[Speedskater]

Oh-Oh:
I wrote:
 A typical 20 Amp breaker may allow 200 Amp continuous current (lots of small print).
But I meant:
 A typical 20 Amp breaker may allow 200 Amp instantaneous current (lots of small print).

[Speedskater]

..............................................
Under sizing the feeder conductors is poor engineering. Start by proving your statement by siting the NEC code article and section.
 40+ years in the commercial/industrial electrical industry I never seen an electrical equipment power riser diagram where the electrical engineer undersized the feeder conductors feeding a power transformer. NOT once! Not even when the transformer was feeding a piece of equipment where the load was known. In the case of million dollar x-ray equipment the transformer feeder was over sized, not under sized.

As for the size of the breaker protecting the feeder and primary of the transformer just going from memory NEC code minimum is 125% of the manufacture's nameplate rating up to 250% of the manufacture's nameplate rating.
....................................... ......

You have minimum and maximum upside down. Not sure how feeders got into the conversation. But small breakers and large transformers or large feeders is always permitted. (small breakers and large receptacles is not permitted)

[Speedskater]

On transformer sizing.
Power transformers are typically rated for resistive loads. Big power amplifiers with linear supplies place an additional burden on transformers.

[amblin]

I have a 40A breaker on the dedicated 12AWG line (part of the house circuit, under 30ft long in seperate tubes) and there's a dedicated 32A breaker for the transformer itself , plus 20A breakers on the two PDU socket boards they installed on the cabinet, both PDUs with 12AWG wiring and solid copper bar as conductor .


I *think* it's more than sufficient to get the thing going, I just hope it'll last long enough considering all the effort put into it....

[jea48]

You have minimum and maximum upside down. Not sure how feeders got into the conversation. But small breakers and large transformers or large feeders is always permitted. (small breakers and large receptacles is not permitted)

You have minimum and maximum upside down.

Do I?

I guess so does NEC Article 450.3(B) Table 450.3(B)

Not sure how feeders got into the conversation.

Granted, usually the feed that feeds a 5KVA power transformer may not be thought of as a feeder but I wouldn't call it a branch circuit either. I definitely would not call a feed feeding a larger power transformer a branch circuit either.

But small breakers and large transformers or large feeders is always permitted.

Really?

Not when it comes to power transformers found in commercial/industrial facilities.
What kind of Electrical Engineer are you? Did you ever design the electrical power distribution system of a building? There is a reason for electrical safety code. Even if it is only bare minimum.

 

[jea48]

I have a 40A breaker on the dedicated 12AWG line (part of the house circuit, under 30ft long in seperate tubes) and there's a dedicated 32A breaker for the transformer itself , plus 20A breakers on the two PDU socket boards they installed on the cabinet, both PDUs with 12AWG wiring and solid copper bar as conductor .

WOW! #12 wire protected by a 40 amp breaker. The only thing code is the 32amp breaker for the disconnecting means at the transformer. Slightly over 125% of the transformer's rating well within electrical code for the overcurrent protection of the transformer. Bet that was designed by the company that built the transformer and supplied the cabinet and associated breaker and internal wiring. Is the internal wiring inside the cabinet #12awg? Or is it at least #10awg. How about the primary and secondary leads of the transformer? #12awg wire or #10?

I look forward to reading Speedskater's response to your post.

[srb]

WOW! #12 wire protected by a 40 amp breaker.

Yes, something's amiss here.  Either the wrong size breaker is installed or the wire is actually 8AWG?

[amblin]

WOW! #12 wire protected by a 40 amp breaker. The only thing code is the 32amp breaker for the disconnecting means at the transformer. Slightly over 125% of the transformer's rating well within electrical code for the overcurrent protection of the transformer. Bet that was designed by the company that built the transformer and supplied the cabinet and associated breaker and internal wiring. Is the internal wiring inside the cabinet #12awg? Or is it at least #10awg. How about the primary and secondary leads of the transformer? #12awg wire or #10?

I look forward to reading Speedskater's response to your post.

To be honest I'm not so sure because it came with the house..  The line was originally used by an air conditioning unit and boiler rated for around 6KW continuous and 10KW max, so I guess I misread the AWG--mm^2 table...

The wiring inside the cabinet is all 10AWG I was told..

A

[Speedskater]

You mean the one that labeled?

Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers
Over 600 Volts (as a Percentage of Transformer-Rated Current)

That's maximum, not minimum.

[jea48]

Yes, something's amiss here.  Either the wrong size breaker is installed or the wire is actually 8AWG?

My bet is the OP installed the dedicated branch circuit wiring as well as the overcurrent protection, the 40 amp breaker.

 That 40 amp breaker will pass quick short duration draws of current in excess of 400 amps all day long without tripping open. What do you think will be happening, in such an event, to the terminations at both ends of the #12 wire as well as the insulation covering the wire? Imagine the fault current that could pass through the 40 amp breaker before it hopefully tripped open. How could such a load, event, ever exist just feeding his audio equipment? One scenario faulty wiring anywhere in the circuit from the 40 amp breaker up to the safety fuse/s protection in the audio equipment. 

EDIT:

The branch circuit is existing.

To be honest I'm not so sure because it came with the house..  The line was originally used by an air conditioning unit and boiler rated for around 6KW continuous and 10KW max, so I guess I misread the AWG--mm^2 table...

The wiring inside the cabinet is all 10AWG I was told..

A

[jea48]

You mean the one that labeled?

Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers
Over 600 Volts (as a Percentage of Transformer-Rated Current)

That's maximum, not minimum.

 NEC Article 450.3, (B)
(B) Transformers 1000 volts, nominal, or less. Overcurrent protection SHALL be provided in accordance with Table 450.3(B).

[Speedskater]

This one?

Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers
600 Volts and Less (as a Percentage of Transformer-Rated Current)

[amblin]

  So should I ask them to change anything before it leave the factory? The unit is still being tested overnight at full load.  Or there's something I should call the electrician and change at the house? I believe I still have some Prysmian 7 AWG lines left over somewhere in the house while installing the new boiler.

They already checked polarity , ground ,  EMI and many interesting things I can't quite understand 

[jea48]

To be honest I'm not so sure because it came with the house..  The line was originally used by an air conditioning unit and boiler rated for around 6KW continuous and 10KW max, so I guess I misread the AWG--mm^2 table...

A

Just a guess the wire is #8awg, good for amps. Just a guess the boiler nameplate showed a minimum circuit ampacity of 40 amps.

The fact that both the air conditioner and boiler shared the same branch circuit probably would not meet code, at least here in the states, today. Who knows in the country you live. I doubt anybody tried to run the boiler and air conditioner at the same. Maybe they had a safety control circuit that prevented them from running at the same tine.

The wiring inside the cabinet is all 10AWG I was told..

And that would meet bare minimum electrical safety standards for a 5KVA transformer.
5000VA / 220V = 22.73amps X 125% = 28.41amps. #10awg copper wire is good for 30 amps.
#12awg copper wire is good for 20 amps.

Does the wire at the wall outlet you will be feeding the transformer look bigger or smaller than the wiring inside the transformer cabinet?

[brucek]

Believe what you want......

For your safety.

After you get it all wired up make sure you check the output for AC polarity.

*You should measure 220V nominal from the hot contact on the receptacle to the cabinet.

* You should measure zero volts from the neutral contact on the receptacle to the cabinet.

It is unsafe to let the secondary of an isolation transformer float above ground without the proper safeguards and monitoring/supervision of qualified personnel.

I'm getting the sense that you may not be familiar with balanced power. Your concerns are absolutely spot on for conventional systems.

A balanced power unit is basically a very high quality isolation transformer with a center tapped low impedance secondary. So instead of a hot of 120 volts and neutral return, you have two 60 volt lines with a center tapped ground. Across the 60 volt lines which are 180 degrees out of phase you have your 120 volts, with each 60 volt lead referenced to the earth ground at the center tap. The reference ground potential has been located at the midpoint between the two mains so there is no "neutral" wire. 

 There are a lot of benefits in this configuration. Any reactive currents developed in the load arrive at the common center tap and are cancelled. Any noise that would normally travel on the safety ground are cancelled at the center tap. There's not much chance of a ground loop. The other benefit of course can be a lower noise floor.

brucek

[jea48]

So should I ask them to change anything before it leave the factory? The unit is still being tested overnight at full load.  Or there's something I should call the electrician and change at the house?

They already checked polarity , ground ,  EMI and many interesting things I can't quite understand 

Are they building/wiring the unit totally? Plug and play? Is it coming with a power cord and plug installed on the end of the power cord? If so the plug will be rated for 250V 30 amps.

Ask them if they wired the secondary side of the transformer as a Grounded Power System or an Isolated Power System? A Grounded Power System is what you have in your home now. An Isolated Power System is used in Hospital OR, ER, rooms or CATH Lab rooms. In the case of an Isolated Power System the secondary of the transformer is left floating above ground and in your case both output power leads would not have any reference to ground. No reference to the cabinet of the transformer or to any safety equipment grounding conductor.

My guess is if the manufacture is building the unit to be installed in a residential dwelling it will be wired as a "Grounded Power System".

Here in the US NEC code requires a separate grounding wire be installed from the neutral conductor of the secondary winding of the transformer and metal cabinet enclosure to the main grounding system of the main electrical service. Here in the states, a transformer the size you are using, the feed safety equipment grounding conductor can not be used for the ground wire. a separate ground wire must be installed. All grounding within the premise has to be on the same ground plane.

 I have no idea how it is done in the country you live. You might ask the manufacture that is building the unit for you. He surely knows.

[jea48]

I'm getting the sense that you may not be familiar with balanced power. Your concerns are absolutely spot on for conventional systems.

A balanced power unit is basically a very high quality isolation transformer with a center tapped low impedance secondary. So instead of a hot of 120 volts and neutral return, you have two 60 volt lines with a center tapped ground. Across the 60 volt lines which are 180 degrees out of phase you have your 120 volts, with each 60 volt lead referenced to the earth ground at the center tap. The reference ground potential has been located at the midpoint between the two mains so there is no "neutral" wire. 

 There are a lot of benefits in this configuration. Any reactive currents developed in the load arrive at the common center tap and are cancelled. Any noise that would normally travel on the safety ground are cancelled at the center tap. There's not much chance of a ground loop. The other benefit of course can be a lower noise floor.

brucek

Balanced power? Where did you read that in this thread?
And yes, I do know what a balanced power system is.

I do thank you for your post though. It made me go back and read the OP's original post where he said he was going to have a 10KVA transformer built.
I really don't want to read this whole thread again to find out if it's a 10KVA or a 5KVA he ended up going with.

[amblin]

Are they building/wiring the unit totally? Plug and play? Is it coming with a power cord and plug installed on the end of the power cord? If so the plug will be rated for 250V 30 amps.

Ask them if they wired the secondary side of the transformer as a Grounded Power System or an Isolated Power System? A Grounded Power System is what you have in your home now. An Isolated Power System is used in Hospital OR, ER, rooms or CATH Lab rooms. In the case of an Isolated Power System the secondary of the transformer is left floating above ground and in your case both output power leads would not have any reference to ground. No reference to the cabinet of the transformer or to any safety equipment grounding conductor.

My guess is if the manufacture is building the unit to be installed in a residential dwelling it will be wired as a "Grounded Power System".

Here in the US NEC code requires a separate grounding wire be installed from the neutral conductor of the secondary winding of the transformer and metal cabinet enclosure to the main grounding system of the main electrical service. Here in the states, a transformer the size you are using, the feed safety equipment grounding conductor can not be used for the ground wire. a separate ground wire must be installed. All grounding within the premise has to be on the same ground plane.

 I have no idea how it is done in the country you live. You might ask the manufacture that is building the unit for you. He surely knows.

It's a complete, ready built plug and play unit fully tested and with warranty. And the plug is huge, So huge even the iron man would say yes, that is a good plug...    Actually they build loads of similar units everyday , with the only modifications being extra insulation and shielding to make it suitable for home use.

And yes , being an isolated transformer the second stage ground is floating, isolated from AC mains . But the engineer told me he provided a chassis ground point for the second stage for 'safety reasons' , not sure what he meant though. He also said although he can wire it to the fully grounded configuration but by doing so pretty much voids all the benefits of having an 'isolated transformer' in the first place.

And btw, with both stages grounded, I'm not sure can it still be called 'isolated transformer' anymore?

A

[Speedskater]

Plug-in trasformers, either balanced or isolation by rule are treated differently than permanent hard-wired transformers.

[jea48]

It's a complete, ready built plug and play unit fully tested and with warranty. And the plug is huge, So huge even the iron man would say yes, that is a good plug...    Actually they build loads of similar units everyday , with the only modifications being extra insulation and shielding to make it suitable for home use.

And yes , being an isolated transformer the second stage ground is floating, isolated from AC mains . But the engineer told me he provided a chassis ground point for the second stage for 'safety reasons' , not sure what he meant though. He also said although he can wire it to the fully grounded configuration but by doing so pretty much voids all the benefits of having an 'isolated transformer' in the first place.

And btw, with both stages grounded, I'm not sure can it still be called 'isolated transformer' anymore?

A

And btw, with both stages grounded, I'm not sure can it still be called 'isolated transformer' anymore?

YES.

Best example is a computer data processing center. Normal mains power is run through a UPS system/s that has an isolation transformer/s and at the end process the neutral of the separately derived system is connected back to the main electrical service's earth grounding electrode system. The neutral is also bonded, connected, to the metal enclosure of the isolation transformer/s or first disconnecting means metal enclosure. This point is where all safety equipment grounding conductors connect to.
Microsoft does not use an "Isolated Power System" to power their Mega dollar server equipment in their data processing facilities. If an "Isolated Power System" is so great why don't they use it to power their equipment?

What are the chances you can get a wiring diagram of the unit you are having built and post it here on your thread. A picture is worth a thousand words.

Does the unit have a "Line Isolation Monitor (LIM)" that will notify the user if one of the 2 hot ungrounded floating conductors come into contact with the metal enclosure of the transformer enclosure itself or a piece of equipment that is fed from the "Isolated Power System" that has a metal enclosure. An audible alarm and or maybe a warning light?  If not how will the user know if such a fault occurs? Or does the user just wait for the other shoe to drop. That can be dangerous and or possibly expensive.