cfrancka
Member
- Location
- Independence, Missouri
Why does the conductors have to be rated at 115% of name plate rating
Generator--115% rule
- Thread starter cfrancka
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
because the code says so?
ceb58
Senior Member
- Location
- Raeford, NC
They don't if the generator has a OCP device built in.
- Location
- Massachusetts
I do not know, but are you aware that is for the conductors between the windings and the first over currentdevice which is generally already installed on the genset.
iceworm
Curmudgeon still using printed IEEE Color Books
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- North of the 65 parallel
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- EE (Field - as little design as possible)
445.12 Overcurrent Protection.
(A) Constant-Voltage Generators. Constant-voltage generators, except ac generator exciters, shall be protected from overloads by inherent design, circuit breakers, fuses,
or other acceptable overcurrent protective means suitable for the conditions of use.
445.13 Ampacity of Conductors.
The ampacity of the conductors from the generator terminals to the first distribution device(s) containing overcurrent protection shall not be less than 115 percent of the nameplate current rating of the generator. It shall be permitted to size the neutral conductors in accordance with 220.61. Conductors that must carry ground-fault currents shall not be smaller than required by 250.30(A). Neutral conductors of dc generators that must carry ground-fault currents shall not be smaller than the minimum required size of the largest conductor.
Exception: Where the design and operation of the generator prevent overloading, the ampacity of the conductors shall not be less than 100 percent of the nameplate current rating of the generator.
This is from the 2008. I don't think it has changed much.
Disclaimer: Following is personal opinion. I work with generator installation a lot - really a lot. It is still only opinion. And since this is the anonomous internet I personally wouldn't think there was any authority.
This is really generalized. One can easily find holes everywhere. And I'm thinking most everyone knows all this.
Genset Iinstallation Design 101 per the worm
The NEC is uncharacteristicaly silent about generator installations. OCP sizing, ampacity sizing are very design dependent. Interestingly, 445 applies equally to a 10kw off-grid residential Lister as to a 10MW, 13.8KV gas turbine. I left out the hardware store Briggs type cause they tend to show up cord and plug
445.13, ampacity from the gen to the first OCP, is directed at generators that did not come with a CB. If the generator shows up with a CB, then 445.13 does not apply. The mfg selected the wire from the gen to the CB. The NEC is not concerned. The conductors from the gen CB is a feeder and, of course, is sized using normal NEC methods.
If the gen does not have an integral CB, then 445.13 now cares. Now the installed conductors have to meet 445.13. Note the 445.13 Exception. The conductors may not need to be 115%. They could be 100% of nameplate. Plenty of generators have marginally sized drivers. Loaded over 100% and the driver runs out of horsepower - slows down and trips. Standby rated units are the worst for this. The drivers are right flat thin. The mfgs don't tend to put in one extra mouse-power if it will save a penny (yes - truth)
And there is another wrench for the soup:
The generator can have a low pf load - lagging is the worst. Rotor current is high, stator current is high, throttle is pulled way back - low real power. Gen and conductors are getting hot, but the driver is just fine. Most gens have heavy derates for heavy lagging pf loads. Now the protections are really design dependent - waving a code book at the genset/load is not going to save anything. Leading side pf loads have other issues - mostly stability, not really part of this discussion.
Here is one example - not one of my customers, but one I saw. It is a standby 2.25MW CAT. The only way it will put out 2.25MW is to tease it up on a resistive load. At 100% power, the least little bump and down she goes. The same driver/generator can be purchased rated for ~2MW continuous or 1750KW prime. The control settings are different, but it is the same driver/alternator.
In this example, one might select the gen to first OCP conductors at 100% for the standby case and at 115% for the continuous or prime duty. Both would meet code. Generally these are design decisions.
Although not asked, the OCP is part of the design equation. 445.12 does not give much guidance. Says the generator has to be protected. Doesn't say anything about the conductors. The first CB is not selected to protect the conductors. It ends up getting picked to not trip on the biggest surge (examples: motor starting, xfm inrush) the gen might see. The driver/gen rotor has enough inertia that it will put out short circuit current for a few cycles - and normal xfm inrush may well hit that. The genset gets jerked to it's knees and staggers back to speed and the CB didn't trip, all is good. One might size the first OCP at 125% and if less than 800A, next size up.
Consider a continuous rated genset, no CB on the genset. Gen to first OCP conductors are rated at 115%. Feeder from OCP to distribution are rated at 125%. A little dumb, but meets code minimum.
So, one might say: 445.13 is not mandating oversize conductors, rather it is allowing the conductors to the first OCP to be undersized.ice
(A) Constant-Voltage Generators. Constant-voltage generators, except ac generator exciters, shall be protected from overloads by inherent design, circuit breakers, fuses,
or other acceptable overcurrent protective means suitable for the conditions of use.
445.13 Ampacity of Conductors.
The ampacity of the conductors from the generator terminals to the first distribution device(s) containing overcurrent protection shall not be less than 115 percent of the nameplate current rating of the generator. It shall be permitted to size the neutral conductors in accordance with 220.61. Conductors that must carry ground-fault currents shall not be smaller than required by 250.30(A). Neutral conductors of dc generators that must carry ground-fault currents shall not be smaller than the minimum required size of the largest conductor.
Exception: Where the design and operation of the generator prevent overloading, the ampacity of the conductors shall not be less than 100 percent of the nameplate current rating of the generator.
This is from the 2008. I don't think it has changed much.
Disclaimer: Following is personal opinion. I work with generator installation a lot - really a lot. It is still only opinion. And since this is the anonomous internet I personally wouldn't think there was any authority.
This is really generalized. One can easily find holes everywhere. And I'm thinking most everyone knows all this.
Genset Iinstallation Design 101 per the worm
The NEC is uncharacteristicaly silent about generator installations. OCP sizing, ampacity sizing are very design dependent. Interestingly, 445 applies equally to a 10kw off-grid residential Lister as to a 10MW, 13.8KV gas turbine. I left out the hardware store Briggs type cause they tend to show up cord and plug
445.13, ampacity from the gen to the first OCP, is directed at generators that did not come with a CB. If the generator shows up with a CB, then 445.13 does not apply. The mfg selected the wire from the gen to the CB. The NEC is not concerned. The conductors from the gen CB is a feeder and, of course, is sized using normal NEC methods.
If the gen does not have an integral CB, then 445.13 now cares. Now the installed conductors have to meet 445.13. Note the 445.13 Exception. The conductors may not need to be 115%. They could be 100% of nameplate. Plenty of generators have marginally sized drivers. Loaded over 100% and the driver runs out of horsepower - slows down and trips. Standby rated units are the worst for this. The drivers are right flat thin. The mfgs don't tend to put in one extra mouse-power if it will save a penny (yes - truth)
And there is another wrench for the soup:
The generator can have a low pf load - lagging is the worst. Rotor current is high, stator current is high, throttle is pulled way back - low real power. Gen and conductors are getting hot, but the driver is just fine. Most gens have heavy derates for heavy lagging pf loads. Now the protections are really design dependent - waving a code book at the genset/load is not going to save anything. Leading side pf loads have other issues - mostly stability, not really part of this discussion.
Here is one example - not one of my customers, but one I saw. It is a standby 2.25MW CAT. The only way it will put out 2.25MW is to tease it up on a resistive load. At 100% power, the least little bump and down she goes. The same driver/generator can be purchased rated for ~2MW continuous or 1750KW prime. The control settings are different, but it is the same driver/alternator.
In this example, one might select the gen to first OCP conductors at 100% for the standby case and at 115% for the continuous or prime duty. Both would meet code. Generally these are design decisions.
Although not asked, the OCP is part of the design equation. 445.12 does not give much guidance. Says the generator has to be protected. Doesn't say anything about the conductors. The first CB is not selected to protect the conductors. It ends up getting picked to not trip on the biggest surge (examples: motor starting, xfm inrush) the gen might see. The driver/gen rotor has enough inertia that it will put out short circuit current for a few cycles - and normal xfm inrush may well hit that. The genset gets jerked to it's knees and staggers back to speed and the CB didn't trip, all is good. One might size the first OCP at 125% and if less than 800A, next size up.
Consider a continuous rated genset, no CB on the genset. Gen to first OCP conductors are rated at 115%. Feeder from OCP to distribution are rated at 125%. A little dumb, but meets code minimum.
So, one might say: 445.13 is not mandating oversize conductors, rather it is allowing the conductors to the first OCP to be undersized.ice
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