Transformer wiring

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Bronco9588

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Happy Thanksgiving!

I have purchased and recently received an acme isolation transformer: 10 KVA 240x480 - 240/120. I was expecting some larger wires inside the xfmr. Primary side was 14ga and secondary was 12 ga! By my calculation I would get approximately 42 amps primary side (240v configuration) and 84 amps secondary side. Assuming 100% load on one phase, I was expecting something 4 ga or larger.

Are they using superconductors here? I plan to give a call to the manufacturer and discuss.
 
No really "super conductors" but conductors with higher than standard insulation.
Although not directly applicable to your application, look at Table 310.15(B)(19) and you will see an example of conductors with high temp insulation and the ampacities allowed.
You can research wire manufacturer data and find #14 has a very high ampacity with the proper insulation.
 
augie47 said:
No really "super conductors" but conductors with higher than standard insulation.
Although not directly applicable to your application, look at Table 310.15(B)(19) and you will see an example of conductors with high temp insulation and the ampacities allowed.
You can research wire manufacturer data and find #14 has a very high ampacity with the proper insulation.
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Yea... I was being somewhat sarcastic with superconductors. Hard to relay that sarcasm on written text. I think the wires were 135 degree C rated.
 
The wiring in the transformer itself is enamel coated. Say you have 120 turns on the secondary side and 60 on the primary. With 120 V on the secondary side the voltage turn to turn is only 1 V! Plus it’s got an air gap all around or strapped to a giant hunk of heat sucking steel. So ampacity looks very different on the core.
 
Bronco9588 said:
Happy Thanksgiving!

I have purchased and recently received an acme isolation transformer: 10 KVA 240x480 - 240/120. I was expecting some larger wires inside the xfmr. Primary side was 14ga and secondary was 12 ga! By my calculation I would get approximately 42 amps primary side (240v configuration) and 84 amps secondary side. Assuming 100% load on one phase, I was expecting something 4 ga or larger.

Are they using superconductors here? I plan to give a call to the manufacturer and discuss.
Click to expand...
That’s because your used to NEC sizing. Look in an appliance you just wired and fed with a 40 amp breaker. All small wire.
appliances, like transformers are built to different standards.
 
Are you talking about internal lead wires, or are you talking about the enamel coated magnet wire used to build the coils?

Note also that this transformer has two primary and two secondary coils, and the 240V primary configuration places the two coils in parallel. Each primary coil is only carrying 21A.

-Jon
 
winnie said:
Are you talking about internal lead wires, or are you talking about the enamel coated magnet wire used to build the coils?

Note also that this transformer has two primary and two secondary coils, and the 240V primary configuration places the two coils in parallel. Each primary coil is only carrying 21A.

-Jon
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Thanks Jon.

Just the wire leads. I don't care too much about the internals of the "black box" at this time.

My concern was more with the secondary side of the xfmr due to the fact that the primary side had two pairs of wires (Your explanation of the 21 amp thing makes sense on the primary side.) The concern is can the transformer handle unbalanced loads between phases up to 10 KVA on a single phase (idk why you would do that on purpose)? Specifically, I think the design would have to account for up to an 84 amp current draw on one phase (plus a factor of safety). In a perfect world, you would put 42 amps on both phases and be done with it. From that 84 amp number, 12 gauge wiring appears to be small.
 
Remember that the transformer requires secondary protection! If the full 10 KVA came out on half of the secondary, then that secondary half would be overloaded and should trip your secondary OCPD.

-Jon
 
winnie said:
Remember that the transformer requires secondary protection! If the full 10 KVA came out on half of the secondary, then that secondary half would be overloaded and should trip your secondary OCPD.

-Jon
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Yes. I am feeding it to a panel as an "independently derrived" system that will have breakers, new ground, bonding...

Hypothetical speaking, can the transformer support an 80 amp load on a single phase? And from that assumption, that is a lot of current running through 12 ga wires...

Unfortunately, there it is also impossible to have breaker protection from the secondary to the panel. I envision a scenario that a transformer is totaled by a short between the transformer and the breaker panel on the secondary side, particularly with transformers that have a larger step down.
 
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Think of the transformer as having 5 kVA secondary coils.

The transformer _could_ push more than 10 kVA through a single secondary coil, but more than 5 kVA per coil is an overload.

Jon
 
Bronco9588 said:
Unfortunately, there it is also impossible to have breaker protection from the secondary to the panel. I envision a scenario that a transformer is totaled by a short between the transformer and the breaker panel on the secondary side, particularly with transformers that have a larger step down.
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Forgot to add: the transformer secondary is essentially considered a 'tap', a set of conductors protected by OCPD at the load end rather than at the supply end.

Yes, if you had just the wrong sort of fault on the secondary you could overload the secondary conductors without tripping OCPD. But the real risk of overload is too many loads turned on, and the secondary panel is providing protection from that. A short circuit in these 'tap' conductors would trip the primary OCPD.

-Jon



You see the
 
winnie said:
Remember that the transformer requires secondary protection! If the full 10 KVA came out on half of the secondary, then that secondary half would be overloaded and should trip your secondary OCPD.

-Jon
Click to expand...
The transformer secondary does not require protection if the primary is protected at 125% or less of rated current.

However the secondary conductors do require protection.
 
The difference is that inside of a piece of equipment, the wiring and methods are controlled and predictable, then the entire unit has to pass testing / listing standards. The NEC is concerned with field wiring in which conditions and installation methods or practices vary widely, so the standards that have to be met are deliberately more conservative as a safety factor. Remember, we call it the National Electric Code, but officially it is NFPA 70, and NFPA stands for the National FIRE Protection Association.
 
Bronco9588 said:
Thanks Jon.

Just the wire leads. I don't care too much about the internals of the "black box" at this time.

My concern was more with the secondary side of the xfmr due to the fact that the primary side had two pairs of wires (Your explanation of the 21 amp thing makes sense on the primary side.) The concern is can the transformer handle unbalanced loads between phases up to 10 KVA on a single phase (idk why you would do that on purpose)? Specifically, I think the design would have to account for up to an 84 amp current draw on one phase (plus a factor of safety). In a perfect world, you would put 42 amps on both phases and be done with it. From that 84 amp number, 12 gauge wiring appears to be small.
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But that is critical to what you can do. When you connect the secondary as 120 volts you put two 5 kVA coils in parallel to one another and can supply the entire 10 KVA rating of the unit. If you wire it as 120/240 you put those same two 5 kVA coils in series with one another. If you put all 120 volt loads on just one side - the max the design can safely handle is only 5 kVA, with the other 5 kVA coil sitting there idle.

But at same time if you wired it for 120 volt output you would have twice the overcurrent protection level you would have if you wired it for 120/240, if you correctly protected it for max rating.

Remember use of 12 AWG for most general wiring - 90 C conductor is rated 30 amps. 240.4(D) often forces us to limit it to 20 amp protection, but ampacity adjustments still can start with the 90C value of 30. Then remember the transformer leads are often higher temp rated. The ampacity is based on what the insulation can take, the conductor itself is generally fine at much higher temperature than the insulation can handle.

General ampacity of NEC conductors is somewhat conservative on the side of safety as well, they can often withstand higher temps, and often are designed to operate at higher temps within listed equipment and/or may also incorporate methods of cooling to gain higher ampacity in listed equipment.

Max rated current on one of those 5 kVA @ 120 volt secondary coils is 42 amps. (full 10 kVA secondary rating of 84 amps includes two of those coils in parallel to one another) Really not all that high if you consider THHN @ 90C has 30 amps ampacity as a general rule.
 
winnie said:
Think of the transformer as having 5 kVA secondary coils.

The transformer _could_ push more than 10 kVA through a single secondary coil, but more than 5 kVA per coil is an overload.

Jon
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When wiring over both phases, 240V power, can I assume that each coil shares the load (i.e. 1/2 the rated load each)?
 
And, remember that conductor temperature ratings are based on avoiding insulation damage, not conductor melting.

Wire contained in a transformer housing doesn't pose the same damage threat that building wire does to a structure.
 
Bronco9588 said:
When wiring over both phases, 240V power, can I assume that each coil shares the load (i.e. 1/2 the rated load each)?
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They are wired in series when voltage is 240. Current is same everywhere in series connected components.

Since this is a source each section is supplying half the power - presuming they are basically identical and have same characteristics coming from their associated primary coils. Start to mess with those identical features and get some imbalance in any way things can start to go south and something could be set up for eventual failure.
 
Measure the coil resistance at ambient temp.
Put a heavy load on it.
Measure the coil resistance again.

Using the formula for the resistance tempco for copper should tell you how hot the wires are getting.

Every time I dig this deep into any product for sale, I get more questions than answers. :(
 
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