Transformer OCPD and 705.65(B)

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bwat

EE
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NC
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New PV site. Let's say I have a 1MVA transformer and (10) 150kW inverters, but the inverters are all de-rated to 100kW on this LV side which is 600V.

The exception in 705.65(B) makes me think that I don't need a main breaker on the 600V side since the de-rated continuous current of the inverters does not exceed transformer rating (1MVA transformer... 10 x 100kW also = 1MVA).

What's giving me pause is that the exception only works in my favor to not need a main breaker if I'm allowed the use the de-rated value. With the standard rated value of the inverter, I'd need a main. It also says inverter (singular). Anybody know if I can use the de-rated value for this?

=== 705.65(B)===
Exception: A power transformer with a current rating on the side connected toward the interactive inverter output that is not less than the rated continuous output current of the inverter shall be permitted without overcurrent protection from that source.
===


Lastly, the whole evaluating both sides as primary trips me up, as I believe it does for many others else as well. It is not a problem that I'll have 10 CBs on the 600V side? And the sum of those CB ratings will total beyond the 125% that is permitted for if it were one device. Is that all irrelevant due to 705.65?

=== Note 2 from 450.3(A).. my underlines===
Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. If both circuit breakers and fuses are used as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses.
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electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
bwat said:
(bump)

I'm sure someone has some insight on this.
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Remember, in a transformer setup, you typically have two separate requirements for protection on the secondary side: Protection of the transformer itself, and protection of the secondary conductors. Transformer protection is per 450.3(A), modified by 705.65(B). What 705.65(B) is telling you is (in your case) you dont need to protect the transformer from the inverter source. You still need to protect it from the utility source as you normally would. None of this says anything about the secondary conductor protection which is per 240.21(C)
 
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Carultch

Senior Member
Location
Massachusetts
electrofelon said:
Remember, in a transformer setup, you typically have two separate requirements for protection on the secondary side: Protection of the transformer itself, and protection of the secondary conductors. Transformer protection is per 450.3(A), modified by 705.65(B). What 705.65(B) is telling you is (in your case) you dont need to protect the transformer from the inverter source. You still need to protect it from the utility source as you normally would. None of this says anything about the secondary conductor protection which is per 240.21(C)
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So, does this mean that 240.21(C) applies independently of 705.65(B)? Assume for this discussion, that the transformer topology does not qualify for the exceptions in 240.21(C).

The way I see this, is that 705.65(B) only allows a single inverter per transformer to connect without secondary OCPD. While if there were multiple inverters on the same transformer secondary, you would then have to have OCPD per 240.21(C). And this would apply, whether you aggregate them in a panelboard, or parallel them on separate circuits from the transformer busbars. 705.65(B) only indicates the singular word inverter, rather than "inverter(s)" that would apply to any number of them.

That is, unless the transformer is otherwise part of a listed assembly of equipment for the application. Like a throat-coupled switchboard/transformer combination, or an inverter with a built-in transformer.
 
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wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
705.65(B) was last seen in the 2017 NEC. For the 2020 NEC, the equivalent section is 705.30(C), which is clearer and addresses some of the issues raised.

Cheers, Wayne
 
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wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
electrofelon said:
Wayne I do not have the 2020, could you post that section?
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You can read it online here in a format easier to use than at nfpa.org:


(2020) 705.30(C):

Transformers with sources on each side shall be provided with overcurrent protection in accordance with 450.3. The primary shall be the side connected to the largest source of available fault current. Secondary protection shall not be required for a transformer secondary that has a current rating not less than the sum of the rated continuous output currents of the power sources connected to that secondary.

Cheers, Wayne
 
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