Implementing Fuse Protection between inverters and 3-phase power source

There are 30 amps running on each phase

Not quite. If everything is normal, each phase will have less than the motor current rating. How much depends on the motor's load.

I don't have the code book handy (it's in the factory library) but I've got a cheat sheet here. This is what I've got:

• 2 hp - 15A breaker, 3.91A thermal overload (heat interlock)
• 5 hp - 15A breaker, 8.74A thermal overload
• 20hp - 50A breaker, 31.05A thermal overload

There's something wrong with your figures, I'm not sure if I understand correctly.

You've got a 460VAC, 3 phase power source with 30A per phase. Is that capable of 30A or already has 30A running through it?

If it's only capable of 30A, I'd say your system is under designed. Your 20hp motor is already rated at 27A. You've only got 3A overhead. You'll bust that fuse when you start your 2 or 3hp motors.

Thanks for your feedback.

Well let's say that the system at this point is capable of providing 30 per phase. And yes there will be 27 amps on the first motor with 20 Hp. I have placed a fuse protection for that rated to 50 amps, but this motor is not my concern because the fuses will take care well of it. As mentioned earlier the other two motors Motor2 and Motor3 repectivelly 2Hp and 5Hp are to be protected with fuses rated to 15amps. I can see the theoritical part of this according to the code book or uggly's book pages 70 and 80. However I am still wondering how this works in the pratical manner when like I said there going to be 30 amps per phase supplying those motors. The inverters for each motors connected between the 460vac 3-phase 30 amp/phase source are set on the same number you are describing.

I just can't see how a 15amp fuse protection won't blow up when 30 per phase are already there.

Is this what you have?

You're worried that the 15A fuses on the 2 and 5 hp motor load will blow? They shouldn't unless you get a short circuit. The motor will only draw as much current as it needs (unless it's overloaded, which is another matter).

Remember your basic electricity? If there are 30amps coming in on the main line, those 30 amps will be distributed among the three loads. You will not have 30A running through all the loads because that would be 90A which your system doesn't have.

That's not what worries me.

You say your system is capable of handling 30Amps but you've got three motors with a combined maximum total of almost 40Amps. You'll blow the main fuse for sure and probably burn up the cables if they're sized for 30Amps.

I'd advise you to get some readily available help over there. The guys here at CR4 are usually willing but helping via typing is going to take a long time and is difficult at best.

Thanks for your comments, yes your drawing is very close to what I have I tried to attach a pdf document that I converted from my cad drawings. I calculated a 30 amps current load on the 460 vac 3phase based on the total watts provided by the motors 2900 and the 1.7 factor used on this calculations. I came out with something very close to 30 amps. That is why I assumed such load. The currents on each motor were calcualted from table 430.150 of the NEC 2000 based on the Hp of each motor.

Please let me know if you have any other feedback

Sir do you have an email I could reach you? I would like to be able to send you a cad drawing of the system for better illustration of what I am attempting to do. Thanks for your time and considerations.

To send a personal message, you click on my user name (Vulcan). You'll find a link there that says "send a message to Vulcan". Then you can create your message without having to use your email address.

Also, don't give out your email address in this forum. That is in the CR4 FAQ.

Don't send me an AutoCAD drawing since I don't have AutoCAD in my PC. If you know how to draw in Powerpoint, you can make your drawing there, copy it to MS Paint and save it as a jpg or gif file. Then, you can insert it into your posts. This gif drawing was made that way:

A jpg drawing will have better color.

I may be wrong but I have a suspicion that reygalindo is getting confused over the nature of current draw. The supply is rated at 30 amps but this does not mean that 30 amps is flowing through the cables. The 15 amp fuses will be quite happy supplying the smaller motors and will only blow should some fault occur causing the motor to draw more than 15 amps. In this regard your design seems quite correct.

My concern is for the section supplying the 20 hp motor. If the supply is rated at 30 amps it is too small for the job. Having a 50 amp fuse between a 30 amp supply and the inverter makes no sense. The supply must have a minimum capacity of the largest protection device attached to it. In this case 50 amps.

Conclusion? Your supply needs to have a minimum rating of 50 amps but the rest of your design is fine.

Vulcan tu diseño esta ok, solo unos detalles, los valores de las termomagméticas no menores a estos valores, una caja de paso para distribución de los conductores y una linea a tierra por protección, se adjunta los grosores de los cables a ser usados, REYGALINDO recuerda que 75 voltios a 25 mA puede ser peligroso y si no sabes como cenectarlos puedes tener bonitos juegos artificiales y quemar a los motores :

Is it ok to use wire size number 8 for all of the 3 motors? I calculated the current to be nearly to 30 amps since the total watts from motors was close to 2900. The NEC suggest to use wire size 8 for current loads between 30 and 40 amps. Thanks for your comments.

Actually, it's not my design. I just drew what I thought his system looked like. He didn't provide the size of his cables but he did say that the system is good for 30A so it is definitely small for this system.

My Spanish is really rusty . I'm stumped on "cenectarlos" and "juegos" mystified me. Doesn't "juegos" mean "games"?

TW 6 AWG is the code cable, sorry my fingers, "cenectarlos" y want said connection, and "juegos artificiale = pyrotechnics games".

I read it as "artificial games".

When running multiple motors from one source, same conductors, the NEC allows the following:

125% current of the largest motor, 20 hp, 27 FLA: 33.75 amps

Full load current of 5 hp motor: 7.6 amps

Full load current of 2 hp motor: 3.4 amps

Total circuit amperage: 44.75 amps

Breaker size for this load: 50 amps

Feeder conductor size to line side of breaker: #8 ga (55 amps)

Conductor size from load side of 50 amp breaker to 20 hp motor: #10 ga (35 amps)

Conductor size from load side of 50 amp breaker to 20 amp breaker (5 hp motor): #12 ga (25 amps)

Conductor size from load side of 50 amp breaker to 15 amp breaker (2 hp motor) #12 ga (25 amps)

Fuses and circuit breakers are for ground fault and short circuit protection. Thermal overloads are for overcurrent protection. These factors should not be confused.

This is the correct way to determine the supply load that you will need to power the 3 motors. A 30 amp circuit will not work for this application.

Good luck with this project!

Jeff

Thanks for your feedback Jeff, I actually planned to use a 100amp service breaker to feed the 460 vac, 3-phase 30 amp power source for all the 3 motors. Of course each motor will its respective fuse protection. Lets say the 27 amp times 2.5 equals 67.5 thus I should use a fuse protection for 70amps on this 20Hp motor.

In a simmilar manner the 2Hp with 3.4 times 2.5 equals 8.5 so I decided to use a 10 amp fuse protection for it. On the 5Hp the current is 7.6. That times 2.5 equals 19 amps so I will use a 20 amps fuse protection. Correct me if I am wrong to what I understand here is that I could also use 300% as a multiple factor with the current load of each motor depending on what kind of fuse I want instead of the 250% multiple factor. This is some of the information I got from table 430.152 on the NEC 2000.

Again I just wanted to make sure that I am in the right track. I greatly appreciate your feedback.

ReyGalindo

ReyGalindo--

A couple errors: 1) the 2.5 multiplier you are using is for inverse-time circuit breakers, not fuses. With the typical dual-element fuses, you are limited to the 1.75 (175%) multiplier (see NEC table 430.52). 2) Regarding wire sizing from another post, the wire size is based on the motor current, set to no less than 125% of the nameplate (NEC section 430.22), even though the fuse is larger. Using 12-awg wires for the smaller motors is OK but not the minimum permitted.

Now, a bigger problem: Your wire sizes and fuse (or breaker) sizes need to be according to the inverter manufacturer's requirements, not just based on the motor sizes. See NEC 110.3(B), which is often ignored by many electricians, but is the real direction you need.

Finally, the posts about total current draw are based on the assumption that the motors are going to be loaded to their full name plate value. Most equipment is designed with the motors somewhat oversized, to allow for voltage dips and aging or other problems with the equipment. With the inverters supplying the power to the motors, they will automatically supply the voltage and current necessary to meet the load the motors are actually driving, so it is quite common to see the inverter's input be a lower number of amps than the motors were drawing when operated directly from the line with conventional starters. In that regard, the inverters are acting as an energy-conserving device (I could discuss more of the theory, but this simpler comment is enough here). Therefore, I would not be surprised to find your installation with the three motors actually draws less than the 30 amps you first posted.

Also, pay careful attention the the inverter manufacturer's instructions about distance from the inverter to the motor, as well as any grounding requirements, and make sure that the motor is rated for use on an inverter (the insulation needs to have a higher than expected voltage rating).

As Vulcan has pointed out, there are ways to reach individuals on CR4 via direct email, so contact me if you want to explore this more. --JMM

If each <...Inverter...> is programmed correctly then it will protect the downstream motor. The <...fuse...> is there to protect the downstream cables. Nothing has been said about the installed cables!

If in doubt, consult a qualified local Electrician - preferably before there is an incident that makes one unpopular with the facility's fire insurance provider.