of 173 amps if each sub-panel has a 100 amp main breaker ?

Or is this one of those 'because the NEC says so' things ?

Again, my apologies for posting what may be a stupid question

Thank you all for what you do, I learn a lot that I will no doubt never use...........

Because the line side overcurrent protection is 225 * (480/208) = 519 amps. One third of that is the minimum secondary conductor ampacity using the 25' secondary conductor rule. 519/3=173.
This is an attempt to make sure that a fault on the secondary conductors that are between the transformer and the first overcurrent protective device are large enough to flow enough current so that the 225 amp primary OCPD opens the circuit before the conductors themselves act as fuses.

 

[wwhitney]

Because the line side overcurrent protection is 225 * (480/208) = 519 amps. One third of that is the minimum secondary conductor ampacity using the 25' secondary conductor rule. 519/3=173.

Just to reiterate, 519A is the equivalent secondary protection for an L-L fault. For an L-N fault, the equivalent secondary protection is 900A, as the transformer turns ratio is 4:1.

This is an attempt to make sure that a fault on the secondary conductors that are between the transformer and the first overcurrent protective device are large enough to flow enough current so that the 225 amp primary OCPD opens the circuit before the conductors themselves act as fuses.

Seems like we should consider the worst case of L-L fault or L-N fault, then.

Cheers, Wayne

 

[jim dungar]

Just to reiterate, 519A is the equivalent secondary protection for an L-L fault. For an L-N fault, the equivalent secondary protection is 900A, as the transformer turns ratio is 4:1.


Seems like we should consider the worst case of L-L fault or L-N fault, then.

You are free to do this with your designs, but it is not how the NEC has been applied for probably the past 100 years.

 

[wwhitney]

You are free to do this with your designs, but it is not how the NEC has been applied for probably the past 100 years.

A version of 240.21(B)(3) (Taps Supplying a Transformer, Primary plus Secondary less than 25' Long) first appeared in the 1971 NEC as 240-15 Exception 7. 240.21(C)(2) (Transformer Secondary Conductors, < 10' Long) and 240.21(C)(3) (Transformer Secondary Conductors, Industrial Establishments, < 25' Long) first appeared in the 1999 NEC. And 240.21(C)(6) (Transformer Secondary Conductors, < 25' Long) first appeared in the 2002 NEC.

So 20-50 years.

I'm still hoping that someone will come up with a technical reason that the "overload factor" (5.2 times) allowed for a delta-wye transformer under the 240.21(C)(6) 25' secondary conductor rule should be larger than the "overload factor" (3.0 times) allowed under the 240.21(B)(2) 25' feeder tap rule. While I understand that the choice of the factor of 3 in writing 240.21(B)(2) is somewhat arbitrary, it just seems unreasonably arbitrary to apply 240.21(C)(6) in a way that allows an even higher overload factor.

Cheers, Wayne

 

[jim dungar]

Prior to the 1971 version, I don't think there were any limitations at all, so how long have transformers been used?
When they decided to add something, the L-L voltage ratio was apparently an acceptable value.

 

[Jpflex]

Allowed by 240.21(C) with a limit of 6 sets per 450.3

If paralleling the secondary into 4 separate panels at 100 amp main breakers each, would not the sum of the total parallel secondary main breakers not be allowed to exceed the total maximum secondary OCPD? i did not see the OP mention primary/secondary voltages or clarify if this is single or 3 phase so i cannot calculate the max secondary OCPD

 

[don_resqcapt19]

....

I'm still hoping that someone will come up with a technical reason that the "overload factor" (5.2 times) allowed for a delta-wye ,,,

Cheers, Wayne

There is no "overload" factor in this. The upstream devices are only providing short circuit and ground fault protection. The overload protection is provided at the load end of the feeder tap or secondary conductor.

 

[don_resqcapt19]

If paralleling the secondary into 4 separate panels at 100 amp main breakers each, would not the sum of the total parallel secondary main breakers not be allowed to exceed the total maximum secondary OCPD? i did not see the OP mention primary/secondary voltages or clarify if this is single or 3 phase so i cannot calculate the max secondary OCPD

If the transformer primary protection exceeds 125% of the primary rate current, then the sum of the protective devices on the secondary are not permitted to exceed 125% of the rated secondary current.
Where the primary protection is 125% or less, there is no limit on the sum of the ratings of the secondary protective devices as no secondary protection is required.

 

[wwhitney]

There is no "overload" factor in this. The upstream devices are only providing short circuit and ground fault protection. The overload protection is provided at the load end of the feeder tap or secondary conductor.

By "overload factor" I mean the ratio of "the actual upstream OCPD rating" to "what the upstream OCPD rating would need to be to provide overload protection" for the feeder tap or secondary conductor. If you prefer we could call it the "OCPD oversizing factor" or the "underampacity factor" or anything else appropriate. For now I'll call it the factor.

240.21(B)(2) clearly sets the maximum allowable factor as 3.0 for 25' of feeder tap conductor.. So it makes no sense to me that the common interpretation of 240.21(C)(6) results in a factor of 5.2 for 25' of delta-wye secondary conductor. Particularly as the transformer itself is an additional impedance, which would suggest lowering the allowable impedance of the secondary conductors by using a smaller factor than 3.0, rather than increasing the allowable factor.

Cheers, Wayne

 

[don_resqcapt19]

By "overload factor" I mean the ratio of "the actual upstream OCPD rating" to "what the upstream OCPD rating would need to be to provide overload protection" for the feeder tap or secondary conductor. If you prefer we could call it the "OCPD oversizing factor" or the "underampacity factor" or anything else appropriate. For now I'll call it the factor.

240.21(B)(2) clearly sets the maximum allowable factor as 3.0 for 25' of feeder tap conductor.. So it makes no sense to me that the common interpretation of 240.21(C)(6) results in a factor of 5.2 for 25' of delta-wye secondary conductor. Particularly as the transformer itself is an additional impedance, which would suggest lowering the allowable impedance of the secondary conductors by using a smaller factor than 3.0, rather than increasing the allowable factor.

Cheers, Wayne

I know what you mean, but I am just taking issue with the use of the defined term "overload" as it is not used per its definition in these comments.
There is no overload protection being provided for these conductors other than by the device at the load end of the conductors.