Feeder size fed from generator receptacle

Status
Not open for further replies.

JP23

Member
Location
California
A 1-line shows 1/0 conductors fed from a 200 amp generator reverse service generator receptacle
in conduit running 60' through a building to a 200 amp non-fused transfer switch and then to an
adjacent 22 Kaic panel with a 150 amp main. I have a few questions regarding this.

1) 240.4 requires conductors to be protected at the point of supply. The generator feeder as shown doesn't comply with
tap rules so shouldn't the feeder conductors fed from the receptacle be required to be 200 amp rated to match the receptacle rating?
Wondering what I'm missing.

2) The transfer switch is rated at 10 kaic and the panel is calculated at 22 kaic. It seems to me that
a transfer switch rated at least 22 kaic would be required or fused protection should be added in front
of it to raise the SCCR. Just wondering how the EE is justifying this or is there something in 110.10 I'm missing?

Any input would be greatly appreciated.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
A 1-line shows 1/0 conductors fed from a 200 amp generator reverse service generator receptacle
in conduit running 60' through a building to a 200 amp non-fused transfer switch and then to an
adjacent 22 Kaic panel with a 150 amp main. I have a few questions regarding this.

1) 240.4 requires conductors to be protected at the point of supply. The generator feeder as shown doesn't comply with
tap rules so shouldn't the feeder conductors fed from the receptacle be required to be 200 amp rated to match the receptacle rating?
Wondering what I'm missing.

2) The transfer switch is rated at 10 kaic and the panel is calculated at 22 kaic. It seems to me that
a transfer switch rated at least 22 kaic would be required or fused protection should be added in front
of it to raise the SCCR. Just wondering how the EE is justifying this or is there something in 110.10 I'm missing?

Any input would be greatly appreciated.

1. If there is a breaker at the generator, that determines the size of conductor needed, not the rating of the generator receptacle.

2. The AIC from the generator is almost certainly far less than 10kA. Generators do not have a high surge output, at least not that high compared to their nominal output. Even if it were higher, it would be reduced by the 60' of feeder. The panel calculation of 22kaic is presumably done based on the utility service AIC. But the transfer switch carries (at different times) both the generator fault current and the service fault current. If the 22kAIC calculation is correct then the transfer switch will need to be rated for 22kA.
Fuses do not raise the AIC rating of a device, they (may) limit the fault current that the device will be exposed to.
 

Smart $

Esteemed Member
Location
Ohio
My take on #1 is compliance is not required until you connect a functional generator... and there would only be a violation if a generator of greater than the ampacity of the 1/0 conductors and protected output or ampacity of 1/0 conductors divided by 115% if unprotected is connected.

As for #2, if the available fault current (AFC) on the service side of the transfer switch is not more than 10kAIC, all is good. Do you know the AFC?
 

JP23

Member
Location
California
The AFC is 22 at the panel which is adjacent to the transfer switch.
Looks like they need to spec out either a fused disconnect in front of the
transfer switch or a fused transfer switch that has a higher rating when fuses
are installed. Seems like common sense that the ampacity of the conductors
fed from a generator receptacle should match the receptacle rating but I guess that's
not the case. Thanks for your replies!
 

Smart $

Esteemed Member
Location
Ohio
The 22k came from the EE. I'm still working on learning all the steps for calculating
the fault current so I have been relying on them to provide that information.
Sorry to be pedant about the matter, but it seems something is amiss.

I find it strange that a transfer switch has a 10kAIC rating if it doesn't have integral OCPD, at least on the normal supply side. Yet you talk of adding fuses.

Then there's the issue of the 22kAFC at the panel according to the EE, but yet he spec'd a 10kAIC switch to go in front of it. Any chance this switch and panel are a listed series combination?
 

JP23

Member
Location
California
I communicated with the engineer and he accidentally left out the rating on the plans.
Usually if there is no rating specified the equipment is 10 kaic. I noticed that he
had 22 kaic for the panel and not the transfer switch. I did a little research and
found that many manual double throw switches are only rated at 10k but if you
add certain fuses in front of it to protect it the rating is increased. Some models
are increased if you use a fused double throw switch and certain fuses specified
by the manufacturer. Check out the info at the top of this page in the link below.
I appreciate your responses and interest.

http://w3.usa.siemens.com/powerdist...switches/double-throw/Pages/double-throw.aspx
 
Status
Not open for further replies.
Top