Fusing and wire size calculations

[bmore12586]

I am using a ABB drive part number is dcs800-s02-0350-05. My situation is i have a 93 KVA 460vac to 240 vac transformer. I have primary fuses which are 150 frs-r and fwp-500 on the secondary. The secondary fuse sizing comes from ABB and these are semiconductor fuses. The ABB documentation tells you to run 500 mcm or parallel 250 mcm on the secondary and the armature wiring for the motor. They are telling you to run a wire size to cover these fuses. The fuses are only sized this way to handle the in rush. My transformer can only put out 234 amps at 240 vac and the drive has a max output of 350 amps DC. Why do i need to run a wire size to cover these fuses when the only time it will ever see the 500 amp load is in a short circuit condition. In my mind i should be ok with sizing wire to the 350 amps dc. I believe that ABB is assuming that there is not any fusing on the primary side. Does anybody have any input and which code would this be handled under.

 

[Carultch]

I am using a ABB drive part number is dcs800-s02-0350-05. My situation is i have a 93 KVA 460vac to 240 vac transformer. I have primary fuses which are 150 frs-r and fwp-500 on the secondary. The secondary fuse sizing comes from ABB and these are semiconductor fuses. The ABB documentation tells you to run 500 mcm or parallel 250 mcm on the secondary and the armature wiring for the motor. They are telling you to run a wire size to cover these fuses. The fuses are only sized this way to handle the in rush. My transformer can only put out 234 amps at 240 vac and the drive has a max output of 350 amps DC. Why do i need to run a wire size to cover these fuses when the only time it will ever see the 500 amp load is in a short circuit condition. In my mind i should be ok with sizing wire to the 350 amps dc. I believe that ABB is assuming that there is not any fusing on the primary side. Does anybody have any input and which code would this be handled under.

It probably has to do with 240.21(C), for transformer secondary conductors.

There might be a rule specific to motor applications that overrides this. I'm not familiar with motors, so I do not know for sure. Motors change everything, for precisely the reason of inrush starting current.

 

[don_resqcapt19]

First per 430.122(A) the conductors on the line side of the drive must have an ampacity of at least 125% of the input current rating of the drive.

Second the conductors between the transformer secondary fuse must have an ampacity equal to or greater than the rating of the fuse. The "round-up" rule in 240.4(B) does not apply to these conductors, so the 500kcmil conductors are not large enough for the 500 amp fuses.

Is the transformer a 3 wire to 3 wire delta/delta transformer? Is so, you could use the primary OCPD to protect both the primary and secondary transformer windings as well as the secondary conductors as long as you size the conductors per 240.21(C)(1).

Note the FWP-500 amp fuse is a special purpose fuse and is not suitable to provide the required protection of the conductors between the transformer and the drive. The following is from the UL Guide Information for "Special Purpose Fuses-Component (JFHR2). That is the listing for those fuses.

The devices covered under this category are incomplete in certain constructional features or restricted in performance capabilities and are intended for use as components of complete equipment submitted for investigation rather than for direct separate installation in the field. THE FINAL ACCEPTANCE OF THE COMPONENT IS DEPENDENT UPON ITS INSTALLATION AND USE IN COMPLETE EQUIPMENT SUBMITTED TO UL.

 

[bmore12586]

The secondary fuses are not there to protect the wire from the transformer to the drive they are there to protect the SCR's in the drive per the manufacture. Per the manufacture they show those fuses in there drawing because they are assuming you are not using an isolation transformer.

 

[don_resqcapt19]

The secondary fuses are not there to protect the wire from the transformer to the drive they are there to protect the SCR's in the drive per the manufacture. Per the manufacture they show those fuses in there drawing because they are assuming you are not using an isolation transformer.

So these fuses are internal or part of the drive? What is protecting the transformer secondary conductors?

 

[bmore12586]

The fuses are not part of the drive but the primary of the transformer would see full load before the secondary ever saw the 500 amps.

 

[don_resqcapt19]

The fuses are not part of the drive but the primary of the transformer would see full load before the secondary ever saw the 500 amps.

The 150 FRS are the primary fuses. Is the transformer a 3 wire to 3 wire (delta/delta) transformer?

 

[bmore12586]

3 PHASE DELTA/WYE

 

[don_resqcapt19]

3 PHASE DELTA/WYE

Then you need overcurrent protection for the secondary conductors and the 500 amp fast acting fuses are not suitable for that purpose. You cannot use the primary OCPD to protect the secondary conductors on a delta/wye transformer.

 

[Jraef]

I... My situation is i have a 93 KVA 460vac to 240 vac transformer. ...

3 PHASE DELTA/WYE

Then you need overcurrent protection for the secondary conductors and the 500 amp fast acting fuses are not suitable for that purpose. You cannot use the primary OCPD to protect the secondary conductors on a delta/wye transformer.

It is 480-240V, there is no way it could be Delta-Wye. It is most likely a Delta-Delta 3 phase 3 wire "Drive Isolation Transformer", they are specifically designed for this singular purpose. That's why it is rated for an oddball 93kVA. This is not a distribution transformer, it is a dedicated part of the drive system. The primary fuses are protecting the entire circuit.


As was said, those fuses are for the protection of the drive thyristors, not the conductors. The UL listing of the drive will require those exact fuses be used.

 

[bmore12586]

It is a drive isolation transformer but it is a 460/240 delta wye transformer. Do you know if there are any exceptions on wire size for these fast acting fuses? Seems crazy to protect for 500 amps when the circuit cant ever produce this amperage.

 

[jim dungar]

It is 480-240V, there is no way it could be Delta-Wye.

Why could it not be a 480-240Y/138?


The OP needs to say what the nameplate wiring diagram shows for winding connections.

 

[don_resqcapt19]

It is 480-240V, there is no way it could be Delta-Wye. It is most likely a Delta-Delta 3 phase 3 wire "Drive Isolation Transformer", they are specifically designed for this singular purpose. That's why it is rated for an oddball 93kVA. This is not a distribution transformer, it is a dedicated part of the drive system. The primary fuses are protecting the entire circuit.


As was said, those fuses are for the protection of the drive thyristors, not the conductors. The UL listing of the drive will require those exact fuses be used.

Jeff,
I have installed a number of 460/230 delta/wye drive isolation transformers (in fact I don't think I have ever installed a delta/delta drive isolation transformer). There is no reason why it can't be a delta/wye with those voltages. It only means that you don't have 120 volts line to neutral. You have 132 if the secondary is 230 line to line and 138 if the secondary is 240. The following is from one transformer manufacturer.

SECTION 4

STANDARD DRIVE ISOLATION TRANSFORMERS

208 Delta Primary Volts 480Y Secondary Volts

230 Delta Primary Volts 230Y Secondary Volts

460 Delta Primary Volts 230Y Secondary Volts

Other primary and secondary voltages available are 240, 380 and 600.

© Hammond Power Solutions Inc. Data subject to change without notice.

 

[don_resqcapt19]

It is a drive isolation transformer but it is a 460/240 delta wye transformer. Do you know if there are any exceptions on wire size for these fast acting fuses? Seems crazy to protect for 500 amps when the circuit cant ever produce this amperage.

You have to provide other fuses to protect the secondary conductors. You don't have a choice since the transformer is delta/wye. The code only requires that the conductors that supply the drive have an ampacity of at least 125% of the rated input current, not of the fuses that are used to protect the drive. These conductors must be protected with a fuse that is suitable for use on a feeder or branch circuit.

 

[bmore12586]

where would i find the codes for the delta/wye fusing of a branch or feeder circuit in the nec

 

[don_resqcapt19]

where would i find the codes for the delta/wye fusing of a branch or feeder circuit in the nec

240.21(C), but since you have a delta/wye transformer you can't use 240.21(C)(1).
Note with transformers there are two sets of overcurrent protection rules. The ones in Article 450 (450.3) apply to the protection of the transformer itself and the ones in Article 240 apply to the protection of the transformer conductors. You have to satisfy both sets of rules, but often one OCPD can serve more than one function. For example where Article 450 requires transformer secondary protection, the OCPD that protects the secondary conductors can also protect the transformer secondary if correct sized.

 

[bmore12586]

great all this helps but i still can not find where it says that Fwp fuses can not be used for secondary protection of a transformer. Anybody?

 

[don_resqcapt19]

great all this helps but i still can not find where it says that Fwp fuses can not be used for secondary protection of a transformer. Anybody?

That is a 110.3(B) issue. See my quote from the UL Guide Information in post #3.

 

[Smart $]

great all this helps but i still can not find where it says that Fwp fuses can not be used for secondary protection of a transformer. Anybody?

Transformer secondary protection is never required... but commonplace where secondary conductors must have overcurrent protection. Where transformer secondary protection is used, the maximum rating is 125% of secondary rated current 9A or greater.

If you do not use transformer secondary protection, the 125% maximum overcurrent protection shifts to the primary protection rating for primary rated current 9A or greater. Your primary is protected with a 150A fuse, which is at the 125% maximum [116.7A × 125% = 145.9A rounded up per Note 1 to Table 450.3(B)]. As such, you do not need to add transformer secondary protection.

However, this does not alleviate the requirement for secondary conductor protection. If you use 500's, the maximum fuse rating you can use is 400A. If you use parallel 250's your conductors would be considered protected by the 500A fuses as long as derating doesn't take the ampacity lower than 451A (or 225.5A each).

And you want to go smaller...??? You feasibly can, but then you are faced with series fusing to provide NEC compliant conductor protection (actually you have that problem using 500's anyway).

 

[Jraef]

Why could it not be a 480-240Y/138?


The OP needs to say what the nameplate wiring diagram shows for winding connections.

What can I say. I must still be suffering from the effects of a raucous 4rth party... :dunce::slaphead: