EVSE DC Cable Requirements

[SparksWillFly]

tldr: does NEC 625.41 apply to field-installed DC conductors on EVSE?


With EV charging there are manufacturers that sell a DC fast charger in 2 pieces. An AC-DC rectifying cabinet and a DC charging pedestal. The manufacturer does not supply the DC wires that interconnect the equipment and they do not instruct you on how to calculate. They simply give you the maximum ampacity and wire size that can be used. Looking for insight on a discussion that has come up about if NEC 625.41 is applicable to this calculation of DC conductors.

From my interpretation, the equipment is considered continuous duty and it shouldn't matter if it's the AC coming in or the DC branch circuit is going out. It's a circuit related to EVSE and should be sized at 125% of the max amps.

Because this is something installed in the field, wouldn't it also fall out of any UL listings? As in, the manufacturer can't advise that the 125% calculation isn't necessary because it's not within their jurisdiction. If they want to control the DC wire size it needs to be a part of their UL stamped product.

Parameters to work with:
Max DC amps output from rectifier: 500a
Max wire size: 4/0
Conduit: 1- 4 inch
Conductors on each terminal (negative and positive) - 3

350a x 125% = 437.5a (is what the conductors should be rated for)

4/0 XHHW-2 rated at 260a x 4 = 780a
Derate for 8 current carrying conductors in a conduit - 780 x 70% = 546a
No derating for ambient temp

 

[suemarkp]

Why 260A x 4? If the lugs take 3 conductors, that is 260 x 3 = 780A. Then that yields 6 current carrying conductors, not 8. Derate is lower with 6 (80% instead of 70%) and is well over 438A.

 

[SparksWillFly]

Why 260A x 4? If the lugs take 3 conductors, that is 260 x 3 = 780A. Then that yields 6 current carrying conductors, not 8. Derate is lower with 6 (80% instead of 70%) and is well over 438A.

Sorry, there's some typos on my end. The question isn't about if the wire is large enough. It's - is the 125% applicable to the DC conductors

Should look like:

Parameters to work with:
Max DC amps output from rectifier: 500a
Max wire size: 4/0
Conduit: 1- 4 inch
Conductors on each terminal (negative and positive) - 3

350a x 125% = 437.5a (is what the conductors should be rated for)

4/0 XHHW-2 rated at 260a x 3 = 780a
Derate for 6 current carrying conductors in a conduit - 780 x 80% = 624a
No derating for ambient temp

 

[jaggedben]

We did have a discussion on this subject before, which I managed to find, although it may only further confuse you.

EV Charger - DC Cable Sizing - Continuous Load

Hi all, A coworker of mine just stated that the 125% continuous load correction factor doesn't matter and isn't taken into consideration on the DC side of a EV charging circuit. This is the first time i'm hearing this and as far as I know, the NEC doesn't differentiate AC vs DC cable sizing...

forums.mikeholt.com

 

[jaggedben]

Note that 625.41 deals with overcurrent protection, which your DC conductors may not have. But there's still the question of how to size your conductors to the NEC, if the manufacturer does not provide precise instructions or refers to local codes.

 

[gene6]

An AC-DC rectifying cabinet

Man how history repeats, many old buildings in NYC had DC services and DC elevators up into the late 80's possibly longer. To convert these when Con Edison shutdown DC service, we installed rectifier cabinets to convert 208 AC to 120/240 volt DC. You can still find those old rectifiers and DC panels mainly for service elevators and also a few other motors and equipment in old buildings.

 

[JoeStillman]

Reposting this here for more visibility

tldr: does NEC 625.41 apply to field-installed DC conductors on EVSE?


With EV charging there are manufacturers that sell a DC fast charger in 2 pieces. An AC-DC rectifying cabinet and a DC charging pedestal. The manufacturer does not supply the DC wires that interconnect the equipment and they do not instruct you on how to calculate. They simply give you the maximum ampacity and wire size that can be used. Looking for insight on a discussion that has come up about if NEC 625.41 is applicable to this calculation of DC conductors.

From my interpretation, the equipment is considered continuous duty and it shouldn't matter if it's the AC coming in or the DC branch circuit is going out. It's a circuit related to EVSE and should be sized at 125% of the max amps.

Because this is something installed in the field, wouldn't it also fall out of any UL listings? As in, the manufacturer can't advise that the 125% calculation isn't necessary because it's not within their jurisdiction. If they want to control the DC wire size it needs to be a part of their UL stamped product.

Parameters to work with:
Max DC amps output from rectifier: 500a
Max wire size: 4/0
Conduit: 1- 4 inch
Conductors on each terminal (negative and positive) - 3

350a x 125% = 437.5a (is what the conductors should be rated for)

4/0 XHHW-2 rated at 260a x 3 = 780a
Derate for 6 current carrying conductors in a conduit - 780 x 80% = 624a
No derating for ambient temp

Where did the 350A come from? 625.41 refers to OCPD and not wire size.

 

[SparksWillFly]

Where did the 350A come from? 625.41 refers to OCPD and not wire size.

350a is what the rectifier output is. It can do a max 500a but the software limits it to 350a DC output. There are 500a DC fuses in the equipment for this output.

 

[gene6]

There are 500a DC fuses in the equipment for this output.

Then you probably have a 500A branch circuit