2020 NEC 625.42 - DC conductor sizing

[ChargedUp]

NEC 625.41 requires AC OCPD to be sized to 125 percent of the maximum load of the EVSE equipment.

NEC 625.42 is related to DC conductor sizing downstream of AC OCPD. i.e. for example level 3 charging cabinet that feeds (2) separate downstream charging posts. The feeder between the cabinet and posts is DC.

Regarding 625.42, the article says "the power transfer equipment shall have sufficient rating to supply the load served. Electric vehicle charging loads shall be considered to be continuous loads for the purposes of this article. Service and feeder shall be sized in accordance with the product ratings. Where an automatic load management system is used, the maximum equipment load on a service and feeder shall be the maximum load permitted by the automatic load management system..."

Do you interpret this article to size DC conductors based on the maximum load listed on the datasheet (product rating) OR the maximum load listed on the datasheet x1.25? This is stemming from the statement in 625.42: "EV charging loads shall be considered to be continuous loads for the purposes of this article".

Thanks all

 

[wwhitney]

The topic of DC Fast Chargers consisting of a power cabinet and field installed conductors from there to separate charging posts has come up before on this forum. Since I understand the power cabinet and charging posts are sold together as a single product, it's not clear to me that these DC conductors are subject to the NEC, rather than being part of the utilization equipment. In which case they would be governed by the manufacturer's instructions.

If they are subject to the NEC, then they are feeders only if the charging post has OCPD in it; otherwise they are branch circuits. Either way, I don't necessarily see them as carrying a continuous load. While 625.42 says that the load is to be considered continuous for the purposes of 625, nothing in 625 tells you how to size conductors. Instead you need to look at Article 210 or 215 for sizing, at which point the stricture in 625.42 no longer applies.

So then whether these DC conductors are carrying a continuous load depends on whether they are "expected" to carry their maximum current for more than 3 hours. I would think that would depend on the DCFC size relative to the population of EVs on the market. If the DCFC is rated only 25 kW or 50 kW, then a monster Hummer EV with a battery size over 200 kWh could easily cause it to run at maximum current for 3 hours. Conversely, a DCFC with a 150 kW or 250 kW rating would not be expected to run at maximum current for 3 hours.

Then if the conductors are subject to the NEC, and are carrying a continuous load, whether a 125% factor is required in sizing them depends on the OCPD, if any, in the power cabinet. [If the rectifier in the power cabinet is somehow inherently current limited, then perhaps there is a section in the NEC that would allow the omission of OCPD if the conductor ampacity is above that current limit. Certainly such provisions are in Article 690 for photovoltaics; I don't know if there is any applicable allowance for this application.] If that OCPD is 100% rated, then no 125% factor is required for conductor sizing. If it is not, then the minimum conductor size will be the greater of (a) what is required to comply with 240.4, including 240.4(B), and (b) 125% of the maximum current.

Cheers, Wayne

 

[ChargedUp]

Thank you so much for your detailed response. For your first paragraph, the DC conductors are often procured and field installed by contractor since the posts are separate from the cabinets, and therefore would be subject to NEC.

I certainly don't see level 3 DC fast chargers as a continuous load, as defined by NEC (i.e. 3 hours). Only level 1 or 2 charging would be subject to continuous load sizing, which happens in the form of sizing the AC supply circuit to said level 1 or 2 EVSE.

Regarding internal OCPD for the DC output within the EVSE equipment, oftentimes that is proprietary to the manufacturer, and the only thing the EOR has to go on are the maximum DC output currents listed in the manufacturer datasheet. In my experience, it is typically fusing in combination with an automatic load management system, which would be 100% rated. But either case, we don't always know the specifics.

For this level 3 application, in a nutshell I think this comes down to whether or not we need to size the maximum DC output currents listed on manufacturer datasheets to 100% of the load listed, or 125%. What is your final opinion on this considering my above follow-ups?

 

[zbang]

For your first paragraph, the DC conductors are often procured and field installed by contractor since the posts are separate from the cabinets, and therefore would be subject to NEC.

Is the type and sizing left to the field installer or is it specified by the manufacturer (e.g. "(2) 1g copper type THHN/THWN in 1 1/2 inch EMT not to exceed 25 ft.")? One could argue that if the mfg spec's it that exactly that they've "engineered" it and it becomes part of the overall approval.

 

[ChargedUp]

I would say it's suggested by manufacturer and needs to be validated/ok'd by engineer of record.

 

[wwhitney]

So the only reason that you'd need to apply a 125% factor in sizing those DC conductors is if all of the following are true:

- The DC conductors are subject to the NEC
- The power cabinet has OCPD which is not 100%-rated
- The DC conductors are supplying a continuous load.

If you have confidence that at least one of those is not true, then you can size the conductors at 100% of the maximum current.

Cheers, Wayne