OK I been an electrician for 30 years and I do not know exactly what you mean by a "time delay fuse box system" is ,especially when you say later on you have "breakers".
What I think you have is a fuse box, with standard Edison base "plug" fuses you screw in, and someone has installed a type of "fuse' which has a small button on the front. This "fuse" is actually a small circuit breaker that once it detects a short or overload,instead of having to replace the fuse with a new one and throw out the old, it allows you to push the button to reset it.
Now on the front may be typed the words "Time Delay". If this is the case ,here is the answer.
A time delay overcurrent protection device, which can be a standard fuse, or a standard circuit breaker, or even what I think you have, a resetable breaker in the shape of a fuse.
The words "time delay" means only one thing with respect to the devices I have mentioned. The device allows for the current to be drawn to peak up high for a brief amount of time. For instance, when any motor starts from a dead stop, the current needed to start the motor will peak up for a few seconds, as time goes on, and the motor begins to get rolling the current will begin to go down and then level off at the amount of current the motor needs to do it's normal work.
This "starting" current can go very high at first, then drop down to normal "running " current. If the motor normal draws say 3 amps running, the starting current can go as high as 30 amps. This occurs each time your refrigerator starts,for example.
If you were using what is called a "one time" fuse rated at 15 amps, the 30 amp starting current, which is normal for every motor, would cause the fuse to open, or blow, if it is a circuit breaker it will open, or trip.
So do not worry,if you are planning to install a new circuit breaker panel all circuit breakers are designed as "time delay" , which is technically called "Inverse Time".
Now the issue is, if you trully have screw in fuses,and you say you have a 20 amp devices, there may be 20 fuses or breakers protecting circuits that cannot exceed 15 amps. This needs to be corrected right away, having a 20 fuse/breaker can cause a 15 amp circuit to draw more than it's allowed amount and be a fire hazard.
So once you change the fuse box to breakers the installer will need to be sure to install the proper breakers. Somehow you need to figure out how to install the right size fuses on the 15 amp wires I suspect you have. Not all of them are 15 amp I am sure but most maybe.
If you need help checking for which wires need 15 amp fuses, let me know.
Otherwise I hope I have answered your question.
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