Overhead cable

A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
Feeding a xfrm from overhead. Run is about 100' and on wood poles. Want to use aluminum cable.

I believe there's' standard SER cable and then some sort of quadraplex aerial cable that is self supporting with a messenger cable. Has anyone heard of it?

I do need 3 hots and a ground, no neutral correct? I'm coming from a switch in a distribution board and feeding a xfrmr. 480v. stepping down.
 
Also if I need to deliver 240A(calculated load) on the 120/208v side I can use a 75kva step down xfrm and run 125A from the 480v source? At least that's what I'm thinking.
 
Alwayslearningelec said:
I believe there's' standard SER cable and then some sort of quadraplex aerial cable that is self supporting with a messenger cable.
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IMO this is what you'll want to use not SER. The cable is supported by the bare messenger conductor. How did you plan on supporting the SER on a horizontal run? 75kva output at 208Y/120 is below 240 amps.
 
infinity said:
IMO this is what you'll want to use not SER. The cable is supported by the bare messenger conductor. How did you plan on supporting the SER on a horizontal run? 75kva output at 208Y/120 is below 240 amps.
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Yes think I'll need 112.5 kva. Was going to use messenger. I will not need a neutral from switch to xfrmr correct?
 
Alwayslearningelec said:
Yes think I'll need 112.5 kva. Was going to use messenger. I will not need a neutral from switch to xfrmr correct?
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Correct, you only need the ungrounded conductors and an EGC. The messenger of quadplex typically serves as the EGC.
Note that as far as I know there is no listed quadplex cables and some inspectors may have an issue with that.
However it is my opinion that quadplex, even though it is not listed, is permitted by 396.10(A)(6).
 
And I pretty sure of this but also like to MAKE CERTAIN as you can tell by my posts....lol.

So if the only thing that is available at the 480v source is a 400A switch.......

1. I need conductors rated for at least 400A.
2. I need to have a 112.5 kva 480v primary side protect by at least a 175A breaker?
3. A 400A breaker could feed the primary side of this xfrm but I would need conductor's rate for at least 400A?
4. 480v 400A switch feeding a 112.5 xfrmr 1,100 away. When I put in VD calcs and enter current at end of cable I put in the xfrm primary side rating??? 175A? When I do it give me either 1 set 750's or 2 sets 300 mcm. Those would are rated for a lot mroe than 400A amps so I would be at the switch end?
 
Alwayslearningelec said:
Thanks. Quad??? I guess they count the messenger as a cable because it would only be 3 ungrounded conductors and that's Tri.
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The messenger is always part of the conductor count in xxx-plex cable. Look at the typical residential service drop---two insulated conductors and one bare (messenger/neutral) conductor and that is tri-plex.
Alwayslearningelec said:
A 400A breaker could feed the primary side of this xfrm but I would need conductor's rate for at least 400A?
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The primary full load for that transformer is 135 amps. The maximum primary OCPD is 250 percent of the primary full load current. That makes the maximum primary OCPD 338 amps. You cannot use a 400 amp OCPD because 350 amps is a standard OCPD.
You are correct that the primary conductors must have an ampacity that is equal to or greater than the rating of the primary OCPD.
 
don_resqcapt19 said:
The messenger is always part of the conductor count in xxx-plex cable. Look at the typical residential service drop---two insulated conductors and one bare (messenger/neutral) conductor and that is tri-plex.

The primary full load for that transformer is 135 amps. The maximum primary OCPD is 250 percent of the primary full load current. That makes the maximum primary OCPD 338 amps. You cannot use a 400 amp OCPD because 350 amps is a standard OCPD.
You are correct that the primary conductors must have an ampacity that is equal to or greater than the rating of the primary OCPD.T
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Thanks a lot. Your electrical knowledge(along with some other on this forum) is really impressive.

So I'm told there is a 400A spare "bucket" available for us to use hence why i said 400A switch but maybe we can fuse it down if it's a switch and fuse type???
 
don_resqcapt19 said:
The messenger is always part of the conductor count in xxx-plex cable. Look at the typical residential service drop---two insulated conductors and one bare (messenger/neutral) conductor and that is tri-plex.

The primary full load for that transformer is 135 amps. The maximum primary OCPD is 250 percent of the primary full load current. That makes the maximum primary OCPD 338 amps. You cannot use a 400 amp OCPD because 350 amps is a standard OCPD.
You are correct that the primary conductors must have an ampacity that is equal to or greater than the rating of the primary OCPD.
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Two questions.

1. What code section, if any, give the xfrmr FLA?
2. What code section gives the maximum OCPD for the primary FLC?
 
The transformer FLA is simply a mathematical process based on the KVA, voltage and ohms law. Art 450 gives you the allowable ratings for overcurrent devices and Art 240 the allowable ratings for protection of the conductors
 
Don noted the 250% max primary is 338 amps. I'm going by memory so he may correct me but I don't think you can go next size up so a 300 OCP would be max. Keep in mind if you size your primary device at 300 ,that's going to determine your conductor size so you might be better off with a smaller primary OCP.
 
augie47 said:
Don noted the 250% max primary is 338 amps. I'm going by memory so he may correct me but I don't think you can go next size up so a 300 OCP would be max. Keep in mind if you size your primary device at 300 ,that's going to determine your conductor size so you might be better off with a smaller primary OCP.
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You are correct, there is no provision to use the next larger OCPD...good catch
 
don_resqcapt19 said:
I don't agree...you still have to use the NEC ampacities.
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I used to think that too, I work with this stuff often its outdoors, installed like a utility does (other than the neutral is insulated).
The quadruplex has XLPE insulation, its not a NEC insulation type rather its a NESC utility type, there is no ampacity table for XLPE in the NEC,
thats why its called 'other' cable in the table 396.10(A)
and if you study the 396.10(A) table there is no code article or section corresponding to the other cable row. Outdoor free air terminations are also rated 90ºC, you switch to a larger NEC wire type in say a mast type raceway, or a SER cable and use NEC cable where it enters a building.
 
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WD40 said:
I used to think that too, I work with this stuff often its outdoors, installed like a utility does (other than the neutral is insulated).
The quadruplex has XLPE insulation, its not a NEC insulation type rather its a NESC utility type, there is no ampacity table for XLPE in the NEC,
thats why its called 'other' cable in the table 396.10(A)
and if you study the 396.10(A) table there is no code article or section corresponding to the other cable row. Outdoor free air terminations are also rated 90ºC, you switch to a larger NEC wire type in say a mast type raceway, or a SER cable and use NEC cable where it enters a building.
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Yes, you use the table 310.17 ampacities, and in some cases can use the 90°C ampacities, but you still use the NEC ampacities.
 
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