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Old 08-27-2011, 10:55 AM   #1
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Wire Gauge for 100 AMP Generator Feed


A few years ago I had a 12kV (100 AMP) Generac backup generator installed in my home by a licensed electrician/handyman. Although this guy was licensed, I have recently begun to question his decisions with this generator installation.

The wiring from the generator into the transfer switch is #8 gauge stranded copper and looks tiny compared to the giant aluminum street power feed. The distance of the generator feed is approx 150 feet.

This seems inadequate to me. Any advice?


Last edited by jtreibick; 08-27-2011 at 10:58 AM.
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Old 08-27-2011, 11:03 AM   #2
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Wire Gauge for 100 AMP Generator Feed


I believe you are correct on the wire sizing, sized at #8 it is only rated for 50 amps

http://lugsdirect.com/WireCurrentAmp...ble-301-16.htm

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Old 08-27-2011, 11:35 AM   #3
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Wire Gauge for 100 AMP Generator Feed


Unless your generator is 120 Volt only, the wire size is correct for a 12 kVA unit. I doubt that your generator has an output of 12,000 volts.

It would be 50 Amps per leg on a 120/240 Volt system.

I just installed the same size system for my place, but used a #6 wire instead.
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Old 08-27-2011, 11:59 AM   #4
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Wire Gauge for 100 AMP Generator Feed


I believe generator feeds need to be rated at 115% of the generator output.
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Old 08-27-2011, 12:12 PM   #5
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Wire Gauge for 100 AMP Generator Feed


Quote:
Originally Posted by jtreibick View Post
A few years ago I had a 12kV (100 AMP) Generac backup generator installed in my home by a licensed electrician/handyman.
Well which is it, handyman or electrician. They are mutually exclusive.

Also, I assume you mean 12kW, not kV.
This is 50A, and while #8THHN in conduit is acceptable for a 50A circuit, I would have used #6.
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Old 08-27-2011, 12:12 PM   #6
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Wire Gauge for 100 AMP Generator Feed


Quote:
Originally Posted by brric View Post
I believe generator feeds need to be rated at 115% of the generator output.
Do you know where this is in the code? I am truly curious as I don't remember seeing this.
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Old 08-27-2011, 12:23 PM   #7
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Wire Gauge for 100 AMP Generator Feed


Quote:
Originally Posted by Speedy Petey View Post
Do you know where this is in the code? I am truly curious as I don't remember seeing this.
Look in section 445.13

Edit to add: There is an exception to this requirement if the generator has proper design and operation that would prevent overloading of the conductors sized at 100% of the nameplate current.
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Last edited by kbsparky; 08-27-2011 at 12:27 PM. Reason: Added postscript
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Old 08-27-2011, 12:26 PM   #8
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Wire Gauge for 100 AMP Generator Feed


Quote:
Originally Posted by Speedy Petey View Post
Do you know where this is in the code? I am truly curious as I don't remember seeing this.
I remember it from somewhere but haven't found it yet.
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Old 08-27-2011, 12:34 PM   #9
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Wire Gauge for 100 AMP Generator Feed


I wonder what they mean by this: "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."

Load shedding?
So a genset with a main transfer switch would need the 115% conductors, but one with a transfer panel and limited circuits could be 100%?
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Old 08-27-2011, 01:12 PM   #10
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Wire Gauge for 100 AMP Generator Feed


The breaker on the generator the first distribution device.

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.
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Old 08-27-2011, 01:21 PM   #11
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Wire Gauge for 100 AMP Generator Feed


Quote:
Originally Posted by Code05 View Post
The breaker on the generator the first distribution device.

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.
That's not how I would interpret it but I guess one could. The NEC does not generally cover listed manufactured equipment so I would say the terminals of the CB would be the terminals they are refering to.

Last edited by brric; 08-27-2011 at 01:26 PM.
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Old 08-27-2011, 01:36 PM   #12
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Wire Gauge for 100 AMP Generator Feed


Quote:
Originally Posted by Speedy Petey View Post
I wonder what they mean by this: "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."

Load shedding?
So a genset with a main transfer switch would need the 115% conductors, but one with a transfer panel and limited circuits could be 100%?
One instance where the exception would apply is a co-generator.

The amount of current produced by a co-generator is limited by its protection relays. Anything over 100% for more than a few seconds will cause the relay to trip, and disconnect the generator from the load.

In this case, there's no need for conductors rated at more than 100% of the nameplate current.

The type of co-generators I work with are usually 12.5 - 13.8KV, and produce 10 - 30 MW. (That's 10,000 - 30,000 KW). The extra expense of larger 15KV cable can amount to quite a bit.

These power plants are privately owned, and therefore subject to the NEC, not POCO rules.
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Old 08-27-2011, 01:40 PM   #13
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Wire Gauge for 100 AMP Generator Feed


Quote:
Originally Posted by Code05 View Post
The breaker on the generator the first distribution device.

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.
Typically, a generator that operates at more than 600 volts will have its circuit breaker remotely mounted. Sometimes quite a distance away.

I've also seen generators under 600 volts that had breakers mounted in the transfer/synchronization gear, not at the generator.
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Old 08-27-2011, 01:41 PM   #14
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Wire Gauge for 100 AMP Generator Feed


Quote:
Originally Posted by brric View Post
That's not how I would interpret it but I guess one could. The NEC does not generally cover listed manufactured equipment so I would say the terminals of the CB would be the terminals they are refering to.
I will agree that is how it is usually interpreted and installed, but IMO, it does not clearly state that.
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Old 08-27-2011, 01:44 PM   #15
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Wire Gauge for 100 AMP Generator Feed


I think micro has the answer.

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