New Fuse Box Needed for 110v Electrics

Rob said:

If the output of the transformer is not referenced to earth then there is unlikely to be much point fitting an rcd at all.

Double pole fusing, mcbs and rcds will be needed if neither pole of the supply is referenced to earth or if it is centre tapped

Reading again, I imagine its a typo and you meant either rather than neither.

Capacitive coupling through the human body.

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I see it now, I was trying to say that double pole protection is needed for both conductors because they are both line conductors when the transformer output isn't referenced to earth, or if its centre tap earthed.

From what I understand in America.
Voltages and types of supply vary from STate to State, town to town and even building to building.
My information, is that there are still buildings in New York that have dc supplies.
To suggest there is a national standard, is a bit like saying everyone agrees on Brexit.

The neutral at the output of the transformer should be referenced to earth (not the centre-tap), as that is what normal US appliances and single-pole-switched lighting circuits are designed for. Thus, only single-pole MCBs would be required. UK-type RCDs and RCBOs will work correctly in the event of leakage, although the test button may not work because of the lower voltage. UK-type MCBs will work correctly in the event of overcurrent or short-circuit, although the short-circuit current at the output of the transformer may be limited, making larger breakers relatively slower to trip in the event of a fault.

Yellow site transformers are continuously rated, but for a lower output than their 'tool rating' which is the one by which they are sold. The continuous rating is usually given on the data plate. However, because it is desirable in the trailer to have the neutral earthed, not the centre-tap, and a yellow tool transformer should never be modified in this way, I would agree they are a bad choice.

Because of the transformer regulation, which is likely to be around 5% for a 1-2kVA unit, a 110V rated secondary will output around 115V when lightly loaded as it will be most of the time, e.g. at night when the aircon is off and only electronic goods are switched on. Whereas, a 120V transformer will output 126V. So, for a 120V circuit, I'd probably opt for a 110V-rated transformer.

In any case, I think we talk about 110V and 120V as a general 'class' of things without meaning an exact voltage. I would often say 'a 110V motor' or 'a 120V motor' indiscriminately, to distinguish it from a 12V one or a 230V one.

The US uses a wider variety of supply configurations than the UK, but they are all recognised by the national standards. Some localities may have one or other three-phase system more readily available, according to the age of its infrastructure, for example. The UK site supply arrangement, 55V-0-55V with centre-tap earthed, is not used for USA mains power. I understood that the last public DC supply in NYC was switched off by Con Ed in 2007.

Yes Con Ed did disconnect their last dc customer in 2007.
However many of the older buildings have had rectifiers installed to convert to dc (primarily for lift winding motors).
Also many buildings, such as the Dakota Apartments had their own electricity supplies.
Not sure if D.C. still has dc (it did in 2012), though it has the same problem with having dc lift winding motors.
From what I understand, San Francisco still has dc in the area covered by the first public electricity supply company.
Again this is primarily for dc lift winding motors.

Gentlemen,
there were too many and complex to ask but since your questions all related to the transformer I thought my next stop should be tech support at Airlink. Perhaps the answer will be self-explanatory. I have been advised that a 240v 32Amp Dual Pole RCBO is the kit I need to protect the 3Kva 110v system.
Thank you so much for all your answers. I think I need to expand my knowledge beyond V over I R

davesparks said:

It's probably like the UK changing from 250V to 240V and now 230V, the actual transformers are still outputting exactly the same 250V they always were, it's just the paperwork that changed.

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No, that is not the case in the US. 110v would be outside the 120v plus or minus 5% spec. Last time I was there in Brooklyn I measured the voltage at 123v.

Guess what. I had a look on the Airlink site and their transformers ARE actually 120v not 110v. OP please note.

230 to 120v Transformer: US Voltage Converters | Airlink Transformers - https://airlinktransformers.com/category/america-uk-voltage-converters

But sadly they are autotransformers not double wound isolated types. Thus there is the possibility of 240v to earth on the output side if input polarity is reversed, a common problem with caravan sites on the continent.

Bearing this in mind I would consider a yellow site transformer a safer bet. Just use one of double the rating required.