Tap rule

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makalroy

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I?m changing out a broken 200amp generator with a 400amp generator. I don?t need 400amps its just what we have. The gen is feeding a 200 amp panel with 3/0 wire. Do I have to resize my feeder or does the tap rule cover me. (240.21b2). The panel is within 25 ft., feeding a 200amp ocpd (and that?s all), and its run underground.
Thanks
 
sight unseen, sounds to me like you are coverd. Possibly even 240.21(B)(5)
 
The tap rules only cover the required OCPD, they do not change the ampacity of the wire. The rule in 445 requires that the conductors between the generator and the first OCPD have an ampacity of at least 115% of nameplate ampacity of the generator.
 
don_resqcapt19 said:
The tap rules only cover the required OCPD, they do not change the ampacity of the wire. The rule in 445 requires that the conductors between the generator and the first OCPD have an ampacity of at least 115% of nameplate ampacity of the generator.
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IMO those would be the conductors between the windings and the generator mounted breaker.
 
iwire said:
IMO those would be the conductors between the windings and the generator mounted breaker.
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That would assume that the generator has a breaker. I don't think I have ever installed one of that size that had a beaker as part of the generator.
 
iwire said:
IMO those would be the conductors between the windings and the generator mounted breaker.
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Just to put my two cents in don't agree with iwire because the gen is a listed piece of equipment and the internal wiring doesn't apply BUT if the gen has a gen mounted breaker then IMO the 115% rule doesn't apply
 
don_resqcapt19 said:
The tap rules only cover the required OCPD, they do not change the ampacity of the wire. The rule in 445 requires that the conductors between the generator and the first OCPD have an ampacity of at least 115% of nameplate ampacity of the generator.
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There is a 400 amp breaker mounted to the gen.
 
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Quick question. If the loads after the main for some unforeseen reason came to more than 200 A wouldn't it trip the main stopping the generator from putting out enough to damage the 3/0 going to the main brkr anyway?

I wouldn't think the generator having a capacity of 400A implies that it's going to kick out 400A or nothing at all.
 
iwire said:
I have not seen any new gen set without a factory mounted breaker.

Only old installations.
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You can order them anyway you want. Most of the ones I have installed have been in a building with the distribution panel in the same room and these were ordered without a breaker on the generator. All of the ones that were in their own outside enclosure had breakers.
 
Twoskinsoneman said:
Just to put my two cents in don't agree with iwire
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That is not an option. :D


because the gen is a listed piece of equipment and the internal wiring doesn't apply BUT if the gen has a gen mounted breaker then IMO the 115% rule doesn't apply
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Actually I agree, my original response was not clear.

If there is no factory mounted breaker on the genset and I have to run conductors from the windings to the first over current device I must comply with the 115% rule.

If however the factory installs the first over current device after the winding then the 115% rule has no effect at all.
 
daleuger said:
Quick question. If the loads after the main for some unforeseen reason came to more than 200 A wouldn't it trip the main stopping the generator from putting out enough to damage the 3/0 going to the main brkr anyway?.
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That is exactly the point of 'tap rules'.

In general, the NEC requires that conductors be protected from overcurrent at their source of supply.

However there are several exceptions to this general rule, where the conductors are protected by some 'downstream' device. If you follow the limits given in the various tap rules, then the tap conductors are considered protected by the downstream breaker.

-Jon
 
iwire said:
That is not an option. :D





Actually I agree, my original response was not clear.

If there is no factory mounted breaker on the genset and I have to run conductors from the windings to the first over current device I must comply with the 115% rule.

If however the factory installs the first over current device after the winding then the 115% rule has no effect at all.
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Gotcha. I understand now and agree with you.

For the record I think the NEC leaves room for doubt whether or not a breaker on the generator really is enough to satisfy the exception of 445.13. However since the NEC isn't clear I look to NECA (National Electrical Contractors Associastion) document NECA 406-2003 Standard for Installing Residential Generator Sets for clarity.
 
winnie said:
That is exactly the point of 'tap rules'.

In general, the NEC requires that conductors be protected from overcurrent at their source of supply.

However there are several exceptions to this general rule, where the conductors are protected by some 'downstream' device. If you follow the limits given in the various tap rules, then the tap conductors are considered protected by the downstream breaker.

-Jon
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Thank you. That's the gist I was getting when I read the tap rules. Just wanted to make sure I understood that correctly.
 
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