EGC conductor question

[wwhitney]

Thanks, but so can I.

Of course, but we can't follow along at home without the details.

Each pedestal has two 40A chargers. There is a load center in the bulding that has six 50A breakers, two each on two phases of 208V; the panel is fed by a 480/277V delta to 208/120V wye transformer.

So I'm not seeing how it could end being close:

A 40A continuous load requires a 50A breaker, and hence conductors with a minimum ampacity of 46A (since 45A is a standard breaker size). With 4 CCCs in the conduit, the ampacity adjustment factor is 0.8, meaning you need a 90C ampacity of at least 46A/0.8 = 57.5A. That rules out #8 Cu, so you'll need (4) #6 AWG Cu and (1) #10 AWG Cu EGC.

For THWN-2, #6 AWG has an area of 32.71 mm², and #10 AWG is 13.61 mm². That makes the total area 144.45 mm². Per Chapter 9 Table 4, the 40% allowable area of Sch 80 PVC is 105 mm² for 3/4", and 178 mm² for 1".

So 3/4" is quite undersized, and 1" is plenty. What do you have?

Cheers, Wayne

 

[wwhitney]

getting 480v chargers

That is not a thing, for normal EVSEs that provide AC to the cars. The charger (AC to DC conversion) is in the car and won't accept 480V. I guess you might find an EVSE with a 480V:240V transformer built in, but then you'd just be substituting 6 small transformers for 1 larger one.

With DC fast chargers, which provide DC to the car and are therefore actual chargers, I expect 480V would be fine, and small ones might be 50A (25 kW), but you'd be unlikely to install 6 of those.

Cheers, Wayne

 

[winnie]

Okay, 4 conductors in each conduit, not enough room for an EGC.

If you the chargers will function 208 (or 240V) L-N, then you could use a single phase center tapped transformer for each pylon, rather than a single three phase transformer for all. That puts 3 circuit conductors in each conduit.

-Jon

 

[wwhitney]

If you the chargers will function 208 (or 240V) L-N,

Seems like ones from European manufacturers (or ones designed to be exportable to Europe) should, since they would typically be supplied 230V L-N. E.g. the installation manual on this Bosch unit explicitly mentions that supply arrangement, although this model is limited to 32A.

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Cheers, Wayne

 

[LarryFine]

Can you use 3p breakers and supply each pair of chargers as an open delta, allowing for only three CCCs per conduit?

 

[petersonra]

Can you get compact conductors in the size you need?

The other possibility is to use the 90 deg f column for the conductors underground and switch to 75 deg above ground.

 

[ggunn]

Never mind. I was thinking aluminum and the guys in the field were counting on copper. The conduit is big enough.

 

[winnie]

Can you use 3p breakers and supply each pair of chargers as an open delta, allowing for only three CCCs per conduit?

Clever approach. The shared phase would carry 40*1.732=69A, and is presumably considered a continuous load. So I think you would need to use #8 #8 #4 for your circuit conductors, and have corresponding unbalanced OCPD. #8 #8 #4 has larger cross section than #8 #8 #8 #8, but you don't hit the 80% derating for 4 CCC, and #8 #8 #4 is smaller than #6 #6 #6 #6.

-Jon

 

[winnie]

Never mind. I was thinking aluminum and the guys in the field were counting on copper. The conduit is big enough.

Hah.

I think everyone here assumed Cu given the space constraint, and no one of us said it

-Jon

 

[petersonra]

Can you get compact conductors in the size you need?

The other possibility is to use the 90 deg f column for the conductors underground and switch to 75 deg above ground.

Can't do this with pvc, I don't think.

 

[electrofelon]

Never mind. I was thinking aluminum and the guys in the field were counting on copper. The conduit is big enough.

Ok well we'll keep brainstorming for another 3 dozen posts......

 

[wwhitney]

Never mind. I was thinking aluminum and the guys in the field were counting on copper. The conduit is big enough.

I still don't see how it could have been close with Al. #6 Al is still too small, as the ampacity is again 44A. #4 XHHW-2 has an area of 52.52 mm², and #8 XHHW-2 has an area of 28.19 mm². That makes a total of 238.27 mm². While the 40% fill area for 1" Sch 80 is 178 mm², and for 1-1/4" is 320 mm².

I guess if you needed to upsize even more for voltage drop, then 1-1/4" would be just a little too small: #2 XHHW-2 has an area of 73.94 mm², and #6 XHHW-2 has an area of 38.06 mm², for a total of 333.82 mm². But then just using Cu for the EGC works: a #8 THWN-2 has an area of 23.61 mm², reducing the total to 319.27 mm² < 320 mm².

Cheers, Wayne