Fuse VS Overload unit

Sorry about my newbie question. How to justify the OCPD price difference as shown in the picture below? Don't both adequately protect equipment?

From the manufacturer's perspective the cost is justified by the fact one costs much more to make.

As to why you would be justified in paying for that more expensive device, I can come up with three quick reasons offhand (there are probably more, but it's too early for thinking).

1) Re-use. Every time a fuse blows you'll be paying $15, potentially up to $45 if all three were to blow at once. The overload can be reset at no additional cost. Depending on circumstances it may simply end up cheaper in the long run.

2) Adjustability. Fuses tend to come in standard sizes; you can replace with smaller/bigger if required but it'll be a jump each time. The overload can be adjusted with much finer resolution within its range.

3) Trip detection. The overload has contacts built in.

EDIT: Perhaps more importantly, the two devices are not for the same purpose so it's not an apples-to-apples comparison in the first place. Fuses protect against current surges, overloads protect against sustained current draw above what a motor can maintain (but which it can handle in the short-term). Generally a motor circuit will have both; a circuit without a motor or equivalent device would only use fuses (and/or circuit breakers).

the first three definitely make sense. the last one is what I am confused about. Don't fuses protect against overload (sustained over current situation)? Thank you for the thorough explanation.

That is also a smart overload. you can connect it into a network and monitor it from a PLC. It can also feed data back to you and can be reset remotely.

Fuses protect against instantaneous current surges. Even slo-blo/time delay fuses designed to allow inrush current are intended for this.

The purpose of the fuse is to protect wiring and circuitry which will rapidly overheat if current is above a certain level.



Most motors are capable of running higher loads (and thus drawing more current) for short periods than they can handle full-time. They will generate more heat than they can dissipate while running at that level, but get a period to cool off between heavy-duty periods. If the overloaded duty period is too long (or the load too high for the duty cycle), the motor will overheat, potentially damaging it.

A fuse is a poor way to protect against this, because we can't simply say 'trip at X current'. It may be fine to run at X current for 5 minutes with 5 minutes between to cool off (50% duty cycle). It may be fine to run at Y higher current for 2 minutes with 8 minutes to cool off (20% duty cycle). How could you fuse it to allow both levels of current but only for the specified periods?

The overload protects against this by mimicking the process -- as higher current levels pass through it begins to heat up, then it cools off as current drops. If it reaches a set temperature it trips.

In other words, as the motor heats so does the overload. If properly set, then if the motor's load is beyond its ability to handle without overheating, then as the motor heats up over time the overload device will overheat first and trip.

tl;dr Fuses protect wiring/circuitry from current spikes/short circuits. Overloads protect the motor against maintained current draw beyond what it can handle. Consult your electrical code for proper sizing of both.

unsaint33 said:

the first three definitely make sense. the last one is what I am confused about. Don't fuses protect against overload (sustained over current situation)? Thank you for the thorough explanation.

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Hi, basically fuses or circuit breakers are for branch or feeder circuit conductor protection as determined by the current in that branch circuit and the conductor size. (NEC)
OL trip units are for protection of motors, which would be downstream of the motor contactor. The OL may be ON the contactor, however it will open the motor circuit if the motor draws more than it's rated FLA x a percentage (on nameplate) for a length of time.
If a branch circuit goes into a panel for example and feeds several devices, then it is basically a feeder which would be fuse or breaker and the individual branch circuits from the panel or internal to the panel would have fuse or breaker protection and then motor protection (OL) if motors are used. Hope this helps.

Since the picture you posted looks to be a motor overload this is specific to Motor Branch Circuits.

Fuses and Overload Devices perform two different functions in a motor circuit. There are a few exceptions where the right type of fuse, properly sized can provide both but usually not.

Fuses generally provide short circuit protection while overload devices provide overload protection. Then there are some combination devices that can provide both of these protections which are required by the NEC (National Electric Code).

Motors have large inrush currents which a circuit must be able to withstand. Most short circuits create huge currents and so the fuses are usually sized more than the full load current of the motor (depends on the type of fuse). For instance, per UL508A, for short circuit protection of a motor branch circuit you can use a non-time delay fuse up to 300% of the full load amps.

Overloads are generally sized at 115% of the full load amps. You could try to use a fuse rated at 115% of full load amps to do both but most likely, the fuses will blow when the motor starts due to the large inrush current.

In a nutshell, overloads protect against a gradual increase of overcurrent (think a mixer with a real thick batter). Fuses protect against short circuits or quicker increase in current.

unsaint33 said:

the first three definitely make sense. the last one is what I am confused about. Don't fuses protect against overload (sustained over current situation)? Thank you for the thorough explanation.

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No. As has already been explained, an overload relay protects a motor by modeling the heat generated within the motor based upon magnitude of current draw over time and disconnects the motor starter to prevent damage to the motor when an overload condition is detected.

Fuses and circuit breakers, on the other hand, are only intended to protect the motor wiring from becoming a fire hazard in the event of a short circuit condition. You don't size circuit breakers or fuses to the expected operating load, but to the wire size.

Don't forget that overloads can also detect phase loss although most do not directly, in the event of phase loss the increased current through the other phases can trip the overload, fuses can also do this but in my experience many times I have witnessed a motor two phasing due to a blown fuse, also fuses are more likely to deteriorate faster than overloads, I actually prefer the MPCB as this protects against most problems.

fuses were always a thing of the past. most homes were built with fuses and not breakers. If you remember 'A Christmas Story', "The old man could replace fuses quicker than a jackrabbit on a date. He bought them by the gross".

Now, we just reset breakers until the breaker mechanically gets weak and trips too soon.

The same applies to this, only the overloads are smarter, you can incorporate NO/NC contacts for feedback to other devices to let people know with lights or PLC when something is tripped and what it is. They have phase loss detection built in similar to a GFCI that measures equally between all 3 legs, which is why if you use a 3 phase OL for a single phase motor, you have to run the power through all the legs by looping it from L2-Motor back into L3-line and then out again, or else it will just trip all the time even though theres no extra load.


Better, features, better protection, more upfront money, but less money overall if the application as people jamming things up or the motor is going to experience a rough life.


Ideally, you just use the OL on motors. and any branch wiring requires it's own fusing that is fused at the wire specs so no wires get burned up. Unfortunately I see bad practice all the time. 25 amp fuses installed on a 1Hp motor because they ran 10gauge wire, no overloads. and then 5 amp fuses branching to multiple motors who are all 1.3 amp FLA, you'll never blow a fuse at that point, just burn motors up.

ndzied1 said:

Since the picture you posted looks to be a motor overload this is specific to Motor Branch Circuits.

Fuses and Overload Devices perform two different functions in a motor circuit. There are a few exceptions where the right type of fuse, properly sized can provide both but usually not.

Fuses generally provide short circuit protection while overload devices provide overload protection. Then there are some combination devices that can provide both of these protections which are required by the NEC (National Electric Code).

Motors have large inrush currents which a circuit must be able to withstand. Most short circuits create huge currents and so the fuses are usually sized more than the full load current of the motor (depends on the type of fuse). For instance, per UL508A, for short circuit protection of a motor branch circuit you can use a non-time delay fuse up to 300% of the full load amps.

Overloads are generally sized at 115% of the full load amps. You could try to use a fuse rated at 115% of full load amps to do both but most likely, the fuses will blow when the motor starts due to the large inrush current.

Click to expand...

Truth on the overloads, and this confuses people to no end. If someone took a 193 AB overload and put it on a 1.2 FLA motor, you turn the dial to 1.2, the factor is built into it already so there's no guessing. I've seen people take them and constantly turn them up "to the number it needs to be at". but now it's way over the spec.


from the install manual

To adjust trip current, turn dial until the desired current is
aligned with the pointer. Trip rating is 120% of dial setting

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To add more complication to the issue is also the cabinet temperature where the Thermal OL is located.... if it is a thermal overload (hotter cabinet will change setting).
And also if it is a Class 10, 20 or 30 overload relay (delay time before trip... depending on % of overload current).