# Breakers vs Fuses in Outdoor Panels



## joe-nwt (Mar 28, 2019)

A fuse has no thermal element to be affected by ambient heat. A breaker does.


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## MoscaFibra (Apr 15, 2021)

You will have to replace fuses instead of waiting for it to cool down. Once the element melts, its gone. I have seen several spots where the inrush of the motor over time (granted motor is starting once a minute) that the standard time delay fuse just gives up and melts after so much inrush current. I did solve this with a specific time delay made by littlefuse for motor starting in particular. 

If possible, maybe try to cool the control cabinet somewhat? I would agree, fuses are probably cheaper....unless they start popping too much


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## paulengr (Oct 8, 2017)

MoscaFibra said:


> You will have to replace fuses instead of waiting for it to cool down. Once the element melts, its gone. I have seen several spots where the inrush of the motor over time (granted motor is starting once a minute) that the standard time delay fuse just gives up and melts after so much inrush current. I did solve this with a specific time delay made by littlefuse for motor starting in particular.
> 
> If possible, maybe try to cool the control cabinet somewhat? I would agree, fuses are probably cheaper....unless they start popping too much


Yeah, right. May I suggest you reread NEMA ICS 2? Inrush lasts for at most 2 cycles. LRC is another matter.

Two big differences between fuses and breakers is type 1 vs type 2 starters. In a type 1 starter the contactor is allowed to be utterly destroyed and must be replaced as long as it interrupts current. Every time the overload trips the starter must be inspected by a qualified electrician unlike type 2. Type 2 is “no damage”. If you overload just reset and go again.

Although current limiting is possible with breakers so far none are rated to meet type 2 requirements. Only fuses such as type CC, J, and RK1 offer this. Type RK5 does not. Despite heavy advertising there is no magic with so called low peak fuses except the magic price increases.

In addition a “breaker” as defined by UL has a nasty property that by the time you size it big enough to avoid nuisance tripping it is incapable of providing sufficient short circuit protection to the feeder with low fault currents. Eaton first solved this with the MCP which is a motor specific magnetic only breaker but it is only component rated so can’t be used outside of a Listed assembly. There are however now other devices such as an MCSP that are usable under 508S and other uses that fix this issue. They all work far better than a general purpose breaker, even with adjustable instantaneous tripping, but the starter will never be type 2. A short circuit study however can be used to verify if type 2 conditions exist irrespective of fuse use.


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## MoscaFibra (Apr 15, 2021)

paulengr said:


> Yeah, right. May I suggest you reread NEMA ICS 2? Inrush lasts for at most 2 cycles. LRC is another matter.


Mersen fuse - https://ep-us.mersen.com/products/series/atdr-class-cc-time-delay
Littlefuse dual element - Class CC Dual Element Time Delay Fuses - Littelfuse

Pull up the time/current curves, check the chart. The motors they are using pull 55-60amps in rush that I can measure. The mersen shows a 1 second melting point on their chart. The littlefuse if I remember using the dual element reach as high as 90amps before melting at the 1 second hold. The fuses are all the same cost too. 

Since I have put the littlefuse fuses in, we haven't blown 1, where we were blowing 1 a month at least. Even tonging the motor I see 60A every time the blower turns on, on my tester. We are turning the motors on once a minute if not faster when we can. 

The sum of what I am trying to say, the process, cycle time, temperature, inrush all have to do with the choice in fuse. And not all fuses are created equal. And this is with overloads on the motor starter, these are just the fuses sized to protect the wire, not the equipment. 


Back to the OPs questions though, I would still avoid changing to fuses if I could. If heat is the problem with the cabinet, solve the problem of heat. Install a fan to provide air movement? Vortex chiller maybe?


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## SpaceMonkey (Sep 19, 2018)

Should I go with a manual motor protector?


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## gpop (May 14, 2018)

I have always seen fuses as a luxury item. I have never used them as the primary means to protect a load but i have used them reduce the arc if something goes wrong. 
First thing i requested when i started this new job was to retrofit all vfd's with fuses on the primary. I would rather pop a fuse then wait for the breaker to react. Also really handy to be able to throw a 1 amp fuse in a 50 hp drive for testing.


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## paulengr (Oct 8, 2017)

SpaceMonkey said:


> Should I go with a manual motor protector?
> 
> 
> Sent from my iPhone using Tapatalk


Manual motor protectors are Listed assemblies so UL508A approved and usable in NEC installs as opposed to MCPs are only RU approved as a component. So yes you can use them but be careful of four things:

1. It’s essentially a slow magnetic only breaker and some models incorporate a bimetallic overload relay. The relay is affected by temperature. So doesn’t solve OPs issue entirely. An electronic microprocessor overload relay is pretty much temperature independent.

2. Some have very bad AIC like 5-10 kA so you need a fused disconnect feeder and accept that you will have failures with every hard fault (dead short).

3. Some models do not give any indication of overload vs overcurrent (instantaneous) trip, forcing an unnecessary inspection for damage. More of a nuisance really but be aware of it. Plus you need to rethink your overload wiring since now it needs to run through the MMP instead of the overload relay.

4. Not that you need them but generally you won’t have a lot of NC aux contacts.


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## SpaceMonkey (Sep 19, 2018)

paulengr said:


> Manual motor protectors are Listed assemblies so UL508A approved and usable in NEC installs as opposed to MCPs are only RU approved as a component. So yes you can use them but be careful of four things:
> 
> 1. It’s essentially a slow magnetic only breaker and some models incorporate a bimetallic overload relay. The relay is affected by temperature. So doesn’t solve OPs issue entirely. An electronic microprocessor overload relay is pretty much temperature independent.
> 
> ...


I was looking at the WEG MPW manual motor protectors on Automation Direct, it does appear they have a bimetallic element but it does say it’s temperature compensated to 140*F. Do you think that can be trusted or would you still not recommend it?


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## paulengr (Oct 8, 2017)

MoscaFibra said:


> Mersen fuse - https://ep-us.mersen.com/products/series/atdr-class-cc-time-delay
> Littlefuse dual element - Class CC Dual Element Time Delay Fuses - Littelfuse
> 
> Pull up the time/current curves, check the chart. The motors they are using pull 55-60amps in rush that I can measure. The mersen shows a 1 second melting point on their chart. The littlefuse if I remember using the dual element reach as high as 90amps before melting at the 1 second hold. The fuses are all the same cost too.
> ...


Inrush is caused by magnetizing the core. At startup you have no counter EMF. It can be as high at 3 times the locked rotor current which is itself 5.5-10x FLA although typical values are around 6. So with energy efficient motors inrush can be as high as around 24x FLA and I’ve measured some IEC motors very close to that. However the core quickly magnetizes and within 1-2 CYCLES or 1/60th to 1/30th of a second it vanishes leaving only the locked rotor current, maxing out at 5.5-10x FLA. it can be higher in plugging applications though as the collapsing magnetic field spikes the current. Either way standard breakers trip on instantaneous unless you crank them to NEC max where they don’t give good protection. Motor specific breakers or tine delay fuses do far better.

NEC calls for a maximum of 175% of FLA for dual element fuses. You can often use 150% and I’ve seen a lot of suppliers try to push 125% but it’s asking for trouble (nuisance trips). I try to stay between the ditches…150-175%. But they all should have no problems with inrush.


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## MoscaFibra (Apr 15, 2021)

paulengr said:


> Inrush is caused by magnetizing the core. At startup you have no counter EMF. It can be as high at 3 times the locked rotor current which is itself 5.5-10x FLA although typical values are around 6. So with energy efficient motors inrush can be as high as around 24x FLA and I’ve measured some IEC motors very close to that. However the core quickly magnetizes and within 1-2 CYCLES or 1/60th to 1/30th of a second it vanishes leaving only the locked rotor current, maxing out at 5.5-10x FLA. it can be higher in plugging applications though as the collapsing magnetic field spikes the current. Either way standard breakers trip on instantaneous unless you crank them to NEC max where they don’t give good protection. Motor specific breakers or tine delay fuses do far better.
> 
> NEC calls for a maximum of 175% of FLA for dual element fuses. You can often use 150% and I’ve seen a lot of suppliers try to push 125% but it’s asking for trouble (nuisance trips). I try to stay between the ditches…150-175%. But they all should have no problems with inrush.


I agree with all of that. That is essentially what we went through, that is why they were spec'd and engineered with standard time delay fuses. Standard time delay fuses in reality did not hold up. Dual element (for the time being) time delay fuses are. I only mention little fuse as they are the only dual element time delay fuse that I know of in the area that does CC size fusing. 

When I worked out the rough inrush (I am no math whiz, I just use my old college formulas) It calculated a rough inrush as high as 165 amps, I can tong and read in the real world as high as 65-70. The time curve of the fuse they are using, puts that at holding 60ampish for 1 second. Different manufacturer have different time/current curves. At least from our supplier cost does not seem to be a difference between them. 

I had to actually pull up the CEC code book, I don't think I have ever read a distinction between dual element and single in ours...just time delay and not time delay. Sizing seems to follow the same as the NEC code though, Max 175% but you can load a inverse time circuit breaker to 250%

@SpaceMonkey - I would still try to change the temperature of the cabinet if you could. At least from panel fans and the like that I have used, it would be good to follow pauls advice with some things, don't take the interrupting rating in stride either. That **** can be serious. You can certainly try fuses, but I don't think that will clear the heat issue entirely (our long winded discussion that is all I was trying to get too)

Like Paul there said, you need an electronic overload to compensate for the heat, this will at least solve that issue. I don't think they will be as cost prohibitive as replacing everything with fuses and fuse blocks and then having spare fuses either. Maybe run by a supplier and see what they suggest?


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