Back feed thru a fusible disconnect

[Dakota]

On another forum, I was following a thread about the back fed breaker heating up
under load.

My Utility requires a lockable OFF AC disconnect, so I am considering using a fusible type
disconnect with a couple of 60 amp fuses for the back fed OCPD and avoid using
a back fed breaker altogether.


I never see any examples of connecting that way --- is there a problem with doing this?

I should add my Utility Service entrance is on the outside of a detached garage and from
the meter/ base enclosure goes to a Siemens #WB2225 Outdoor Circuit Breaker Enclosure
with a 225 Amp max Rating and a 200 AMP breaker installed. This then feeds a Midwest
T1220 Terminal box which I will also be feeding from the fusible AC disconnect.

Planning on using a Fronius 11.4-1 inverter with a Max continuous rating of 47.5 Amps.
Any problems with doing it this way?

[Naptown]

You are actually a bit above what you could backfeed and will need to do a line tap which in your case is fairly easy since everything is outside.
The issue in your original question is some manufacturers of breakers do not work well with backfeeding. Specifically if the circuit breaker is marked with line and load designations on the terminals they may not be backfed.
A line tap is something that should be performed by a licensed electrician who is familiar with the procedure

[inetdog]

...The issue in your original question is some manufacturers of breakers do not work well with backfeeding. Specifically if the circuit breaker is marked with line and load designations on the terminals they may not be backfed.

You may ask what difference it makes which way the power is flowing, since AC alternates the direction of both current and voltage anyway.
An oversimplified answer is that it makes no difference when the breaker is closed (unless it is a GFCI or AFCI breaker, which we will ignore as not appropriate for this use.) The problem arises while the breaker is opening and after it has opened. Those two situations are not symmetric between forward and back feeding.
Backfeeding from GTI is inherently different from backfeeding from a source such as a generator which can operate independently, but the NEC does not make any distinction between the two types of backfeeding.

[Dakota]

No, that should meet the NEC 120% rule --- that Siemens #WB2225 Outdoor
Circuit Breaker Enclosure has a 225 AMP Bus rating, so 225A X 1.2 = 270 Amps
and with a 200 AMP breaker installed could go to a 70 A OCPD.

The inverter has a Max continuous rating of 47.5 Amps so should be OK with
60 amp fuses.

I'm still a little uncertain if it's kosher to back feed thru that fusible AC disconnect
because I can't find any examples of doing that. I can't see a problem with that
method as long as the Utility is on the Line input and the inverter on the Load terminal
so that when the AC Disconnect is in the OFF position the fuse terminals are not hot.

You are actually a bit above what you could backfeed and will need to do a line tap which in your case is fairly easy since everything is outside.
The issue in your original question is some manufacturers of breakers do not work well with backfeeding. Specifically if the circuit breaker is marked with line and load designations on the terminals they may not be backfed.
A line tap is something that should be performed by a licensed electrician who is familiar with the procedure

[Zee]

On another forum, I was following a thread about the back fed breaker heating up
under load.

My Utility requires a lockable OFF AC disconnect, so I am considering using a fusible type
disconnect with a couple of 60 amp fuses for the back fed OCPD and avoid using
a back fed breaker altogether.


I never see any examples of connecting that way --- is there a problem with doing this?

I should add my Utility Service entrance is on the outside of a detached garage and from
the meter/ base enclosure goes to a Siemens #WB2225 Outdoor Circuit Breaker Enclosure
with a 225 Amp max Rating and a 200 AMP breaker installed. This then feeds a Midwest
T1220 Terminal box which I will also be feeding from the fusible AC disconnect.

Planning on using a Fronius 11.4-1 inverter with a Max continuous rating of 47.5 Amps.
Any problems with doing it this way?

You will need to connect to the MAIN panel. The rating of the breaker that feeds the subpanel, for CODE reasons, will block subpanel tie-in.

You do not need to do a line tap. ====>

NAptown: 120% x 225A bus > 200A + 60A breakers
Should be fine.

You do not sound like someone qualified to do a line tap in any case, i agree 100% with NAPTOWN.

[Zee]

but how are you connecting in the MAIN PANEL, w/o a breaker?

And why fusible?
Just put a 2p60 brkr. in the main and use a non-fusible disco.

[inetdog]

I'm still a little uncertain if it's kosher to back feed thru that fusible AC disconnect
because I can't find any examples of doing that. I can't see a problem with that
method as long as the Utility is on the Line input and the inverter on the Load terminal
so that when the AC Disconnect is in the OFF position the fuse terminals are not hot.

Where in or outside the panel will you be connecting your fused disconnect? If it is not landing at a breaker in the main panel, then it is necessarily a line side tap.
As long as you follow the conductor size and other limitations for a line side tap, I do not see it being significant whether you use a fused disconnect or a subpanel with a main breaker.

A lot of standard illustrations and guides do not seem to get that when the disconnect is open there is no power on the GTI side.

[Zee]

oh, good you are diligent and checked: yes it is kosher to backfeed both fusible and non-fusible ac discos.
Like ya said: LINE side of disco= utility!
Breakers have rare but real issues with backfed currents as NAPTOWN said very well.

[Naptown]

No, that should meet the NEC 120% rule --- that Siemens #WB2225 Outdoor
Circuit Breaker Enclosure has a 225 AMP Bus rating, so 225A X 1.2 = 270 Amps
and with a 200 AMP breaker installed could go to a 70 A OCPD.

The inverter has a Max continuous rating of 47.5 Amps so should be OK with
60 amp fuses.

I'm still a little uncertain if it's kosher to back feed thru that fusible AC disconnect
because I can't find any examples of doing that. I can't see a problem with that
method as long as the Utility is on the Line input and the inverter on the Load terminal
so that when the AC Disconnect is in the OFF position the fuse terminals are not hot.

You are correct I forgot about the 200A breaker. Now the trick is to prove it to the AHJ who typically takes one look at the breaker label and goes from there. Are there markings on the panel that specify that the buss bars are rated for 225A?

[Dakota]

Yep, the label is still on the backside of the enclosure door and specs the
225 Amp bus bars. I also went to Siemens web site and printed out all
the specs on that model.

I'm not sure I am getting across how the power is coming in here.
I have an underground service that of course first feeds a meter on the back of
a detached garage. From the meter enclosure it feeds, right next to it, this
Siemens Outdoor Circuit panel which has only one the one 200 Amp breaker
in it.
This feeds, right next to it a large Midwest Terminal or junction box
and the main power lines to my house, over a 100 feet away, are
attached to the lugs in this enclosure and then go underground.

Seeing as I will have the AC disconnect right next to this terminal box,
the simplest way is to just feed directly out of the disconnect into
these same lugs which do have empty spots available.

[Naptown]

Yep, the label is still on the backside of the enclosure door and specs the
225 Amp bus bars. I also went to Siemens web site and printed out all
the specs on that model.
Good that will get passed the inspector

I'm not sure I am getting across how the power is coming in here.
I have an underground service that of course first feeds a meter on the back of
a detached garage. From the meter enclosure it feeds, right next to it, this
Siemens Outdoor Circuit panel which has only one the one 200 Amp breaker
in it.
This feeds, right next to it a large Midwest Terminal or junction box
and the main power lines to my house, over a 100 feet away, are
attached to the lugs in this enclosure and then go underground.

Seeing as I will have the AC disconnect right next to this terminal box,
the simplest way is to just feed directly out of the disconnect into
these same lugs which do have empty spots available.

Yes that will work and since the breaker is next to the meter the backfed breaker should also serve as the utility disconnect. Question is about the panel is the cover lockable?
Remember to feed the solar into the panel at the opposite end of the panel from the main breaker and label accordingly.

[Dakota]

Yes, you could put a padalock on that, but I don't that would be a good idea.
That is the Main Breaker for all the power to the entire site. With a fire in the
home, the first thing the Fire Dept will want to do is close that Breaker and cut
all power so they can chop and saw without risk.

I agree that the Utility wanting another AC Disconnect for the inverter input
is some what redundant, but it does give them the option to lockout the inverter feed
without putting the entire site in the dark.

Yes that will work and since the breaker is next to the meter the backfed breaker should also serve as the utility disconnect. Question is about the panel is the cover lockable?
Remember to feed the solar into the panel at the opposite end of the panel from the main breaker and label accordingly.

[Naptown]

Actually the FD pulls the meter. Most main disconnects are located inside.

[russ]

The FD doesn't have bolt cutters on the truck?

[Sunking]

The FD doesn't have bolt cutters on the truck?

No they do not. They have fire axes, and one of the first things they do when they pull up to a house fire with solar panels is smash every solar panel with a fire axe. It is the only way to turn them off.