# OCPD Brain Freeze!

#### fossil112

##### Member
Guys(Gals),
I'm having "engineer's block" so to speak. We have (3) line commutated solar inverters that kick out 12kW AC each. Each inverter is rated single phase 240V, so there's one phase and a neutral (and EGC, not shown on image). Inverter A lands on phase A, B on B, and C on C inside a panelboard, so as to create a three phase system.
My struggle is sizing the inverter OCPD in the panelboard. Logic tells me I have true 12kW per phase, so 12,000/240=50A*1.25=62.5A=70A 3-pole breaker. My mind is drifting towards a calculation that involves 416V (240*sqrt(3)=416), but it doesn't feel right.
I've attached a little sketch detailing. Thanks ahead of time.

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#### ron

##### Senior Member
I'm guessing this is not a US based installation due to the 240V Phase to neutral reference.

The 70A breakers seems correct.

#### mbrooke

##### Batteries Not Included
These look like separate single phase inverters, are they slaved together?

#### fossil112

##### Member
I'm guessing this is not a US based installation due to the 240V Phase to neutral reference.

The 70A breakers seems correct.
This is a US installation.

#### fossil112

##### Member
These look like separate single phase inverters, are they slaved together?
Slaved together in that they are "line commutated". Simply means each inverter syncs it's voltage and phase automatically with the grid.

#### iceworm

##### Curmudgeon still using printed IEEE Color Books
.... Logic tells me I have true 12kW per phase, so 12,000/240=50A*1.25=62.5A=70A 3-pole breaker.
Yes, works fine

My mind is drifting towards a calculation that involves 416V (240*sqrt(3)=416), but it doesn't feel right.
Also should feel fine
(36kw/(416))/sqrt(3) = 50A

Do the algebra, it's the same
ice

#### iceworm

##### Curmudgeon still using printed IEEE Color Books
(36kw/(416))/sqrt(3) = 50A

Do the algebra, it's the same
I guess that is a bit cryptic.

((3 x 12kw)/(240 x sqrt(3)))/sqrt(3) = 12/240 = 50
But if I tell you, you will:slaphead:

ice

#### fossil112

##### Member
@ice

Silly me didn't take 36kW, I used 12kW...knew something was missing!

#### mbrooke

##### Batteries Not Included
Slaved together in that they are "line commutated". Simply means each inverter syncs it's voltage and phase automatically with the grid.
Got it. For a moment I was thinking as if it were stand alone hence my confusion.

#### Smart \$

##### Esteemed Member
This is a US installation.
Seems to me they'd be connected line-to-line at 240V and not line-to-neutral at 416V...???

A 240/120V 3? 4W system has a center-tap neutral, i.e. neutral with respect Lines A and C.

#### GoldDigger

##### Moderator
Staff member
Seems to me they'd be connected line-to-line at 240V and not line-to-neutral at 416V...???

A 240/120V 3? 4W system has a center-tap neutral, i.e. neutral with respect Lines A and C.
If you have a 240Y/132 (non-standard) system, then each phase to neutral will be 240V.
This allows you to use some models of 240V inverters without having to wire them phase to phase, which might be undesirable for some reason.

Other inverters are designed or configured for 208 volt output and wired line to line in a standard 208Y/120 system.

An inverter with a 416 volt output would be harder to find and if transformerless would require a higher DC string voltage.

The 240 volt configuration of a GTI is more commonly used with a 120/240 three wire single phase output, and not all such inverters are rated for 240 above ground/neutral in the same way that 120/240 breakers cannot be used on a 240 to ground environment.

#### Smart \$

##### Esteemed Member
If you have a 240Y/132 (non-standard) system, then each phase to neutral will be 240V.
This allows you to use some models of 240V inverters without having to wire them phase to phase, which might be undesirable for some reason.

...
That don't sound right. If you have a 240Y/132V system, the line-to-neutral voltage is 132V... not 240V. FWIW, I've not seen such a system... though I can see how the inverters could be that way on a standard 240/120 3? 4W system... but would you run the inverter's neutral to the panel (i.e. distribution panel, not AC combiner panel)?

Also, wouldn't the breaker then be based on 12kW/132V assuming the neutral point is grounded...???

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#### GoldDigger

##### Moderator
Staff member
Sorry, brain slip.
416Y/240.
And since the inverters are single phase units you could connect them either to three single phase breakers or to the four wires of a three phase wye MWBC.
One potential benefit of the MWBC connection would indeed be the elimination of voltage drop in the neutral when all three inverters are working.

Tapatalk!

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#### Smart \$

##### Esteemed Member
Sorry, brain slip.
416Y/240.
And since the inverters are single phase units you could connect them either to three single phase breakers or to the four wires of a three phase wye MWBC.
One potential benefit of the MWBC connection would indeed be the elimination of voltage drop in the neutral when all three inverters are working.
All the same... what would you use a 416Y/240V 3? 4W system for in the US...???

#### GoldDigger

##### Moderator
Staff member
All the same... what would you use a 416Y/240V 3? 4W system for in the US...???
Connecting to a set of 240 V inverters maybe.

I am not sure from what the OP showed whether this is a service voltage or a transformer secondary just for this purpose.
It could be that the OP has it wrong too.

#### fossil112

##### Member
Connecting to a set of 240 V inverters maybe.

I am not sure from what the OP showed whether this is a service voltage or a transformer secondary just for this purpose.
It could be that the OP has it wrong too.
You guys are spot on.
The inverter output from phase to neutral is 240V. That doesn't do me much good because we're installing at a commercial site, standard 480V. So, I've got literally hundreds of these little 240V inverters that need stepped up to 480. So with only a phase and neutral at 240V, I'm forced to look at a 416:480V transformer. Grounded wye on low side so as to pickup possible phase-ground faults.

#### ron

##### Senior Member
You guys are spot on.
The inverter output from phase to neutral is 240V. That doesn't do me much good because we're installing at a commercial site, standard 480V. So, I've got literally hundreds of these little 240V inverters that need stepped up to 480. So with only a phase and neutral at 240V, I'm forced to look at a 416:480V transformer. Grounded wye on low side so as to pickup possible phase-ground faults.
As I mentioned in an earlier post, it seems like European equipment (415/240V). Be sure it is UL listed.

BTW, we do use 415/240V equipment in data centers for higher efficiency operation of the data center equipment.

#### Smart \$

##### Esteemed Member
You guys are spot on.
The inverter output from phase to neutral is 240V. That doesn't do me much good because we're installing at a commercial site, standard 480V. So, I've got literally hundreds of these little 240V inverters that need stepped up to 480. So with only a phase and neutral at 240V, I'm forced to look at a 416:480V transformer. Grounded wye on low side so as to pickup possible phase-ground faults.
416:480 doesn't seem like it'd be a very commonly available transformer. Haven't researched though...

What about three (3) 240:480 with one line on low side grounded and all three high sides configured in delta.

Would buck/boost work in this scenario... (3) 240:277?

#### fossil112

##### Member
416:480 doesn't seem like it'd be a very commonly available transformer. Haven't researched though...

What about three (3) 240:480 with one line on low side grounded and all three high sides configured in delta.

Would buck/boost work in this scenario... (3) 240:277?
416 is not a common config. A buck boost will work, but if you look at the exact winding configs, they are 244.4:277. So if the utility runs hot (which could be within allowable IEEE standards), the inverters will go offline because the voltage is out of range. I'm in big trouble if that happens. They protect themselves by the same IEEE standards.

#### Smart \$

##### Esteemed Member
416 is not a common config. A buck boost will work, but if you look at the exact winding configs, they are 244.4:277. So if the utility runs hot (which could be within allowable IEEE standards), the inverters will go offline because the voltage is out of range. I'm in big trouble if that happens. They protect themselves by the same IEEE standards.
I understand.

So the 416:480 transformer is not a problem, even though uncommon?

And what about using three 240:480 1? xfmrs?

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