generator overcurrent protection.

[tim89s]

I am very confused. I am trying to size my breaker that feeds a 30 hp 460 volt 34 fla generator. I am having trouble with 445.12(A) and 445.13. Let's assume first that the generator doesn't have a means to prevent overloading. Then I size my conductors at not less than 115% of fla. Then size the breaker to protect the conductors. Is this correct? Now let's assume it has a means to prevent overloading. I then size the conductors at not less than 100% fla, then size the breaker to protect the conductors. Is this correct?

 

[Dubledox]

I size my conductors at not less than 115% of fla. Then size the breaker to protect the conductors.

Correct, but protecting the conductors isn't the only thing you care about. Your overcurrent protection should first and foremost protect the generator (see the thermal damage curve on its TCC for example), and also not trip for inrush current.

Now let's assume it has a means to prevent overloading.I then size the conductors at not less than 100% fla, then size the breaker to protect the conductors. Is this correct?

As per the exception in 445.13, you would be correct. Conductors get sized at at least 100% FLA here, rather than 115%, because you have taken measures of making sure it cannot, such as programming in the generator's controller. Throwing on a MCCB does not count as prevention of overloading.

See the above note about protection again. Sizing the conductors isn't protecting the generator really, it's making sure the FLA from the generator won't weaken the structural integrity of your cable (or start any fires).

 

[tim89s]

Correct, but protecting the conductors isn't the only thing you care about. Your overcurrent protection should first and foremost protect the generator (see the thermal damage curve on its TCC for example), and also inrush current.

This is where I get very confused. You state the overcurrent is to protect the generator as well, but the code doesn't state you can use a maximum size overcurrent device like it does for motors. For example, could I feed the generator mentioned above with #2 thhn and put it on a 100 amp breaker? I am not saying I would do this, I am just wondering what in the code stops you from doing this.

 

[Dubledox]

You state the overcurrent is to protect the generator as well, but the code doesn't state you can use a maximum size overcurrent device like it does for motors. For example, could I feed the generator mentioned above with #2 thhn and put it on a 100 amp breaker? I am not saying I would do this, I am just wondering what in the code stops you from doing this.

So you are asking if you could theoretically oversize the conductors and the overcurrent protection device because the code does not specifically say you can't? I do not think anything in the code stops you from doing this. The code just provides minimum safety guidelines. But your overcurrent protection is not protecting anything if you size it too high. At that point you may as well not put on a circuit breaker on the line at all. You want it to trip at 115% to protect equipment further down the line. Standard practice is take the immediate size up after calculating 115% FLA. Here, for example, a 40A circuit breaker would do nicely.

There's nothing inherently wrong with oversizing the wire, as long as your terminations are good. In fact, larger cables will help lower the voltage drop to the load so in some applications it may even be necessary.

 

[kwired]

One must also remember with a small generator the impedance of the source is often pretty high compared to utility supplied sources. So starting surges from motor loads will not reach same peak current levels - but duration of acceleration will last longer because of lower voltage during starting.

And we are not feeding the generator - we are protecting it from delivering too much current.

You shouldn't need to provide a 100 amp conductor just because a generator has 100 amp output capacity - like any other circuit you need at least a conductor that can carry minimum required ampacity - but can go with more conductor. A generator with a 100 amp output capacity however needs protection for the generator itself from overload regardless of what it is supplying. Many smaller capacity gensets already have overcurrent protection as an integral part of the unit. Some more basic generators and/or larger capacity units though you may need to select your own devices to protect it.

 

[tim89s]

kwired you stated "A generator with a 100 amp output capacity however needs protection for the generator itself from overload regardless of what it is supplying"
How is the size of the overcurrent determined? 240.3 refers you to article 445. 445.12 doesn't state how high or low you can go.
I have a gen that is rated 34 amps. I just so happen to have a 50 amp breaker. Can I use the 50 amp breaker or does it have to be on a 40 amp breaker assuming no overcurrent inside the controller or 35 amp breaker assuming overcurrent inside the controller.

 

[kwired]

kwired you stated "A generator with a 100 amp output capacity however needs protection for the generator itself from overload regardless of what it is supplying"
How is the size of the overcurrent determined? 240.3 refers you to article 445. 445.12 doesn't state how high or low you can go.
I have a gen that is rated 34 amps. I just so happen to have a 50 amp breaker. Can I use the 50 amp breaker or does it have to be on a 40 amp breaker assuming no overcurrent inside the controller or 35 amp breaker assuming overcurrent inside the controller.

NEC covers protection of the conductors, design of the generator will dictate how much overloading it can take and I would think you need to look at any instructions for that generator to determine how to protect it. This is usually not a problem for an installer with most smaller portable generators or for units designed to be residential or light commercial standby units as they typically already have an overcurrent protection device installed in them.