Proper Fuse Sizing for Multi-Tap Transformers
Proper Fuse Sizing for Multi-Tap Transformers
(OP)
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?





RE: Proper Fuse Sizing for Multi-Tap Transformers
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.
RE: Proper Fuse Sizing for Multi-Tap Transformers
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.
RE: Proper Fuse Sizing for Multi-Tap Transformers
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?\\\
RE: Proper Fuse Sizing for Multi-Tap Transformers
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".]
RE: Proper Fuse Sizing for Multi-Tap Transformers
RE: Proper Fuse Sizing for Multi-Tap Transformers