[RE-wrenches] Undercurrent Protection?

Tue Nov 1 12:52:49 PDT 2016

Eric,

No worries on the education side. I’ve been educating for nearly 30 years in this field. I’m learning new stuff every day so I expect others to have the same commitment to learning. I’m glad to see that you do.

One personal frustration with the listing label on inverters is that they did away with the “maximum overcurrent device” rating on the inverter (there is a maximum circuit breaker rating for inverters). This rating, often buried in the specifications of the installation instructions of the inverter, is the breaker that the inverter was tested with. 

An inverter with a one-amp continuous output current can be connected to a 20-A circuit breaker (if it has that rating). In fact you can connect up to 16 of those inverters on one 20-A breaker—we call that a microinverter branch circuit (not a code title).

Similarly, we can take four 40-amp inverters and connect them in parallel on a 200-A breaker as long as the inverter has a 200-amp maximum overcurrent device rating. The safety standard tests for the maximum short circuit current that a specified circuit breaker can present to an inverter to see if the bonding path in the inverter can withstand the inrush and trip the breaker without causing a fire. That’s all the safety standard cares about. 

If you run a circuit breaker that is connected to an inverter at more than 80% of its rating, that’s just stupid because you knew how much current it could generate in the first place—it is defined in the ratings. If you run a circuit breaker connected to a transformer at more than 80% of its rating continuously, you not only violate the listing of the circuit breaker, you also violate the listing of the transformer. The only way to fix the transformer issue is to lower the load on the transformer. The inverter and the transformer have circuit breakers for very different reasons.

The circuit breaker connected to an inverter output circuit is really only there to protect against a fault in the inverter or a fault in the conductor. It does not need to deal with “overcurrent” since the inverter has a defined output current.

Bill.

From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of eric at harvesthesun.com
Sent: Tuesday, November 01, 2016 11:01 AM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Subject: Re: [RE-wrenches] Undercurrent Protection?

Bill,

OK. I think I get it. We're protecting against nuisance tripping due to heat build up over time at breaker terminals resulting from continuous loads/sources of current. I understand that you mean an inverter, as a continuous power source, needs to be "allowed" greater flexibility (125%) of current constraint in order to function when you mention having to "withstand" a certain size breaker. However, we're still missing the actual "overcurrent" protection part, no? Obviously there's Article 240. But that's still not setting a max OCPD size, right?...unless I am missing something.

I won't blame you if you decide you have more important things to attend than educating me on the nuances of the NEC. I'm still learning and I do greatly appreciate your insight!

Eric

--------- Original Message --------- 

Subject: Re: [RE-wrenches] Undercurrent Protection?
From: billbrooks7 at sbcglobal.net <mailto:billbrooks7 at sbcglobal.net> 
Date: 10/31/16 5:41 pm
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org <mailto:re-wrenches at lists.re-wrenches.org> >

Eric,

Your reasoning sounds logical, but it is missing the point. Transformers have to be protected by overcurrent devices, similar to wire. Inverters are current limited devices so they only have to be able to withstand a certain size circuit breaker. The current from the inverter is considered continuous so most circuit breakers are designed to operate at 80% of their rating continuously (therefore the requirement to multiply inverter output current by 125%).

Does that make any more sense?

Bill.

Bill Brooks, PE

Principal

Brooks Engineering

3949 Joslin Lane

Vacaville, CA 95688

707-332-0761 (office and mobile)

bill at brooksolar.com <mailto:bill at brooksolar.com>  (email)

www.brooksolar.com <http://www.brooksolar.com>  (web)

From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of eric at harvesthesun.com <mailto:eric at harvesthesun.com> 
Sent: Monday, October 31, 2016 3:34 PM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org <mailto:re-wrenches at lists.re-wrenches.org> >
Subject: [RE-wrenches] Undercurrent Protection?

I realize this is potentially a stupid question, and one that is long overdue, but here goes:

Why in 690(B) are we setting a minimum threshold on OCPD when we're protecting against over current?

Shouldn't we be setting a maximum threshold...as is done in pretty much all other cases? The way I see it, we should take the array/inverter max output AC current and multiply by 125% to set our min ampacity on wire,

and then calc down to find the breaker trade size that lands between the max AC output current and max AC output current times 125%. That way, we avoid nuisance tripping but achieve the max overcurrent protection possible.

By following 690(B) we're actually decreasing OCP: Literally, 690(B) is stating that we can have a breaker 100x over the circuit rating, so long as it's not under the circuit rating times 1.25. To me this seems antilogical.

Mr. Brooks?

690.9(B) Overcurrent Device Ratings. Overcurrent device ratings

shall be not less than 125 percent of the maximum

currents calculated in 690.8(A).

455.7(B)...The overcurrent protection determined

from this section shall not exceed 125 percent of the

phase converter nameplate single-phase input amperes.

450.4(A)...Such overcurrent device

shall be rated or set at not more than 125 percent of

the rated full-load input current of the autotransformer.

450.5(A)(2) Overcurrent Protection. An overcurrent sensing device

shall be provided that will cause the main switch or commontrip

overcurrent protection referred to in 450.5(A)(1) to open if

the load on the autotransformer reaches or exceeds 125 percent

of its continuous current per-phase or neutral rating.

450.5(B)(2)(b) Ampere Rating. The overcurrent protection shall be

rated or set at a current not exceeding 125 percent of the 

autotransformer continuous per-phase current rating or

42 percent of the continuous-current rating of any seriesconnected

devices in the autotransformer neutral connection.

Eric Stikes

Founding Director & CEO

Good Sun Solar, A CA non-profit corp.

www.goodsun.life <http://www.goodsun.life> 

(530) 559-5023

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