350kw standby generator parallel conductors run in close proximity to transformer

[highpowered]

Couple questions regarding the Installation of the conductors for 1,216 ampmax standby generator(350kw) 120/208VAC

1) If I use PVC pipe on retaining wall distance of 200 feet from main service to generator i can put each phase isolated per conduit run? Can I run the ground and neutrals independently in its own conduit? I'm also confused about the ground size I interpreted the code as the size of the service which is 3,000amps?I dont want to do this but in terms of connecting to the terminals this may be easier. The size wire I would need would be if I use 3 conductors per pipe is 700kcm. If I run all 3 phases plus nuetral and ground in 1 pipe it would be 1,000kcm.

2) these conduits would pass behind the main transformer for the building. 2-3feet is the clearance to the wall I would be mounting the pipes on. I'm not sure the term to use for the magnetic field the transformer makes. Would this be a problem causing interference/heat to the back up generator? I believe this transformer would not be on when the power goes out and the generator kicks on.

 

[texie]

Couple questions regarding the Installation of the conductors for 1,216 ampmax standby generator(350kw) 120/208VAC

1) If I use PVC pipe on retaining wall distance of 200 feet from main service to generator i can put each phase isolated per conduit run? Can I run the ground and neutrals independently in its own conduit? I'm also confused about the ground size I interpreted the code as the size of the service which is 3,000amps?I dont want to do this but in terms of connecting to the terminals this may be easier. The size wire I would need would be if I use 3 conductors per pipe is 700kcm. If I run all 3 phases plus nuetral and ground in 1 pipe it would be 1,000kcm.

2) these conduits would pass behind the main transformer for the building. 2-3feet is the clearance to the wall I would be mounting the pipes on. I'm not sure the term to use for the magnetic field the transformer makes. Would this be a problem causing interference/heat to the back up generator? I believe this transformer would not be on when the power goes out and the generator kicks on.

A lot more info will be needed.
Non SDS or SDS?
ATS SUSE or not?
The conductor sizing is not based KW but rather KVA and breaker size due to power factor.
No, you can not do isolated phase parallel in this case, regardless of the PVC.
Sounds like you should have some engineering assistance.

 

[ron]

I guess you want to use the exception to 300.5 (I)

Exception No. 2: Isolated phase, polarity, grounded conductor
and equipment grounding and bonding conductor installations
shall be permitted in nonmetallic raceways or cables
with a nonmetallic covering or nonmagnetic sheath in close
proximity where conductors are paralleled as per,nitted in
310.10(H), and where the conditions of 300.20(8) are met.

Are your conduit underground?

 

[highpowered]

I guess you want to use the exception to 300.5 (I)

Exception No. 2: Isolated phase, polarity, grounded conductor
and equipment grounding and bonding conductor installations
shall be permitted in nonmetallic raceways or cables
with a nonmetallic covering or nonmagnetic sheath in close
proximity where conductors are paralleled as per,nitted in
310.10(H), and where the conditions of 300.20(8) are met.

Are your conduit underground?

Yes and no. It will be underground for 5 feet then above ground 180'

 

[ron]

Yes and no. It will be underground for 5 feet then above ground 180'

I think the exception only exists for underground. What code section are you planning to use if not 300.5 (I) underground?

 

[don_resqcapt19]

I have never understood why that exception exists...it appears to me that 300.3(B)(3) permits an isolated phase installation in any location.

 

[highpowered]

I have never understood why that exception exists...it appears to me that 300.3(B)(3) permits an isolated phase installation in any location.

I read this somewhere long ago and I cant find it now. It has to do with the magnetic properties in metal heating up if the conductors are out of phase

 

[GoldDigger]

Among other things the stray magnetic field close to the raceway will be orders of magnitude higher if each raceway contains only one phase. If the raceway is in the wall of a building or above ground, the potential exposure of sensitive equipment (or people) to that field is pretty large.
If the ducts are underground, the closest possible approach will be beyond the region of maximum field.
But for a project where magnetic field strength is critical, the engineer may choose to add a requirement even for underground duct runs.

Tapatalk!

 

[highpowered]

Among other things the stray magnetic field close to the raceway will be orders of magnitude higher if each raceway contains only one phase. If the raceway is in the wall of a building or above ground, the potential exposure of sensitive equipment (or people) to that field is pretty large.
If the ducts are underground, the closest possible approach will be beyond the region of maximum field.
But for a project where magnetic field strength is critical, the engineer may choose to add a requirement even for underground duct runs.

Tapatalk!

Ok so not a good idea then. I'm not seeing anything in the code for Sizing parallel conductors? What would be the formula at 200' for 1,216amps

 

[rcwilson]

Ok so not a good idea then. I'm not seeing anything in the code for Sizing parallel conductors? What would be the formula at 200' for 1,216amps

Parallel conductors are assumed to share load equally. If 1216 is the amapcity required: 3/phase = 1216/3 = 405.3 A. 4/phase = 1216A/4 =304 Amps. 5/phae = 1216/5 = 243.2 A

(Verify that 1216 A meets 445.13 requirement that conductor ampacity = 115% of generator nameplate amps).

Since the circuit is 208/120V, 4 wires + ground are needed in each conduit and the neutral is current carrying so an 80% conductor derate is needed.

Assume 4/phase. Required ampacity after derating for 80% = 304 A/ 80% = 380A. 500 kCMIL copper would work at the 75C rating..

Check: 500 kcmil 75C = 380A. x 80% = 304A. x 4/phase = 1216 A. You can actually derate from the 90C rating and use 400 kcmil copper 4/phase.

(I haven't tried to buy 400 kcmil in many years). 4- 600 kcmil aluminum would work also. This all assumes a 0C (86F) environment.

You can down size the neutral conductor. I didn't checked the ground wire sizing.

Routing next to the transformer should not be an issue.

 

[texie]

Parallel conductors are assumed to share load equally. If 1216 is the amapcity required: 3/phase = 1216/3 = 405.3 A. 4/phase = 1216A/4 =304 Amps. 5/phae = 1216/5 = 243.2 A

(Verify that 1216 A meets 445.13 requirement that conductor ampacity = 115% of generator nameplate amps).

Since the circuit is 208/120V, 4 wires + ground are needed in each conduit and the neutral is current carrying so an 80% conductor derate is needed.

Assume 4/phase. Required ampacity after derating for 80% = 304 A/ 80% = 380A. 500 kCMIL copper would work at the 75C rating..

Check: 500 kcmil 75C = 380A. x 80% = 304A. x 4/phase = 1216 A. You can actually derate from the 90C rating and use 400 kcmil copper 4/phase.

(I haven't tried to buy 400 kcmil in many years). 4- 600 kcmil aluminum would work also. This all assumes a 0C (86F) environment.

You can down size the neutral conductor. I didn't checked the ground wire sizing.

Routing next to the transformer should not be an issue.

I won't argue with your method here. But I will say that the conductors need to be sized to the breaker on the genset, not 445.13 which only covers conductors from the terminals of the generator to the breaker. These are taken care of by the manufacturer.

 

[highpowered]

I won't argue with your method here. But I will say that the conductors need to be sized to the breaker on the genset, not 445.13 which only covers conductors from the terminals of the generator to the breaker. These are taken care of by the manufacturer.

sounds gound Thanks! Any one ever used a buss duct? my engineer wants to use this but the look pretty impossible to work with? if not he wants 3 sets of 700kcm?

 

[kingpb]

Here is some isolated phase bus duct (IPB) we use this all the time. :roll:



A little higher capacity than you need but it shows that it is above ground and not in PVC conduit.

 

[highpowered]

Here is some isolated phase bus duct (IPB) we use this all the time. :roll:

View attachment 9811

A little higher capacity than you need but it shows that it is above ground and not in PVC conduit.

How many amps those carry!

 

[rcwilson]

Isophase bus is used to connect large generators to step-up transformers. 1200A-44,000A continuous ratings are available. Large hydro generators use the high ampacity bus. Momentary short circuit ratings can be as high as 900kA.

The picture looks like something in the 10,000A range, 18 kV for a 300 MVA generator.

Isolated Phase Bus is designed like a coax cable. The energized aluminum bus for each phase is held in the conductive aluminum tube enclosure by insulators. The enclosures from each phase are shorted together at the generator and transformer ends and grounded. The current flowing in the energized conductors induces counter currents in the enclosures that flow opposite to the phase currents, cancelling external magnetic fields. That eliminates magnetic and induced current problems in surrounding equipment and structures.

Enclosure thickness is designed to minimize losses from the circulating currents. A 7500A bus will have about 80 watts loss per foot per phase in the enclosure and about the same in the bus. Enclosure temperatures can be as high as 80C, but are required to be lower in accessible areas. Forced air cooling is also possible to increase ampacity.

Each phase is isolated from other phases by its grounded enclosure. It is impossible to have a phase-phase fault which can damage a large generator. For high ampacity, high fault levels, IPB is the most economical design. But it is usually not cost effective for generators smaller than 100MVA unless magnetic field considerations are important.

For the OP's 350 kW, 208V genset, I would look at either 3-5 cables per phase or sandwich style bus duct (1600A?). If room is tight, sandwich bus duct can fit. (1600A is about 9"x5"). But it has to be designed exactly to the field conditions. Conduit & wire is more forgiving of dimensional errors.

 

[don_resqcapt19]

sounds gound Thanks! Any one ever used a buss duct? my engineer wants to use this but the look pretty impossible to work with? if not he wants 3 sets of 700kcm?

Bus duct is not bad to work with, but you have to be very careful with your measurements when you place the order. They have sections that will give you an inch or so adjustment. You should try to include one of those sections between each change in direction.

Also based on some experience and talking with others, I am not sure I would select bus duct for an outside installation. It is difficult to keep water out of the joints for long term and there have been a number of failures caused by water in the joints. I think I would be looking at cable bus or cable tray, with a preference for cable bus.

 

[kingpb]

How many amps those carry!

Largest so far was for a 900MVA generator at 27kV, so around 20,000A.

 

[highpowered]

Isophase bus is used to connect large generators to step-up transformers. 1200A-44,000A continuous ratings are available. Large hydro generators use the high ampacity bus. Momentary short circuit ratings can be as high as 900kA.

The picture looks like something in the 10,000A range, 18 kV for a 300 MVA generator.

Isolated Phase Bus is designed like a coax cable. The energized aluminum bus for each phase is held in the conductive aluminum tube enclosure by insulators. The enclosures from each phase are shorted together at the generator and transformer ends and grounded. The current flowing in the energized conductors induces counter currents in the enclosures that flow opposite to the phase currents, cancelling external magnetic fields. That eliminates magnetic and induced current problems in surrounding equipment and structures.

Enclosure thickness is designed to minimize losses from the circulating currents. A 7500A bus will have about 80 watts loss per foot per phase in the enclosure and about the same in the bus. Enclosure temperatures can be as high as 80C, but are required to be lower in accessible areas. Forced air cooling is also possible to increase ampacity.

Each phase is isolated from other phases by its grounded enclosure. It is impossible to have a phase-phase fault which can damage a large generator. For high ampacity, high fault levels, IPB is the most economical design. But it is usually not cost effective for generators smaller than 100MVA unless magnetic field considerations are important.

For the OP's 350 kW, 208V genset, I would look at either 3-5 cables per phase or sandwich style bus duct (1600A?). If room is tight, sandwich bus duct can fit. (1600A is about 9"x5"). But it has to be designed exactly to the field conditions. Conduit & wire is more forgiving of dimensional errors.

I just revisited the site today, I dont see it possible to use buss ducts. Theres just to many variables like the panel room being a big one absolutely no room. I'm going to have to knock down and rebuild the adjacent wall just to fit the transfer switches in. I talked the owners into moving the generator 50' from the electrical room next to the utility transformer. this way I can trench from the parking garage to to generator without involving the utility companys unground lines to the transformer. The only problem I have now is how the hell im going to get from the transfer switch to the main buss with 5 3" conduits :?