I am looking at possibly using 2 transformers for a new office addition. 1 for the computer/lighting equipment and 1 for the heatpump. A co-worker stated that I could just use 1 larger transformer and de-rate it, but I cannot find a reference for this in the code. Is this true and if so, where is the de-rating formula in the code book?gail.s.hicks@usace.army.mil
derating of a transformer
- Thread starter ghicks
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Re: derating of a transformer
I was originally going to use 1 25kva transformer for the whole addition, computers, lights, and heat pump. My coworker stated that I needed a k-rated transformer for the computers and lights. He mentioned both the harmonics and the heat. I had not heard of this requirement and cannot find it in the code. He then told me that I could use a larger transformer and re-rate it and be okay. He cannot tell me how to de-rate it or find it in the code either, but he says he knows it is there.
How can a de-rated transformer take care of the harmonics?
I was originally going to use 1 25kva transformer for the whole addition, computers, lights, and heat pump. My coworker stated that I needed a k-rated transformer for the computers and lights. He mentioned both the harmonics and the heat. I had not heard of this requirement and cannot find it in the code. He then told me that I could use a larger transformer and re-rate it and be okay. He cannot tell me how to de-rate it or find it in the code either, but he says he knows it is there.
How can a de-rated transformer take care of the harmonics?
james_mcquade
Senior Member
Re: derating of a transformer
check with your transformer distributor, tell him your application in detail. he then can direct you as to which one to use. he also has a derating table for percentage of use versus, load,temperature, mounting location, cooling.
check with your transformer distributor, tell him your application in detail. he then can direct you as to which one to use. he also has a derating table for percentage of use versus, load,temperature, mounting location, cooling.
scott thompson
Senior Member
- Location
- Anaheim, California
Re: derating of a transformer
There are several ways to approach this design - each depends on what works best overall for the Client and the Installer (best options, cost, space, availability, future, codes, etc.).
First option would be the use of one Polyphase Delta/Wye Transformer for all the loads mentioned (Computers, Lights, HVAC).
This Transformer should be sized per all coincidental connected loads, plus some growth factor. Also, allow for hard starts on the HVAC equipment.
Since there appears to be Harmonic Creating loads, the use of K rated Transformer is advised. As an alternate to K rating, increased capacity - such as 30-50% higher capacity than maximum loaded KVA might be possible.
Need to know the expected Harmonic current levels, then design accordingly for K rating or oversizing.
Ventillation of the Transformer's room is important also.
Second Option would be to place the Lighting on higher voltage system - assuming the location is serviced with a 480Y/277VAC 3? 4W system, then use a common Polyphase Transformer for Computer loads and HVAC loads.
Still need to follow the steps in the "first option" as to sizing per design loads and K values.
Third Option would be the use of two separate Transformers for these loads.
This option has a few different approaches:
1st approach uses two individual 3? 4W Transformers for the loads. One driving Computers and some Lighting, the other driving HVAC equipment and remaining Lighting. Sizes depend on designed loads.
2nd approach uses two individual 3? Transformers - one 3? 4W Wye Transformer for Computers and Lighting (likely to be K rated), and one 3? 3W Delta Transformer for HVAC equipment (Corner Grounded Delta Xformer is option).
Again, sizes depend on designed loads.
3rd approach uses two individual Transformers - one 1? 3W for loads with High Harmonic Distortion (Computers and some Lights), and one 3? 4W Wye for Lights and HVAC equipment.
(if lighting / 1 phase loads are low, then possibly 3 ph 4 wire Delta Xformer).
Size according to designed loads.
These items are areas which an Electrical Engineer (in the power systems field) designs commonly. Might be worth your time to consult an EE about this project.
Scott35
There are several ways to approach this design - each depends on what works best overall for the Client and the Installer (best options, cost, space, availability, future, codes, etc.).
First option would be the use of one Polyphase Delta/Wye Transformer for all the loads mentioned (Computers, Lights, HVAC).
This Transformer should be sized per all coincidental connected loads, plus some growth factor. Also, allow for hard starts on the HVAC equipment.
Since there appears to be Harmonic Creating loads, the use of K rated Transformer is advised. As an alternate to K rating, increased capacity - such as 30-50% higher capacity than maximum loaded KVA might be possible.
Need to know the expected Harmonic current levels, then design accordingly for K rating or oversizing.
Ventillation of the Transformer's room is important also.
Second Option would be to place the Lighting on higher voltage system - assuming the location is serviced with a 480Y/277VAC 3? 4W system, then use a common Polyphase Transformer for Computer loads and HVAC loads.
Still need to follow the steps in the "first option" as to sizing per design loads and K values.
Third Option would be the use of two separate Transformers for these loads.
This option has a few different approaches:
1st approach uses two individual 3? 4W Transformers for the loads. One driving Computers and some Lighting, the other driving HVAC equipment and remaining Lighting. Sizes depend on designed loads.
2nd approach uses two individual 3? Transformers - one 3? 4W Wye Transformer for Computers and Lighting (likely to be K rated), and one 3? 3W Delta Transformer for HVAC equipment (Corner Grounded Delta Xformer is option).
Again, sizes depend on designed loads.
3rd approach uses two individual Transformers - one 1? 3W for loads with High Harmonic Distortion (Computers and some Lights), and one 3? 4W Wye for Lights and HVAC equipment.
(if lighting / 1 phase loads are low, then possibly 3 ph 4 wire Delta Xformer).
Size according to designed loads.
These items are areas which an Electrical Engineer (in the power systems field) designs commonly. Might be worth your time to consult an EE about this project.
Scott35
Re: derating of a transformer
ghicks, what voltage is the service, and what voltage is the HVAC and lighting operating at?
Scott gave you some good advice. What I would add is if you have a substantial computer load, I would put the computing load on a dedicated "K-13" transformer, and run the HVAC, & lighting directly off the service.
Basically you divide the load up by critical and non-critical.
[ February 05, 2004, 10:01 AM: Message edited by: dereckbc ]
ghicks, what voltage is the service, and what voltage is the HVAC and lighting operating at?
Scott gave you some good advice. What I would add is if you have a substantial computer load, I would put the computing load on a dedicated "K-13" transformer, and run the HVAC, & lighting directly off the service.
Basically you divide the load up by critical and non-critical.
[ February 05, 2004, 10:01 AM: Message edited by: dereckbc ]
Re: derating of a transformer
Dereck, The incoming service is 480. The heat puimp is 208/230 and the lighting is 120. This is a small office addition with only 4 offices in one open room. I am a EE, but just out of school and this is the first time I have had to work with this. Thanks for the help, Gail
Dereck, The incoming service is 480. The heat puimp is 208/230 and the lighting is 120. This is a small office addition with only 4 offices in one open room. I am a EE, but just out of school and this is the first time I have had to work with this. Thanks for the help, Gail
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: derating of a transformer
If your computer loads are limited to "general purpose" personal computers and there are not a large number of laser printers (i.e. you have a general office not a CAD room), you should not need a K rated transformer.
In 25 years, I have never seen a general purpose transformer fail due to harmonics caused by general office loads.
Put in a single normal transformer.
If your computer loads are limited to "general purpose" personal computers and there are not a large number of laser printers (i.e. you have a general office not a CAD room), you should not need a K rated transformer.
In 25 years, I have never seen a general purpose transformer fail due to harmonics caused by general office loads.
Put in a single normal transformer.
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