EV Charger - DC Cable Sizing - Continuous Load

[JMFSE]

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. Can anyone please confirm this?

Thanks!

 

[tom baker]

If the DC cable is part of a listed EV charger, then the cable size is done under the UL standard, your concern is the supple circuit size
Welcome to the forum!

 

[JMFSE]

If the DC cable is part of a listed EV charger, then the cable size is done under the UL standard, your concern is the supple circuit size
Welcome to the forum!

Hi Tom. For clarification, I'm referring specifically to the DC branch cables between the power conversion cabinet and the charging station, not the charging cord itself.

 

[ggunn]

Hi Tom. For clarification, I'm referring specifically to the DC branch cables between the power conversion cabinet and the charging station, not the charging cord itself.

For continuous current ampacity adjustments, I don't know of any exception for DC circuits.

 

[wwhitney]

Hi Tom. For clarification, I'm referring specifically to the DC branch cables between the power conversion cabinet and the charging station, not the charging cord itself.

Right, but if the cabinet and the station were listed together, those cables, even if field installed, would not be subject to the NEC. They would controlled by the product listing standard and the manufacturer's instructions. At least that's a reasonable interpretation.

Cheers, Wayne

 

[jaggedben]

Right, but if the cabinet and the station were listed together, those cables, even if field installed, would not be subject to the NEC. They would controlled by the product listing standard and the manufacturer's instructions. At least that's a reasonable interpretation.

Cheers, Wayne

I would agree if the cabinet, station, and cables are listed together.

 

[wwhitney]

I would agree if the cabinet, station, and cables are listed together.

What exactly does that mean?

I would think if the installation instructions specify the conductor type and size to use between the cabinet and the station, and the cabinet and station are listed together, that would suffice.

I mean, if the outlet is at the cabinet, then the conductors between the station and the cabinet aren't branch circuit or feeder conductors.

Cheers, Wayne

 

[jaggedben]

What exactly does that mean?

I would think if the installation instructions specify the conductor type and size to use between the cabinet and the station, and the cabinet and station are listed together, that would suffice.

I mean, if the outlet is at the cabinet, then the conductors between the station and the cabinet aren't branch circuit or feeder conductors.

Cheers, Wayne

I've never seen equipment instructions specify an exact wiring method unless it was manufacturer supplied. Whereas most installation instructions say 'refer to your local codes' when it comes to wiring not supplied by the manufacturer.

For example Enphase trunk cables are provided by the manufacturer, but once you connect them in a j-box to extend the circuit to the ground that wiring is 'refer to local codes'. And with Solaredge, even though the optimizers and inverter are listed together and cannot be used with anything else, the wiring between them is 'refer to local codes'.

So there seems to be agreement between NFPA and UL not to step into each other's jurisdictions in this area. Not sure how formal that is, but it seems to hold IME.

 

[jaggedben]

Or perhaps UL just won't include stuff in listings if the manufacturer does not supply it and thus exert some level of control.

 

[wwhitney]

I've never seen equipment instructions specify an exact wiring method unless it was manufacturer supplied.

So I decided to look at the installation manual of one of these, the ABB "Terra HP Generation 3 UL 350 kW Dynamic DC system". It has the detailed information below. This is just one example, but the sort of thing I was expecting.

Also, if you say that the DC power cables between the power cabinet and the charge post are subject to the NEC, that makes them branch circuit conductors, presumably. That raise a host of additional questions: how does the NEC regulate the power conversion equipment in the DC cabinet? Does it have the appropriate branch circuit OCPD? Etc.

Versus the interpretation that the outlet occurs in the wiring compartment of the power cabinet where the AC supply conductors are landed, and that the power cabinet, DC power cables, and charge post are all part of the utilization equipment.

Cheers, Wayne

12.19.2 DC power installation cables

The below data are specific for the DC power cables between the power cabinet and the charge post.

• Parameter: Specification
• Type: DLO, Type W²
  2 DC cables per DC+ and 2 DC cables per DC-
  Flexible, with voltage sensing lines
  Unshielded
• Number of cores: 1
• Cross section range (minimum): 125 mm² (250 kcmil)
• Cross section range (maximum): 300 mm² (592 kcmil)
• Diameter: 27 to 35 mm (1.1 to 1.4 in)
• Maximum length: 60 m (197 ft)
• Conductor: Stranded in tin plated copper or aluminum fine wire, according to VDE 0295 cl.5/IEC Cl.5
• Diameter of the phase conductors: Refer to the local rules.
• Insulation: Rubber or PVC that is serviceable for outdoor use, UV-protected, and oil resistant
• Voltage rating Uo³/U⁴: 1000 V
• Minimum test voltage: 6 kV
• Ambient temperature range: -40°C to 80°C (-40 °F to 176 °F)
• Maximum cable temperature (on the surface): +90°C (+194 °F)
• Bending radius: 6x outer diameter

² You can only use XHHW-2 cables after an approval of ABB E-Mobility B.V..

 

[310 BLAZE IT]

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

A 100% rated fuse or breaker for a DC system is quite challenging because it has an enclosure temperature requirement that is very challenging to meet.

It is possible that they are 90C rated 'terminals' and 'devices' if they are liquid-cooled internally but you will need to check on both ends.

You can use PV Wire (UL 4703) too, DLO is not your only option.

PV Inverters in section 690.7(A)(1)(1) offer a similar precedent on this issue and that is to require 125% on the main DC supply circuits. Only under condition 2 does it allow 87.5 percent.

I have a similar frustration with this code section and have posted about it before.
Some inverter nameplates but not all have current limits. code should provide an exception for these inverters since most inverters have at least 3% accurate input current MPPT and are adjusting to below this limit.

In the end the code is the code and you must follow it.

 

[wwhitney]

PV Inverters in section 690.7(A)(1)(1) offer a similar precedent on this issue and that is to require 125% on the main DC supply circuits. Only under condition 2 does it allow 87.5 percent.

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

 

[310 BLAZE IT]

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

right but that gets you to 109 percent instead of 156 percent although the modules are still connected - not that it is entirely relevant but it is the only way to have a lower current than 125 percent on a DC circuit that I know of.

Maybe one day these sensors in the charge cords will get embedded in all conductors so they are protected thermally and have coolant loops.

 

[jaggedben]

So I decided to look at the installation manual of one of these, the ABB "Terra HP Generation 3 UL 350 kW Dynamic DC system". It has the detailed information below. This is just one example, but the sort of thing I was expecting.

...

So if that were the equipment, then as it pertains to the OP, I would be right. "Refer to the local rules." I see nothing in 625 so we are back to 210.19(A)(1) (or maybe 215.2(1), makes no difference).

So we can tell our OP 'your coworker is wrong'. Maybe with the caveat 'unless the listing instructions contain explicit alternative guidance on which exact conductors to use or how to size them.'

 

[wwhitney]

So if that were the equipment, then as it pertains to the OP, I would be right. "Refer to the local rules." I see nothing in 625 so we are back to 210.19(A)(1) (or maybe 215.2(1), makes no difference).

You haven't addressed any of the consequences of saying that the DC conductors are a branch circuit (or feeder if the pedestal has integral OCPD). What article covers the power cabinet? Is the DC side subject to Article 712? Does the equipment have DC circuit breakers that are UL 489 listed as branch circuit protection? Etc.

So while I agree that your interpretation is at least somewhat defensible, the interpretation that the DC conductors are part of the "listed utilization equipment," and not the premises wiring system remains reasonable.

I wonder how AHJs are typical parsing this.

Cheers, Wayne

 

[jaggedben]

You haven't addressed any of the consequences of ...

Why do I need to? All of that can still be covered by the listing instructions while those same instructions still defer to the local code for conductor size. I don't see how the code necessitates any logical interference between one set of issues and the other. For example, article 712 contains no more conductor sizing instructions than 625, so that question makes no difference. Moreover, if it did, we might have a problem of code interpretation, but 110.3(B) would still not give us a blanket exemption from 210, 215, or any other applicable article containing conductor size requirements. Only the listing instructions themselves, or factory installation, can give us that exemption.

Bottom line, if people want to appeal listing instructions to avoid using larger conductor sizes then it's going to be up to the manufacturers to design and test for those conductor sizes, and UL standards to account for it. (The extra cost and time involved might be a reason this doesn't happen.) There's no blanket exemption in the NEC. 90.7 refers to 'factory installed internal wiring' and this falls outside of factory installed.

 

[wwhitney]

Why do I need to? All of that can still be covered by the listing instructions while those same instructions still defer to the local code for conductor size.

The UL listing standard for the product is not going to cover branch circuit conductors. So if you are saying the DC conductors are branch circuit conductors, and therefore 210.19(A)(1) requires a 125% continuous use factor, then all the other NEC consequences for the installation apply.

Conversely, if the conductors are part of the listed utilization equipment, they are not branch circuit conductors, and there's nothing in the NEC that would require a 125% continuous use factor, even when sized according to "local rules".

Only the listing instructions themselves, or factory installation, can give us that exemption.

I would say the listing instructions can't provide exemptions from NEC sections that don't allow for it, they can only add additional requirements.

Cheers, Wayne

 

[jaggedben]

The UL listing standard for the product is not going to cover branch circuit conductors.

I don't think I agree, or that it is necessarily all one or the other.
The ABB specs you posted above seem to place a lot of restrictions on the conductors even while still deferring to local rules for some parameters. Or take Enphase, where the branch circuit wiring is covered by the NEC between the breaker and the j-box on the roof, and then the manufacturer's cable with the UL listing takes over between the J-box and the inverters. Same circuit. (Enphase calls it a branch circuit, I think you've argued it should be considered a feeder. Makes no difference to the original question in this thread as long as it's one or the other.)

So if you are saying the DC conductors are branch circuit conductors, and therefore 210.19(A)(1) requires a 125% continuous use factor, then all the other NEC consequences for the installation apply.

What consequences? Why would it matter to the original question about conductor size? Also I'm not sure I agree that the listing instructions can't take precedence over particular aspects of the installation just because the NEC says it's a branch circuit.

Also they might be feeders. Haven't you argued that anything that isn't clearly a branch circuit or a service conductor must be a feeder? Doesn't it have to one or the other in the NEC, so the 125% factor will come into play if NEC rules apply?

Anyway, what they are and which NEC rules apply seems contingent on details of the equipment (such as where OCPD is) that aren't specified in the original question, which was general. If we can't figure out what they are in the code when we look at the equipment in detail then that's another problem. And maybe the answer to the question of whether a 125% factor applies gets answered on a case-by-case basis depending on the equipment design and instructions. But I still essentially stand by my point in post #6. Unless the listing instructions cover everything and say otherwise, NEC rules can come into play. And you've got to interpret those rules somehow and make a determination, and the 125% factor may apply. As to the reasoning in the OP, there's no blanket reason the 125% does not apply just because they happen to connect two pieces of equipment that are listed together. Not unless the listing clearly spells out conductor size without deferring to local rules.

 

[wwhitney]

Or take Enphase, where the branch circuit wiring is covered by the NEC between the breaker and the j-box on the roof, and then the manufacturer's cable with the UL listing takes over between the J-box and the inverters. Same circuit.

I wouldn't say it takes over, I would say it augments. Surely Enphase's cable meets all the NEC rules for a branch circuit or feeder cable (that don't have exceptions of the form "or as per the listing instructions")? If it didn't, that would be an NEC violation.

Also they might be feeders. Haven't you argued that anything that isn't clearly a branch circuit or a service conductor must be a feeder? Doesn't it have to one or the other in the NEC, so the 125% factor will come into play if NEC rules apply?

It has to be one or the other for conductors between the service equipment and the outlet. But my argument here is that the outlet is at the "power cabinet", so that doesn't apply to the field installed DC conductors. I agree those conductors aren't exempted from the NEC entirely by 90.7, as they are not factory installed. But they can still plausibly be considered internal to the utilization equipment.

So I am warming to the idea that the DC conductors are subject to the NEC rules, such as Articles 300 and 310, but not to rules specific to branch circuits or feeders. As such you can determine their ampacity per the usual procedure, but 210.19 and 215.2 would not apply. Presumably some other part of the manual tells you what current they will be carrying based on the equipment configuration, and you can select the ampacity accordingly, with no 125% factor.

BTW, this situation of conductors under the NEC that don't have a 125% continuous use factor applies also to service conductors that are not service entrance conductors. Although there the NEC specifies the requisite ampacity.

Now if the power cabinet does have a UL 489 branch circuit type DC breaker protecting the DC conductors, then that at least permits the interpretation that the DC conductors are branch circuit conductors. Conversely, if it doesn't, then the DC conductors as branch circuit conductors interpretation means such equipment could not be installed in an NEC compliant manner without adding such OCPD.

Cheers, Wayne

 

[JMFSE]

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?