Hello everyone. I would like to know if I am completing this task per the NEC. I have a 220V system and Hermetic refrigeration compressors that I would like to buck down to 200V. The amp draw for the 2 compressors are 18 amps each, plus a 1.5 amp condenser fan. I have found the correct bucking transformer (9KVA) and now need to make sure I am fusing it correctly. Would I size the primary side of my transformer normally (9KVA/220V = 40.9 * 1.67=68, so rounding down to 60A), or would I size it per the motor loads? Should the entire transformer branch, since it is a buck boost transformer, be considered an individual branch circuit or can I treat it as a normal transformer circuit. My initial approach is to size the primary side of the transformer, and then size individual branch protection for each compressor (18*1.75 rounding down is 30A each). So in the end, I have two 60 amp fuses protecting my primary, and 2 30 A fuses protecting each compressor (4 fuses total). Please let me know if I need to clarify anything, and I appreciate the help.
Buck Boost Transformer and Compressor
- Thread starter fifty60
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Have you gone to the B-B xfmr mfg web sites? I know that Eaton Electrical, SqD, Acme as well as other make it fairly simple when selecting a B-B transformer such as your application. The will also include the correct wiring diagram to use.
Art 450 of the NEC includes the correct means of providing over current protection also. The incorrect placement of an OCPD con prove to be catastrophic and as such the wiring diagram as well as Art 450 must be followed.
ophic.
I understand how to fuse the primary. My question is regarding the prohibition of shunt winding fuses. Looking at Figure 450.4 in the NEC 2011 rule book, does this mean that I cannot fuse the load that is being supplied from the taps at A and B? Or does it mean that I cannot put fuse on the actual winding of A and B? Should I only fuse the primary side? I have a multi motor load on the secondary of the BB transformer. I would like to fuse them separately. They already have internal thermal overload protection, but I would also like to fuse the conductors supplying the compressors. Can I fuse the loads on the secondary side of the BB transformer per NEC 450.4? Thanks.
It appears you don't...
You mention 450.4, so I assume no have no problem understanding what it says the max ocpd rating can be... yet your first post doesn't demonstrate that...:happysad:
Smart $ you are correct. For auto-transformer, Per 450.4, I need to use 125% of the primary side current, which rounding up will give me the 60A. This is definitely different than a sub 600V isolation transformer, which % wise differs regarding what is on the secondary. What about the secondary of the auto transformer? If I have branch motor loads on the secondary of the BB transformer can I fuse conductors (compressors already internally thermally protected) or do I have to size them to the primary OCPD?
- Location
- Wisconsin
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- PE (Retired) - Power Systems
Imagine your B-B transformers being properly connected and installed in a single box.
There are three primary leads into the box, these need protection per 450.4.
There are three secondary conductors leaving the box, these need protection per 240.21(C).
Per 450.4 no fuses are allowed internal to the 'box'.
There are three primary leads into the box, these need protection per 450.4.
There are three secondary conductors leaving the box, these need protection per 240.21(C).
Per 450.4 no fuses are allowed internal to the 'box'.
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- Location
- Bremerton, Washington
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- Master Electrician
220V is not a standard secondary voltage. Is it really 220 or is it 208 or 240?
curt swartz
Senior Member
- Location
- San Jose, CA
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- Electrical Contractor
Would you please share some more info about this install.
As Tom mentioned 220 Volts is not a standard US voltage.
Do these compressors really require 200 volts?
How are you calculating the size of the BB transformer? 9KVA sounds way too big for the loads you are feeding.
As Tom mentioned 220 Volts is not a standard US voltage.
Do these compressors really require 200 volts?
How are you calculating the size of the BB transformer? 9KVA sounds way too big for the loads you are feeding.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Problem: There are two possible ways of rating a BB transformer.
The simple one is to look at the rating just like any other transformer, namely the amount of power that it can transfer from the primary to the secondary winding.
By that definition, the BB transformer only needs to transform (transfer from primary to secondary) the power corresponding to the 20 volt difference in voltage between the input and output voltages times the amp figure (with or without the 1.73 depending on whether you count line current or phase current). The full power of the load is NOT going through the primary.
The second way is to consider it the wired up BB system, as a black box with input and output terminals, and rate it on the basis of the amount of power in (or out.) That is useful since that is what you want to do with it, but it gives a misleadingly inflated number when you are looking to get a transformer with which to build such a system.
Correct.
But add the full power of the load is not going through either the primary or secondary winding(s)... only the kVA of the difference of primary and secondary voltage is. So if you have say a 1kVA load at 240V and bucking or boosting 24V...
1kVA ? 240V ? 24V = 0.1kVA
The OP'er probably only needs a 1kVA buck-boost
This is a great explanation of how to describe a BB the way that you have. I have worked with transformer design engineers from Westinghouse and Eaton as well as a smaller manufacture for over 20 years and have never heard it described as you have.
It is a very simple rule for those who don't understand or are not familiar BB or don't install BB xfmrs but on rare occasions.
Thanks
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You're welcome.
Been doing this for more than 20 yrs, first time I've used this example (well at least that I remember).
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
A very clear, concise description of the rules to be applied!
Just as an informational note (not normative or citable in court), the primary input rating of the autotransformer constructed by connecting the secondary in either buck or boost in series with the primary (and referred to in 450.4) will NOT generally be the primary input rating of the original transformer itself. You have created an autotransformer in which different parts of the interconnected windings have different current carrying capacity.
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Input and output, might have been a better choice of terms.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Possible it is a system being manufactured to be shipped out of the US (I hope) as 220 was an old voltage system that has long been removed from most places, I think you can find it around Pensylvainia as they still have some old 2-phase systems there.
I hope this is not being use for a voltage drop as that could be a problem if more then one load is fed.
Another point is if you have line to neutral load such as controls buck/boost systems will also change the line to neutral voltage so either you have to obtain the 120 volts ahead of the BB transformers or install a transformer for the 120 volt loads.
Smart is correct in the sizing of the BB transformer as you only size it for the voltage change so the size of the transformer will be well lower then the actual load kva, most manufactures will have a sizing chart to size the BB transformer to on their web site for the actual load it will see.
Some more info from the OP is needed to go any farther
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