Proper Fuse Sizing for Multi-Tap Transformers

[jaustin21]

Can anyone give me a good rule of thumb to use when trying to determine proper fuse sizing of a multi-tap transformer. I know that NEC and UL allows for 250% max on the input side of the transformer and if under 9 amps output then the max is 167% and if 9 amps and over then it is 125%. However, what if I double tap the output, say tap the 120v and the 24v. The NEC and UL state that if using parallel taps I am limited to six and the sum must not exceed the max allowed for one. But, the 120v and 24v are seperate and not in parallel. Taking in consideration of the VA of the transformer, do I treat these as two seperate circuits, staying within the max allowed limits for each or do I need to prorate them for the expected VA each will provide the control circuits and then use that to calculate max fuse or circuit breaker sizing. Also, though NEC allows the use of circuit breakers for both the load and line side of the transformer, UL limits the protection to only fuses for the load side if the current is under 9 amps. Is this correct or am I misreading UL?

 

[busbar]

Assuming the transformer is a 55°C machine-tool type, a conservative approach would be to fuse the primary and respective secondaries at 125% rated winding current {possibly 167%} with dual-element fuses. Having more than a 2-wire secondary on a 1ø transformer otherwise restricts your options.

 

[rhatcher]

Busbar, I believe that jaustin21 is saying that his transformer has a single secondary winding with taps for 24V and 120V as (crudely) shown below.

N 480V source (for example)
|________| primary winding
__________
| | | secondary winding
N 24V 120V
|__| |
|________| secondary circuits (loads)


Normally you would only use one tap and fuse it for full transformer kVA according to the factors you gave. But, if you use both taps at the same time as shown above, wouldn't you have to prorate the fuses (and obviously the loads) for each secondary circuit so that the total kVA for both does not exceed the transformer kVA?

I think that was jaustin21's question. However, if I misunderstood his question or your response, I apologize in advance.

 

[jbartos]

Questions/suggestions to jaustin21 (Electrical) Jan 4, 2004 marked ///\\Can anyone give me a good rule of thumb to use when trying to determine proper fuse sizing of a multi-tap transformer. I know that NEC and UL allows for 250% max on the input side of the transformer and if under 9 amps output then the max is 167% and if 9 amps and over then it is 125%. However, what if I double tap the output, say tap the 120v and the 24v. The NEC and UL state that if using parallel taps I am limited to six and the sum must not exceed the max allowed for one. But, the 120v and 24v are seperate and not in parallel.
///Please, would you clarify the separate taps and parallel taps? Normally, taps may be paralleled in special cases only, depending on the secondary transformer winding. Then, it is better to refer to the transformer secondary windings rather than taps.\\ Taking in consideration of the VA of the transformer, do I treat these as two seperate circuits,
///Do you mean separate transformer secondary windings?\\ staying within the max allowed limits for each or do I need to prorate them for the expected VA each will provide the control circuits and then use that to calculate max fuse or circuit breaker sizing.
///The multiwinding transformer secondaries or multitap transformer secondary winding should not be loaded more than the transformer rated VA. However, the manufacturer should provide the secondary windings or tap rated loading and protection device rating.\\ Also, though NEC allows the use of circuit breakers for both the load and line side of the transformer, UL limits the protection to only fuses for the load side if the current is under 9 amps. Is this correct or am I misreading UL? ///The power distribution circuit breakers have been available from 10A up in the past for power distribution panels. The circuit breakers are now available for lower amp rating than 10A. Apparently, the UL may be still considering 10A smallest rated circuit breakers. Do you have a current UL standard?\\

 

[busbar]

rhatcher—I overlooked "...the 120v and 24v are separate and not in parallel". My apologies.

[For the sake of discussion there are components like Micron Impervitran "group J".]

 

[jaustin21]

UPDATE: When I asked ABB this question, (they supply the multi-tap transformers we use), their reply was to fuse the 120V and 24V IAW UL and NEC guidelines as seperate supplies not parallel. The winding of the secondary will provide the VA to both. i.e. for a 50VA transformer, 50VA will be provided to both the 120V and the 24V taps independent of each other. Therefore no prorating of the device is necessary, however, other tranformers may not be wound the same so it is best to consult the manufacture concerning proper fusing. UL does address this in their UL508A section governing over current protection for power transformers, which states that for multi-secondary voltages, fused the secondary side IAW the table provided. UL guidelines do not provide for CB protection for secondary currents of less than 9 amps, only fusing, even though NEC does. This is due to UL's lowest standard size of a CB being 15A. I am talking with the UL rep in my area and will forward this question to their main office to see if they can shed any more light on the subject.

 

[jbartos]

Comment on the previous posting: Traditionally, transformer taps are meant to be conductive exits from one transformer winding. The independent taps may be interpreted as different transformer winding that may potentially be connected on the transformer output.