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offgridgirl avatar image
offgridgirl asked

Parallel Multiplus II ground relay Max current

I would like to use 3 Multiplus II 48/3000 120V wired in parallel in my vehicule for educational purpose and public demonstrations. As I understood, the neutral to earth bonding should only be made at one point in the system. This means only one Multiplus will switch its ground relay, the others (slaves) will be disabled. Is that correct?

Since this protective bonding must handle a default current corresponding to the total installed inverter power, in my case, the internal relay should handle continuously :3x3000VA/120V = 75A.

The maximum number of devices in parallel specified in the datasheet is 6, so in reality one ground relay may handle up to 150A...

Can you confirm that the internal ground relay supports these currents?

Thank you for a helpful answer.

Best regards

Priscilla

Multiplus-IIMultiPlus Quattro Inverter Chargermultiplus in parallelGroundingsafety
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2 Answers
snoobler avatar image
snoobler answered ·

Don't forget the transfer switches and the potential for power assist.

Each inverter can draw 50A from shore and provide an additional 25A in power assist. That's 75A/inverter.

6 * 75A = 450A.

So, by your logic, the ground relay AND the ground wire must be able to handle 450A. Is your ground wire going to be two 4/0 cables? Can your chassis handle 450A?

I don't know the NEC 250 well at all, but I don't believe your claimed requirement exists as you present it.

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offgridgirl avatar image offgridgirl commented ·

Yes, you are right but if I am shore the N-PE bonding is done on grid side, not by ground relay. I only use 3 multi in my application.

According to the manual, it seems that the power assist is disabled on the multi II 48/3000/35-50:

"With its PowerAssist feature the Multi can add up to 3 kVA (that is 3000 / 120 = 25 A) to the output during periods of peak power requirement. Together with a maximum input current of 50 A this means that the output can supply up to 50 + 25 = 75 A. On the 48/3000/35-50 unit, AC-out-1 is limited to 50A. "


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snoobler avatar image snoobler offgridgirl commented ·
That only changes numbers by 1/3rd... that's still 300A the ground relay must handle, with only a single high current rated 4/0 as your ground cable... so the question remains, is your assumption about the ground relay being required to handle 300A correct?
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offgridgirl avatar image offgridgirl snoobler commented ·

The relay will not see 300A with 6 inverters in parallel. The scenario where you have 300A on output is only if you are connected to the grid and in this case, the inverter don't close the relay. With 6 inverters off grid (in this case N-PE relay is closed) the output current can reach 150A.

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Kevin Windrem avatar image
Kevin Windrem answered ·

My thought is that the "single point" should include the collection of inverter/chargers and all ground relays should activate together.

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snoobler avatar image snoobler commented ·
There MUST be only one N-G bond in any system. Multiple N-G bonds will result in current on the ground wire and will trip any GFCI or similar devices.


If inverter relays are used in a parallel inverter installation, only one relay should be on.


The alternative would be to have all inverter relays open and make the N-G bond at the panel.

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