EV Charger - DC Cable Sizing - Continuous Load

[wwhitney]

Does this mean that DC conductors don't need to be 125% rated if load software regulates the amperage?

For that question and similar ones you have determine whether the DC conductors are feeders, branch circuits, or neither (internal to the equipment). Which is what we have been debating.

Can you provide a reference to the equipment in question? And/or tell us what internal OCPD it may or may not have on the DC conductors, and what standard that OCPD is listed to?

Cheers, Wayne

 

[JMFSE]

The power cabinet that converts AC in to DC out is fused with a short circuit current rating of 65kA.
The OCPD is at the switchgear (AC) upstream from the power cabinet. The input current rating is 260A and recommended breaker size is 350A.
The DC conductors in question are downstream of the power cabinet and feed the charging station itself.

 

[wwhitney]

The power cabinet that converts AC in to DC out is fused with a short circuit current rating of 65kA.

By this you mean it has fuses in it on the AC input conductors, in addition to the switchgear OCPD? Is there any further OCPD on the DC side? In the power cabinet or the charge pedestal?

The OCPD is at the switchgear (AC) upstream from the power cabinet. The input current rating is 260A and recommended breaker size is 350A.

That part is per the NEC, as the AC wiring supplying the power cabinet is certainly either a feeder or a branch circuit. DC fast chargers meet the definition of EVSE (even though that term is often used just to mean Level 1/2 EVSEs), and so 625.41 requires OCPD of at least 260 * 1.25 = 325A (not a standard size, next size up is 350A).

As to 625.42 which you quoted, it has no bearing on the DC conductors unless you deem them a feeder. [Which is maybe plausible if the charge pedestal has branch circuit listed OCPD in it.] 625.42 only refers to feeders and services, not branch circuits or other conductors. Even so, 625.41 would still require a 125% factor for that OCPD based on that "maximum load permitted by the automatic load management system."

Cheers, Wayne

 

[wwhitney]

A further comment:

Closely parsing 625.41 and 625.42, they never say that the EVSE is a continuous load for purposes other Article 625.

(2017) 625.42 says "Electric vehicle charging loads shall be considered to be continuous loads for the purposes of this article," which implicitly says we don't need to consider them to be continuous loads for any other article.

625.41 doesn't say anything about ampacity, just OCPD size. The first sentence tells us the OCPD of branch circuits and feeders supplying the EVSE "shall be sized for continuous duty and shall have a rating of not less than 125 percent of the maximum load of the equipment." It further states "Where noncontinuous loads are supplied from the same feeder, the overcurrent device shall have a rating of not less than the sum of the noncontinuous loads plus 125 percent of the continuous loads." That last part about continuous loads includes the EVSE because of 625.42.

So even if 210.19 or 215.2 applied to the DC conductors, those sections wouldn't be instructing us to use a 125% continuous use factor. If 240.4 applies to the DC conductors, and the DC conductors have OCPD sized per 625.41, then the DC conductors would need sufficient ampacity, which would be more than 100% of their rating. [Other than the trivial case of 15A to 20A, the jump in standard OCPD sizes is always less than 25% of the smaller OCPD size.]

Cheers, Wayne

 

[JMFSE]

By this you mean it has fuses in it on the AC input conductors, in addition to the switchgear OCPD? Is there any further OCPD on the DC side? In the power cabinet or the charge pedestal?


That part is per the NEC, as the AC wiring supplying the power cabinet is certainly either a feeder or a branch circuit. DC fast chargers meet the definition of EVSE (even though that term is often used just to mean Level 1/2 EVSEs), and so 625.41 requires OCPD of at least 260 * 1.25 = 325A (not a standard size, next size up is 350A).

As to 625.42 which you quoted, it has no bearing on the DC conductors unless you deem them a feeder. [Which is maybe plausible if the charge pedestal has branch circuit listed OCPD in it.] 625.42 only refers to feeders and services, not branch circuits or other conductors. Even so, 625.41 would still require a 125% factor for that OCPD based on that "maximum load permitted by the automatic load management system."

Cheers, Wayne

Clarification: the power cabinet has UL248-13 listed fuses on the DC side. The same fuses are ALSO found within the the actual charging station right after the DC input terminals. In other words, the DC conductors are protected "at both ends" by UL248-13 listed fuses.

 

[JoeStillman]

Clarification: the power cabinet has UL248-13 listed fuses on the DC side. The same fuses are ALSO found within the the actual charging station right after the DC input terminals. In other words, the DC conductors are protected "at both ends" by UL248-13 listed fuses.

That means it doesn't meet the definition of a branch circuit.

Branch Circuit. The circuit conductors between the final overcurrent
device protecting the circuit and the outlet(s).

It's a feeder.

Feeder. All circuit conductors between the service equipment,
the source of a separately derived system, or other power supply
source and the final branch-circuit overcurrent device.

As a feeder, its ampacity is governed by 215.2(A)(1)(a). Exception 1 says that if the overcurrent device is 100% rated, the continuous load doesn't have the 125% factor in the ampacity calculation. I'm pretty sure fuses are all 100% rated, so the 125% factor doesn't apply.

 

[topgone]

Article 625.42 states "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."
Does this mean that DC conductors don't need to be 125% rated if load software regulates the amperage?
For example, can I run conductors rated for 200A if the load management system is set for 200A? Or would the load management software have to be set at 160A so the 200A cables are 125% oversized?

Remember that you apply a fudge factor if the conductors you are using are going to deliver power to a "continuous" load! If you put your EV charger on "fast-charging", and you designed your conductors for fast-charging amps, your DLO cables won't have to be designed for 125%, IMO. Just size your cables according the fast-charging amps. It is because your EV charger will only be running for about 20 to 30 minutes.

 

[tortuga]

Conductors can never be UL because the adjustment and correction factors depend on the conditions of use, something out of control of UL

This is hands down the right answer, and this issue comes up every so often for me, recently with minisplits;
With no exceptions for UL, 90.2(A) says the code covers the installation of all electrical, signaling, or communications cables and or conductors and or a raceway at any of the following locations :
(1) Public and private premises, including buildings, structures, mobile homes, recreational vehicles, and floating
buildings
(2) Yards, lots, parking lots, carnivals, and industrial substations
(3) Installations of conductors and equipment that connect to the supply of electricity
(4) Installations used by the electric utility, such as office buildings, warehouses, garages, machine shops, and recreational buildings, that are not an integral part of a generating plant, substation, or control center.

Historical factoid: Con Edison had 120/240 DC grid(s) that were widely used on the east coast for a long time, the last one was shut down in NYC in 2007. Most of the NEC rules just specify voltage and not AC or DC for that reason.

 

[310 BLAZE IT]

an update on this topic. I spoke with Southwire and ABB and they only recommend class C stranding due to their wiring box, it is not a firm requirement. seems that this has created much confusion for many people.

Sizing for 500A boost with a known duty cycle when only 300A is continuously drawn seems like a waste. I see this analogous to a motor inrush but longer duration but am struggling to figure out how the code would allow someone to deal with this more reasonably.

 

[ggunn]

I think you mean 690.8(A)(1)(1). That 125% factor is not a continuous use factor, it is presumably a factor related to the behavior of PV modules under exceptional conditions.

Cheers, Wayne

On the DC side of PV systems there are two 125% factors that need to be included in the calculations. One is for the potential of solar irradiation in excess of 1000W per square meter and the other is for continuous use of the conductors.

 

[Sberry]

Just out of curiosity,, who is providing the dc wire, the equipment vendor or field installer?

 

[jaggedben]

Just out of curiosity,, who is providing the dc wire, the equipment vendor or field installer?

That seems to be very much the question. In a couple examples upthread it seems it would be the installer.