OCP Question for an Autotransformer

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Little Bill

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We are having a circuit run for a new machine. The machine requires 230V 3pf and we only have 208 3ph. The EC (his crew) is installing a field installed autotransformer, using 2 transformers. They will be fed from a panel right next to them, and then feed straight to the machine. They planned on using an 80A breaker for OCP. This was based on info that was given them before the machine arrived. They had wired another smaller machine we have a few years back and was given the same specs as was used then. I was looking at the wiring diagram and requirements for the 208V-230V configuration. It said to fuse (or breaker) the primary side at 125V. I believe the paper said under this set up it would be 30KVA open Delta. I know from the code in Table 450.3(B) that the primary protection should be 125% of the transformer rated current. I'm not very good with transformers but I know that 125% is more than the 80A they were planning on using for OCP. I kindly pointed out to the lead man the info on the wiring diagram, and if that is correct, then the conductor size would also need to be sized accordingly. I don't think I made him very happy or at least his boss. He called his boss and told him what I showed him and his boss said well the machine only needs 80A protection. That would be true had we not had to use a transformer to get that voltage. I didn't tell him he had to do anything nor did I argue with him. I told him I was simply pointing out what is on the transformer requirements. Does what I pointed out sound correct or am I way off base?
 

jim dungar

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You are dealing with a 'buck-boost transformer' configuration.
The 125% OCPD is a maximum limit, it is perfectly acceptable, although maybe not ideal, to use a smaller device.
 

Little Bill

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You are dealing with a 'buck-boost transformer' configuration.
The 125% OCPD is a maximum limit, it is perfectly acceptable, although maybe not ideal, to use a smaller device.


Thanks Jim! Why does the configuration chart in the transformer papers say to use 125A breaker?
 

jim dungar

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Thanks Jim! Why does the configuration chart in the transformer papers say to use 125A breaker?
Most likely, their experience has been that a 125A is needed to handle the inrush of the transformer and/or to be able to provide 100% of the transformer's capacity.
 

Little Bill

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Thanks Jim! Why does the configuration chart in the transformer papers say to use 125A breaker?

I failed to mention that the instructions on wiring this configuration referenced
Table 450.3(B). After going back and looking at 450.4(A) it says "not more than 125%" Is that what Jim meant when he said a lower rated OCP would be ok? I guess I'm asking if the 80A breaker is sufficient. I don't know if you consider the machine as a continuous load or if that even applies, but calculating from the nameplate the machine is 29kva which at 230V 3 ph would be 72.80A. So if you had to apply the cont. load rule of 125% that would be 91A. Now the 80A OCP is in question. Any further help out there?
 

Little Bill

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Most likely, their experience has been that a 125A is needed to handle the inrush of the transformer and/or to be able to provide 100% of the transformer's capacity.

Thanks Jim. You and I must have been posting at the same time. I didn't find your 2nd post until today. If the machine is run 8-16 hrs. a day does it need to be considered a continuous load, or in your opinion is the 80A OCP sufficient?
 

hurk27

Senior Member
I failed to mention that the instructions on wiring this configuration referenced
Table 450.3(B). After going back and looking at 450.4(A) it says "not more than 125%" Is that what Jim meant when he said a lower rated OCP would be ok? I guess I'm asking if the 80A breaker is sufficient. I don't know if you consider the machine as a continuous load or if that even applies, but calculating from the nameplate the machine is 29kva which at 230V 3 ph would be 72.80A. So if you had to apply the cont. load rule of 125% that would be 91A. Now the 80A OCP is in question. Any further help out there?

You have a slight problem in the math.

the 72.8 amps is after the buck/boost transformers, you have to calculate the amperage at the breaker @208 volts to know how to size the breaker.

29kva/208/1.732 is 80.5 amps * 1.25 ends up with 100.62 amp breaker, so you need to wire for the load (80.5a) and breaker for the start up (100.62) next size up breaker 125 amp.
 

Little Bill

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Tennessee NEC:2017
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We are having a circuit run for a new machine. The machine requires 230V 3pf and we only have 208 3ph. The EC (his crew) is installing a field installed autotransformer, using 2 transformers. They will be fed from a panel right next to them, and then feed straight to the machine. They planned on using an 80A breaker for OCP. This was based on info that was given them before the machine arrived. They had wired another smaller machine we have a few years back and was given the same specs as was used then. I was looking at the wiring diagram and requirements for the 208V-230V configuration. It said to fuse (or breaker) the primary side at 125V. I believe the paper said under this set up it would be 30KVA open Delta. I know from the code in Table 450.3(B) that the primary protection should be 125% of the transformer rated current. I'm not very good with transformers but I know that 125% is more than the 80A they were planning on using for OCP. I kindly pointed out to the lead man the info on the wiring diagram, and if that is correct, then the conductor size would also need to be sized accordingly. I don't think I made him very happy or at least his boss. He called his boss and told him what I showed him and his boss said well the machine only needs 80A protection. That would be true had we not had to use a transformer to get that voltage. I didn't tell him he had to do anything nor did I argue with him. I told him I was simply pointing out what is on the transformer requirements. Does what I pointed out sound correct or am I way off base?

You have a slight problem in the math.

the 72.8 amps is after the buck/boost transformers, you have to calculate the amperage at the breaker @208 volts to know how to size the breaker.

29kva/208/1.732 is 80.5 amps * 1.25 ends up with 100.62 amp breaker, so you need to wire for the load (80.5a) and breaker for the start up (100.62) next size up breaker 125 amp.

Thanks Hurk,
I did think that, don't know what I was thinking on that post. I even said as much in my original post. I don't like to look over anyone's shoulder but the EC's crew ask me to give them a hand and showed me the transformer's diagram for the set-up we have. That's when I noticed the 125A OCP. They haven't pulled the wire yet, so wire sized for the load plus 125%, would that be correct?
 

hurk27

Senior Member
Thanks Hurk,
I did think that, don't know what I was thinking on that post. I even said as much in my original post. I don't like to look over anyone's shoulder but the EC's crew ask me to give them a hand and showed me the transformer's diagram for the set-up we have. That's when I noticed the 125A OCP. They haven't pulled the wire yet, so wire sized for the load plus 125%, would that be correct?

Yes you would size your wire per load (80.5a) (240.4(G) ) and size breaker for the required 125% (100.62a) so you end up with a #4 copper on a 125 amp breaker.

Now if this equipment has its own OCPD's then these are feeders so you will have to size the wire per the breaker.

I hope the equipment dosen't have 120 volt loads coming from the boosted side of the transformers?
 
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Little Bill

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Yes you would size your wire per load (80.5a) (240.4(G) ) and size breaker for the required 125% (100.62a) so you end up with a #4 copper on a 125 amp breaker.

Now if this equipment has its own OCPD's then these are feeders so you will have to size the wire per the breaker.

I hope the equipment dosen't have 120 volt loads coming from the boosted side of the transformers?

The only other voltage I know of is from an internal transformer supplying 24V from the input 230V for control relays and temperature controller. There are some breakers in the control cabinet but I think they are overload/temperature protectors. I will check for sure tomorrow.
 

Smart $

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Location
Ohio
Yes you would size your wire per load (80.5a) (240.4(G) ) and size breaker for the required 125% (100.62a) so you end up with a #4 copper on a 125 amp breaker.
Hmmmm.... I believe the installation must still meet the requirements of 240.4(F) which permits the installation to be protected by the primary ocpd meeting 450.3 but cannot exceed secondary conductor ampacity multiplied by the secondary to primary voltage ratio. Working that backwards from a 125A ocpd: 125A ? (230V ? 208V) = 113A. Looks like #2 copper is required... or secondary ocpd to run #4 after the secondary ocpd.
 

Little Bill

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Hmmmm.... I believe the installation must still meet the requirements of 240.4(F) which permits the installation to be protected by the primary ocpd meeting 450.3 but cannot exceed secondary conductor ampacity multiplied by the secondary to primary voltage ratio. Working that backwards from a 125A ocpd: 125A ? (230V ? 208V) = 113A. Looks like #2 copper is required... or secondary ocpd to run #4 after the secondary ocpd.

Where does 450.4 for Autotransformers come into play?

The EC's crew did go ahead and pull #2 wire, but had to order the breaker. They didn't pull that size wire for the reasons given in 240.4 but just for the ampacity of the 125A breaker. I don't think they understand transformers anymore than I do. I know the lead man didn't. I had to read the wiring diagram to him for him to make the connections. They also didn't use a trough or gutter to make the connections. You should have seen them trying to stuff those large conductors into the small space in the bottom of the 2 boost transformers, very sad!:confused:
 

david luchini

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Hmmmm.... I believe the installation must still meet the requirements of 240.4(F) which permits the installation to be protected by the primary ocpd meeting 450.3 but cannot exceed secondary conductor ampacity multiplied by the secondary to primary voltage ratio. Working that backwards from a 125A ocpd: 125A ? (230V ? 208V) = 113A. Looks like #2 copper is required... or secondary ocpd to run #4 after the secondary ocpd.

I don't see that 240.4(F) applies. There isn't a delta-delta connected three phase transformer in the buck-boost arrangement, and there aren't really any transformer secondary conductors. In fact, one of the 208V input conductors is connected directly to the 230V output, while the other two inputs are connected in series with the low voltage windings of the 2 transformers to the 230V output.

If the buck-boost transformer is 30kVA as indicated in the OP, then the 125A c/b would violate 450.4(A)...30kVA @208V = 83.27 rated input current...83.27 * 1.25 = 104A, max allowable c/b would be 110A (next standard size.)

However, it is more likely that the open delta buck-boost is achieved using two 2kVA, 240-24V transformers which would give a 33kVA rating to the autotransformer. For a 33kVA autotransformer, 125A would be the max. allowable c/b.
 

Smart $

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I don't see that 240.4(F) applies. There isn't a delta-delta connected three phase transformer in the buck-boost arrangement, and there aren't really any transformer secondary conductors. In fact, one of the 208V input conductors is connected directly to the 230V output, while the other two inputs are connected in series with the low voltage windings of the 2 transformers to the 230V output.

...
Where does 240.4(F) state what type of transformer arrangement is required?

If the arrangement does not qualify under 240.4(F) as secondary conductors being protected by primary ocpd, then secondary ocpd is required. Is this what you are trying to say?

However, from my perspective the setup can be described as such:
Primary is delta 3-wire, single voltage.
Secondary is delta 3-wire, single voltage.​
I see that as meeting the criteria of 240.4(F) allowing secondary conductors to be protected by the primary ocpd (...and it does not matter that one secondary conductor is directly connected to a primary conductor).
 

Little Bill

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I don't see that 240.4(F) applies. There isn't a delta-delta connected three phase transformer in the buck-boost arrangement, and there aren't really any transformer secondary conductors. In fact, one of the 208V input conductors is connected directly to the 230V output, while the other two inputs are connected in series with the low voltage windings of the 2 transformers to the 230V output.

If the buck-boost transformer is 30kVA as indicated in the OP, then the 125A c/b would violate 450.4(A)...30kVA @208V = 83.27 rated input current...83.27 * 1.25 = 104A, max allowable c/b would be 110A (next standard size.)

However, it is more likely that the open delta buck-boost is achieved using two 2kVA, 240-24V transformers which would give a 33kVA rating to the autotransformer. For a 33kVA autotransformer, 125A would be the max. allowable c/b.

Hi David, Thanks for chiming in. You are exactly right on the autotransformer set up. I tried to put the info in my OP from memory and got it wrong. The info on the transformers is:
120/240
12/24
2.0 KVA
The info on the 208 to 230 configuration is:
open delta
33.1 KVA
83.3 load Amps
125A fuse(or breaker)


Where does 240.4(F) state what type of transformer arrangement is required?

If the arrangement does not qualify under 240.4(F) as secondary conductors being protected by primary ocpd, then secondary ocpd is required. Is this what you are trying to say?

However, from my perspective the setup can be described as such:
Primary is delta 3-wire, single voltage.
Secondary is delta 3-wire, single voltage.​
I see that as meeting the criteria of 240.4(F) allowing secondary conductors to be protected by the primary ocpd (...and it does not matter that one secondary conductor is directly connected to a primary conductor).

Now Smart$, both you and David are out of my league. I'll sit back and see what you 2 come up with. I've wired up transformers before, but everything was in place and I just made the connections. I'm hoping to learn something here.:)

Oh, I just noticed the new icons!
 

david luchini

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Where does 240.4(F) state what type of transformer arrangement is required?

Here, 240.4(F) "Conductors supplied by the secondary side of...a three phase, delta-delta connected transformer having a 3-wire secondary, shall be permitted to be protected by the overcurrent protection provided on the primary..."

If the arrangement does not qualify under 240.4(F) as secondary conductors being protected by primary ocpd, then secondary ocpd is required. Is this what you are trying to say?

No, I'm saying the circuit breaker on the input side of the buck-boost transformer protects the conductors on both the input side and the output side of the buck-boost transformer. The input and output conductors must both have an ampacity properly protected by the 125A c/b in the OP, ie, #2 in, #2 out. There is no "primary" and "secondary" to the transformer, there is an input and an output. If the primary and secondary windings of the transformer are interconnected, which winding is primary and which is secondary?

However, from my perspective the setup can be described as such:
Primary is delta 3-wire, single voltage.
Secondary is delta 3-wire, single voltage.​
I see that as meeting the criteria of 240.4(F) allowing secondary conductors to be protected by the primary ocpd (...and it does not matter that one secondary conductor is directly connected to a primary conductor).

You can describe the setup as you have, but 240.4(F) says that you must have a 3 phase delta-delta connected transformer. If you connect Input Line 1 (IL1) to H2 and X1 of the first transformer, IL2 to H1 of both transformers, IL3 to H2 and X1 of the second transformer; and you connect Output Line 1 (OL1) to X2 of the first transformer, OL2 to H1 of both transformers, and OL3 to X2 of the second transformer (sketch it out,) I don't see anything that looks like a delta-delta connected transformer. (Nor, for that matter, do I see how can describe a primary or secondary side of the transformer, since the "primary" and "secondary" windings are interconnected.)
 

Smart $

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Here, 240.4(F) "Conductors supplied by the secondary side of...a three phase, delta-delta connected transformer having a 3-wire secondary, shall be permitted to be protected by the overcurrent protection provided on the primary..."



No, I'm saying the circuit breaker on the input side of the buck-boost transformer protects the conductors on both the input side and the output side of the buck-boost transformer. The input and output conductors must both have an ampacity properly protected by the 125A c/b in the OP, ie, #2 in, #2 out. There is no "primary" and "secondary" to the transformer, there is an input and an output. If the primary and secondary windings of the transformer are interconnected, which winding is primary and which is secondary?



You can describe the setup as you have, but 240.4(F) says that you must have a 3 phase delta-delta connected transformer. If you connect Input Line 1 (IL1) to H2 and X1 of the first transformer, IL2 to H1 of both transformers, IL3 to H2 and X1 of the second transformer; and you connect Output Line 1 (OL1) to X2 of the first transformer, OL2 to H1 of both transformers, and OL3 to X2 of the second transformer (sketch it out,) I don't see anything that looks like a delta-delta connected transformer. (Nor, for that matter, do I see how can describe a primary or secondary side of the transformer, since the "primary" and "secondary" windings are interconnected.)
Compelling argument(s), but I don't buy it.

For one, yes, 240.4(F) says delta-delta connected... but it does not say delta-delta configured, as in the windings must be connected in delta fashion... and what about two isolation transformers connected in open delta fashion? The winding configuration would not actually be a delta connection either. A delta connection can simply be described as three connections with equal voltage between connections and having a 120? out-of-phase relationship to each other.

As for primary and secondary, the terms are relative to the connections and not the windings themselves. If you take an ordinary 3? step-down transformer and connect it in reverse, are the X terminations still secondary connections? Are the H terminations still primary connections?

Buck-boost transformers, though field-connected as autotransformers still have two windings like isolation transformers and can even be used as such if so desired. When connected as isolation transformers, they have primary and secondary winding connection terminals. Just because a jumper is added from one primary terminal to one secondary terminal to form an autotransformer does not preclude the transformer(s) from having primary and secondary connections (IMO).

Perhaps an issue for agreeing to disagree. Ultimately not our decision for an actual installation...

That said, I'm curious what others have to say on the matter...???
 
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david luchini

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For one, yes, 240.4(F) says delta-delta connected... but it does not say delta-delta configured, as in the windings must be connected in delta fashion... and what about two isolation transformers connected in open delta fashion? The winding configuration would not actually be a delta connection either. A delta connection can simply be described as three connections with equal voltage between connections and having a 120? out-of-phase relationship to each other.

I don't think I can agree with the distinction you are making between connected and configured. Using you description of a delta connection, if I had a 240 Delta service and had a 208V, 3ph machine to install, and decided to use a 240 Delta-208/120V Wye transformer to stepdown to 208V for my machine, but only made 3 connections from the transformer to machine (120? out-of-phase relationship to each other,) I'd have a delta - delta connected transformer. I don't buy that.

As for primary and secondary, the terms are relative to the connections and not the windings themselves. If you take an ordinary 3? step-down transformer and connect it in reverse, are the X terminations still secondary connections? Are the H terminations still primary connections?

Buck-boost transformers, though field-connected as autotransformers still have two windings like isolation transformers and can even be used as such if so desired. When connected as isolation transformers, they have primary and secondary winding connection terminals. Just because a jumper is added from one primary terminal to one secondary terminal to form an autotransformer does not preclude the transformer(s) from having primary and secondary connections (IMO).

I agree with your description of primary and secondary connections (in the case of an isolating transformer) but the problem, it seems to me, is that with the autotransformer one of your connections would be both primary and secondary...it is the same connection. I think this is where "primary" and "secondary" descriptions break down with autotransformers, and IMO, is why section 450.4 uses the descriptions of "input" and "output" for autotransformer, rather than primary and secondary.
 
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