# Fuses and overloads



## Wirenuting (Sep 12, 2010)

besc said:


> I come from the instument world but often get asked to troublshoot motor controls. Here's an example of what I run into: A five horse 208 3 pahse motor has overload heaters of (just an example) say seven amps. Upstream from this GE starter is a 30 amp disconnect. (By the way. Is a knife switch just a disconnect without the fuses? Darn instrument guy.) Now. Across the room, in the breaker panel is a thre phase 20 amp breaker that feeds the fuses. Can someone give me an overview of how you match up breakers, fuses and overloads for motor circuits? Right now I'm just being asked to troubleshoot this stuff but I know I will be asked to put one together and would like some guidelines or literature to reference. Thanks. Mike.


I see it as a 3hp motor that pulls 9.6 amps w/ heaters set to 11.4 on a 20 amp breaker & # 12 wire in a size 0 starter. 
From here you can look at its load and lower the heaters if you like to match while remembering that ambient compensation might be needed. The unfused disconnect is fine as a local disconnect as long as it's not fed by a freq drive, then I would install an aux contact in it to kill the drive before the blades open the feeds.


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## ponyboy (Nov 18, 2012)

Motor circuits are provided with a ocpd that will allow the motor to run without nuisance tripping on startup. So it's very normal to have a motor with a fla of 7 amps to be on a breaker of 20 amps or more. This is possible because the overload protection provided for the motor will shut it down once it starts pulling more current than it should. 

Your 30 amp knife switch is just the required disconnect for the motor


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## Wirenuting (Sep 12, 2010)

ponyboy said:


> Motor circuits are provided with a ocpd that will allow the motor to run without nuisance tripping on startup. Do it's very normal to have a motor with a fla of 7 amps to be on a breaker of 20 amps or more. This is possible because the overload protection provided for the motor will shut it down once it starts pulling more current than it should


 Right, Breakers & fuses are for short circuit protection & o/l's are for over current protection.
We didn't mention different classes of overload's. But that's for a later discussion.


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## JRaef (Mar 23, 2009)

The mismatch between the 30A fuses and the 20A breaker is the sort of thing that often happens out of necessity, but should be investigated to be corrected so that someone in the future doesn't misinterpret and cause problems if anything changes.

If the 20A breaker is feeding ONLY this motor circuit, then it is a "Branch", so if it's holding in during startup, then it's fine (assuming all conductor sizing rules were followed). At that point, the 30A fuses become essentially irrelevant, it could have just been an non-fused disconnect, because the 20A CB was ALREADY doing the job as the Branch OCPD. But if at installation time they didn't have a non-fused disconnect available, but still needed one within sight, using a fused one with fuses that will never blow is a work around. As mentioned, it's the OVERLOAD relay that is providing the long term over current protection here, not the fuses or even the CB. So the fuses in that disconnect could have been any size, but given it is a 30A, that's the largest it will hold.

But if that 20A CB is feeding anything ELSE besides that one motor starter, then it becomes a FEEDER circuit instead of a BRANCH, and different rules apply. Then you will need to investigate the conductor sizes used against those new rules along with the appropriateness of the protective devices.


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## glen1971 (Oct 10, 2012)

besc said:


> *I come from the instument world but often get asked to troublshoot motor controls.* Here's an example of what I run into: A five horse 208 3 pahse motor has overload heaters of (just an example) say seven amps. Upstream from this GE starter is a 30 amp disconnect. (By the way. Is a knife switch just a disconnect without the fuses? Darn instrument guy.) Now. Across the room, in the breaker panel is a thre phase 20 amp breaker that feeds the fuses. Can someone give me an overview of how you match up breakers, fuses and overloads for motor circuits? Right now I'm just being asked to troubleshoot this stuff but I know I will be asked to put one together and would like some guidelines or literature to reference.
> 
> Thanks. Mike.


Get yourself an electrical apprenticeship and become dual ticketted..


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## besc (May 16, 2010)

I get what you say JRaef. Even you, glen1971. If they're going to throw me into this electrician world I'm going to have to bone up and get a code book. That's tomorrow. For now, can someone give me how to figure a breaker size for a motor. I see they are generally higher than the full load motor current so how do you figure the correct size. Locke rotor current or what. Mike.


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## micromind (Aug 11, 2007)

Here's how to figure wire/fuses/breakers/overloads for most motors. You'll need the HP, voltage, #of phases and FLA (Full Load Amps)

1) Wire size. 110.14(C)(1)(a)(4) states that conductors are sized using 75ºC unless any part of the system is rated at a lower temp. So we use the 75ºC column in table 310.16

According to 430.6(A)(1), the current is based on the tables in 430, usually 430.248 (1ø) or 430.250 (3ø), or the nameplate current, whichever is higher.

If it's a single motor, 430.21(A) states that the conductors need to be sized at 125% of the motor current as determined above. 

240.4(D) states that the 15, 20, and 30 amp limitations for #14, 12 and 10 does not apply to motors. Use the actual table 310.16 values. 

2) Breakers/fuses. Table 430.52 states that the maximum breaker size (basic standard breaker) is not more than 250% of the current calculated above. The maximum time-delay fuse size is 175%. There is no minimum size. 

The one and only time that nameplate current is used is to figure overloads. 

Here's a sort of extreme example; odd as it may sound, it is code-compliant.

Using table 430.248, the current of a 1HP 1ø 115 volt motor is 16 amps. 16 X 1.25 = 20. The 75ºC column of table 310.16 allows 20 amps for #14. 240.4(D) exempts the 15 amp requirement, so #14 is the smallest wire that can be used. Table 430.52 allows the breaker to be 40 amp. Yes, #14s and a 40 amp breaker. 

Here's another; 430.250 gives the current of a 5HP 3ø 208 volt motor as 16.7 amps. 16.7 X 1.25 = 20.875. 310.16 requires minimum #12s. 430.52 allows a maximum breaker of 45 amp. It also allows a maximum fuse of 30 amp. 

In both cases, the overloads are required to be selected based on the nameplate current on the motor. 

Of course there are exceptions to the above, but they're fairly rare. 

P.S. Figuring motors can be complicated, I hope I got it right.......If not, please correct.


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## RIVETER (Sep 26, 2009)

besc said:


> I come from the instument world but often get asked to troublshoot motor controls. Here's an example of what I run into: A five horse 208 3 pahse motor has overload heaters of (just an example) say seven amps. Upstream from this GE starter is a 30 amp disconnect. (By the way. Is a knife switch just a disconnect without the fuses? Darn instrument guy.) Now. Across the room, in the breaker panel is a thre phase 20 amp breaker that feeds the fuses. Can someone give me an overview of how you match up breakers, fuses and overloads for motor circuits? Right now I'm just being asked to troubleshoot this stuff but I know I will be asked to put one together and would like some guidelines or literature to reference.
> 
> Thanks. Mike.


It is best to explain to them that the electrical industry is such a wide...and varied industry that not all electricians can "DO IT ALL". Don't get yourself into trouble.


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## JRaef (Mar 23, 2009)

So what you should notice from micromind's post is that this actually isn't something you calculate, it's something you look up in the code. There are calculations you need to do as well, but without a code book, you can make errors that will mean doing it again.


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