# 15 Hp Motor



## Soilentblue9 (Apr 11, 2014)

Afternoon,
Im wiring up an upgrade to an exhaust fan system. The new motor is 480v 3 phase 15 hp. I currently have a 40 amp cb. I know I can go up to 250% of the fla out of the codebook. My question is, is there a way to figure out if the 40 amp cb will handle the inrush. I know the locked rotor current of the motor. The circuit breaker is an EHB3040. Is there a graph of the instantaneous trip curve that anyone knows of? 
Thanks


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## Wirenuting (Sep 12, 2010)

A refurbished breaker is about $180-ish
But a 40 is fine depending on distance and other site conditions.

what was the old motor?


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## 460 Delta (May 9, 2018)

I’ll assume it’s a 480 vac system? The 40 amp cb should hold, but seeing it an older Westinghouse, who knows. @micromind has a rule of thumb that holds pretty true, 2x the horsepower on a 480 system will usually get you close enough to work and still be under the max the NEC will allow. 
I use the 4x rule on 230 vac systems and it is pretty true.


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## Soilentblue9 (Apr 11, 2014)

Wirenuting said:


> A refurbished breaker is about $180-ish
> But a 40 is fine depending on distance and other site conditions.
> 
> what was the old motor?


I think 7.5hp. Not 100% sure. Breaker cost isn’t the issue. The mcc is incredibly old and sometimes when ya take it down control transformers end up smoking. I want to avoid that 


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## Wirenuting (Sep 12, 2010)

A 7.5 would have been a 20 amp breaker. 
check your distance and any other field conditions that might require a different sized breaker. The wire would a 10 minimum before you calculate VD.

what size wire is in there now? Have you repulled?


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## Soilentblue9 (Apr 11, 2014)

Wirenuting said:


> A 7.5 would have been a 20 amp breaker.
> check your distance and any other field conditions that might require a different sized breaker. The wire would a 10 minimum before you calculate VD.
> 
> what size wire is in there now? Have you repulled?


It will get 10 gauge when it is upgraded. It could have been a 10 hp I do not know. 


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## Mbit (Feb 28, 2020)

Can you just use an MCP with adjustable instantaneous? Is has an O/L right?

High efficiency motors can have very high inrush, not sure if that's what you're installing.


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## micromind (Aug 11, 2007)

460 Delta said:


> I’ll assume it’s a 480 vac system? The 40 amp cb should hold, but seeing it an older Westinghouse, who knows. @micromind has a rule of thumb that holds pretty true, 2x the horsepower on a 480 system will usually get you close enough to work and still be under the max the NEC will allow.
> I use the 4x rule on 230 vac systems and it is pretty true.


I've connected a number of 15 HP motors operating at 480 to 30 amp breakers and have had no issues at all. The 40 should hold just fine. 

As for the inrush, yes, high-efficient motors do indeed have higher inrush as well as higher locked-rotor currents. One easy way to combat this is to use the smallest wire that is allowed by code. In this case, it'd be #10. Smaller wire = higher resistance. Higher resistance = higher voltage drop, especially at inrush and locked-rotor currents. Higher voltage drop = lower current.


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## Dennis Alwon (May 9, 2009)

There is no way to know if the motor will handle the current but in general a breaker at 250% is unlikely to cause an issue or even at 125% in some cases. The motor may say what the minimum size overcurrent protective device that should be installed.


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## Wirenuting (Sep 12, 2010)

I would almost think that if a 7.5 was in place now and the MCC had a 40 amp breaker in it, that the 7.5 was a new motor installed a few years ago.

Since the OP doesn’t like shutting the MCC down, I would look at the o/l’s to see if they are original and sized to hold a 15hp. If so I would guess the 7.5 was a quick fix replacement.

If the exhaust fan is original it’s possible that the 7.5 was undersized and burnt out quicker when the o/l’s didn’t see the motor.
Check the fans bearings while your at it, insure the sheeves are aligned and that the proper belts are installed. 
I hate BX belts btw installed on old AHU’s.


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## 460 Delta (May 9, 2018)

Soilentblue9 said:


> I think 7.5hp. Not 100% sure. Breaker cost isn’t the issue. The mcc is incredibly old and sometimes when ya take it down control transformers end up smoking. I want to avoid that
> 
> 
> Sent from my iPhone using Tapatalk
> ...


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## SteveBayshore (Apr 7, 2013)

micromind said:


> I've connected a number of 15 HP motors operating at 480 to 30 amp breakers and have had no issues at all. The 40 should hold just fine.
> 
> As for the inrush, yes, high-efficient motors do indeed have higher inrush as well as higher locked-rotor currents. One easy way to combat this is to use the smallest wire that is allowed by code. In this case, it'd be #10. Smaller wire = higher resistance. Higher resistance = higher voltage drop, especially at inrush and locked-rotor currents. Higher voltage drop = lower current.


This is the first time I have ever heard of someone recommending cable voltage drop to be used to limit the current to a motor on purpose. High voltage drop = inefficient motor operation and overheating. What if the run is 400' long?


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## SWDweller (Dec 9, 2020)

I went and looked for the curve for your breaker. No Joy. I am sure if you give the 
local Eaton sales office a ring on Monday they can get one for you. I know that there is a server in Cherryton PA. where the data should be stored. At least that was the mother ship when I was there.


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## micromind (Aug 11, 2007)

SteveBayshore said:


> This is the first time I have ever heard of someone recommending cable voltage drop to be used to limit the current to a motor on purpose. High voltage drop = inefficient motor operation and overheating. What if the run is 400' long?


Sizing wire for a motor is more of an art than a science, all factors need to be looked at. 

Lets say the run is 400'. 

400' of #10 is a bit less than 1Ω, lets say it is 1Ω. If the full-load amps is 20, the voltage drop will be 20 volts. If the source is 480, the motor will see 460. 

Lets say the locked-rotor (starting) current is 6X. The motor will see 360 volts during starting. This is where it gets tricky........in this case, the load is a fan. The torque required starts at pretty much zero and increases as the speed comes up. There's no question that the voltage applied to the motor will start the fan. 

Look at it this way; suppose the starter is an autotransformer and the taps are set at 65%. (Most have 50, 65 and 80% settings). This starter will deliver 312 volts to the motor during starting. What's the difference between using wire to reduce starting voltage vs. a reduced-voltage starter? 

If the motor drives a fan or centrifugal pump or anything else where the load starts small and builds with speed, the motor will start it with a LOT of voltage drop. 

If the load is something like a fully loaded conveyer or a positive-displacement pump that starts against full head pressure, of course you'll need to hit the motor with higher voltage or it won't develop enough torque to start. 

Generally speaking, and depending on basic design, most motors will have peak efficiency at close to nameplate voltage. Higher than that and it'll be less efficient, same with lower. Another disadvantage to higher running voltage is that the higher the voltage, the lower the power factor. If the PUCO bills for VARS, low PF can get expensive. 

Lastly, the magnetizing current (about 1/60th of a second when it's first energized) can easily be 20X full-load current. If the motor is hit will full voltage here, it causes considerable mechanical stress on the windings.


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## paulengr (Oct 8, 2017)

Soilentblue9 said:


> Afternoon,
> Im wiring up an upgrade to an exhaust fan system. The new motor is 480v 3 phase 15 hp. I currently have a 40 amp cb. I know I can go up to 250% of the fla out of the codebook. My question is, is there a way to figure out if the 40 amp cb will handle the inrush. I know the locked rotor current of the motor. The circuit breaker is an EHB3040. Is there a graph of the instantaneous trip curve that anyone knows of?
> Thanks
> 
> ...


Yes, sort of. But this is where we get into the Twlight Zone of breakers and motors. Many motors have a locked rotor Code, usually H or J for 15 HP. There are tables showing the most common for a given horsepower. This gives you a range of locked rotor currents. A typical value would be say 8 times FLA. You can get a maximum FLA right from tge FLA charts in article 430. Beyond this motors may exceed this value on inrush for about 1-1.5 cycles before it drops down to locked rotor current. Inrush can be as high as 2.5 times the locked rotor current. Neither of these has anything to do with load. Now the motor will run right at locked rotor current until it reaches about 85% of speed at which point it drops down quickly to wherever it runs at depending on the no load current and the load current. Either way at this point the breaker and overload should not trip.

So there are 2 points to consider with the breaker curve. First there is the instantaneous region. On UL 489 breakers this depends on the trip curve that is A, B, or C which corresponds to 5, 10, or 20 times the long term rating. So a curve C, 40 A breaker trips instantaneously above 800 A. This may seem high but with NEC FLA of 21 A, code J motor might be almost 8 times FLA or 168 A. Inrush will be up to 2.5 times that or 420 A. So we are OK. But with the more common curve B breaker it’s only 400 A and it trips. Curve C breakers are meant for motor loads.

Second key spot is not so easy to figure out. The “knee” of the starting curve SHOULD be between 2 and 10 seconds with mist in the range of 2-4 except on high torque loads. There are ways to estimate starting time with only crude information or if you know the application is not high inertia you can just arbitrarily pick 6 or 10 seconds. So if you look at the breaker time-current curve (every manufacturer has this on their web sites) and look at the trip point in the range of 6-10 seconds as long as the locked rotor current is less, it won’t trip on starting.

This is with “regular” breakers. Supplemental motor protection devices such as manual motor starters and MCPs are different. With these devices they have some current limiting so you can ignore inrush. You set the MCP based on locked rotor and you don’t have to do anything with the MMS other than set the FLA for the ones with built in overload protection. That’s why UL 1077 supplemental protection is so much easier. UL inherently limits this stuff to 10x for instantaneous but they current limit so it works even with very high inrush.

With high inertia cases I’d recommend avoiding regular 488 breakers if you can. Fused (J or RK1) is another great option. But otherwise you must estimate the acceleration time accurately which is a royal pain without software assistance. Even then estimating Wk2 (inertia) is not easy and you must know this to do the calculation. Frankly it’s easier to just buy a soft start or VFD instead of messing with marginal breaker setups.

Another option is just use the largest allowed breaker. Just make sure the available short circuit is high enough to trip the breaker and it misses inrush as I described earlier.


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## mayanees (Jan 12, 2009)

Soilentblue9 said:


> Afternoon,
> Im wiring up an upgrade to an exhaust fan system. The new motor is 480v 3 phase 15 hp. I currently have a 40 amp cb. I know I can go up to 250% of the fla out of the codebook. My question is, is there a way to figure out if the 40 amp cb will handle the inrush. I know the locked rotor current of the motor. The circuit breaker is an EHB3040. Is there a graph of the instantaneous trip curve that anyone knows of?
> Thanks


I plotted a 15 hP motor with 8.5X Locked Rotor Amps and the EHB3040 can tolerate that level of inrush for around 5.5 seconds. 
I suspect you'll have no problem using the EHB 3040.


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## VELOCI3 (Aug 15, 2019)

FLC [email protected] 3 PH= 21 Amps 
INV time breaker 21 X 250%= 52.5
60 Amp breaker

Use the nameplate FLA to calculate wire and O/L size.
@ 125%


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## SteveBayshore (Apr 7, 2013)

VELOCI3 said:


> FLC [email protected] 3 PH= 21 Amps
> INV time breaker 21 X 250%= 52.5
> 60 Amp breaker
> 
> ...


According to the NEC you *must* use the NEC listed FLA when calculating wire, breaker and O/L size, not the nameplate FLA.


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## VELOCI3 (Aug 15, 2019)

SteveBayshore said:


> According to the NEC you *must* use the NEC listed FLA when calculating wire, breaker and O/L size, not the nameplate FLA.


NEC provides the FLC. Nameplate provides the FLA. O/L is based upon nameplate


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## paulengr (Oct 8, 2017)

VELOCI3 said:


> NEC provides the FLC. Nameplate provides the FLA. O/L is based upon nameplate
> 
> 
> Sent from my iPad using Tapatalk


That is a common interpretation. Overload I agree. The FLC/FLA thing is ambiguous. From a practical point of view though it makes sense to size breaker and wire on NEC table because then any motor should work.


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## VELOCI3 (Aug 15, 2019)

paulengr said:


> That is a common interpretation. Overload I agree. The FLC/FLA thing is ambiguous. From a practical point of view though it makes sense to size breaker and wire on NEC table because then any motor should work.













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## Soilentblue9 (Apr 11, 2014)

Thanks for y’all,s help on this one. I’m just gonna shut down the mcc and install new breakers.


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