Generator Hookup

Old 03-18-04, 07:29 AM
Visiting Guest
Posts: n/a
Generator Hookup

I recently acquired a 20KW generator that is rated at 90AMP/240V. I also have a 200 amp single phase automatic transfer switch. I am installing the generator in a shed that is not connected to my house. The transfer switch will be installed in my basement next to my service panel. I have an existing 2 1/2" conduit between my house and the shed that was installed when the shed was built. I have a few questions about how to properly connect the generator to the transfer switch.

I assume I will need a service disconnect switch at the generator. I assume this should be 100 amp but I have noticed these come in many flavors most reference HP ratings what size would I need? Also should this be fused or non-fused.

Can I run the generator control wires in the same conduit as the power lines?

What size wire should be used. The total cable run will be about 120' from the generator to the transfer switch. Also what type of wire copper aluminum solid stranded? The conduit is sealed end to end but there is no guarantee that water has not penetrated it or will not in the future. Also my preference would be to use a cable that would be conducive to pulling through the conduit (Non-jacketed, stranded). When the generator says 90amp/240v does that mean 45 amps per hot leg or 90amps each?

Last question is configuration of the grounding. I was under the impression I could use three wire (2 Hot, 1 Neutral) from the shed to the house and then use a grounding rod attached to the generator. After reading other posts I now think I may need to use 4 wire (2 Hot, 1 Neutral, 1 Ground) as there will be a 120v run from the house to the shed for lights in the shed. There is no water pipe, cable TV, data etc.

Thanks in advance for any advice
Old 03-18-04, 07:27 PM
Speedy Petey's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Nov 2003
Posts: 2,262
Upvotes: 0
Received 0 Upvotes on 0 Posts
WOW! You have alot going on there.

You do need a disconnect at the genset. With 4 wires going to the house.
You do need a ground rod at the genset.
You cannot run the control wiring with the feed wires.
You also cannot run the 120 volt circuits in either conduit. I normally run 3 conduits to "bigger" residential generators.
It is 90 amps per leg.
You CANNOT run cable in this conduit.
You need #4/4/6/8cu or #2/2/4/6al. I recommend copper THHN/THWN.

Are the genset and switch designed for use with each other?
Were there any instructions with the switch? Many of these sets are package units to only be used with each other.

Lastly. I do not wish to undermine you ability but this is a VERY complex undertaking. I HIGHLY recommend having a LICENSED and EXPERIENCED electrician doing this job. There are many detailed codes to follow and many mistakes to be made if you don't know what you are doing.
From the sound of your questions this is not something you have done before. I don't think an automatic transfer 20kw genset is a place to "learn as you go".
Old 03-19-04, 10:03 AM
Join Date: Feb 2002
Location: port chester n y
Posts: 1,983
Upvotes: 0
Received 0 Upvotes on 0 Posts
It's important that the tranfer panel control circuits be compatible with the generator control circuits. The tranfer panel controls will "detect" a power-loss and close a circuit that will start the generator. A control at the generator will operate the transfer switch after the generator is running, and the emergency circuits will be switched to generator power.

This is an "automatic" tranfer, but you have the option of manual-transfer-- press a button that cranks the generator, and then manipulate a manualy-opertated transfer-switch.

Conductors of different voltages can occupy the same conduit if all conductors have the same insulation and are the same type .Art. 300.3 (C), Conductors of different systems.

POSSIBLY you may have in the 2-1/2" raceway three #2 power-conductors, three #10 Feeder-conductors, one #8 Green Equipment Grounding Conductor, and 4-6 (?) #12 control-conductors, all conductors Type THWN rated at 600 volts.

These conductors will diverge to different connection-points at both ends of the 2-1/2" raceway. For example, in the shed, the #2 Power-conductors will terminate on a 2-pole generator circuit-breaker, the #10 Feeder-conductors will terminate on a breaker-panel, the control-conductor will terminate at the generator. and the Equiptment Grounding Conductor will be "Bonded" to a Grounding Electrode and all metallic surfaces that enclose "live" wires. This probably is best accomplished using a "Wireway", i.,e., a "trough" which contains all the conductors. The Enclosures and conductor raceways are conncected to the Wireway with approved wiring methods, and the conductors in the Wireway "branch-off" to the repective connection-points.

The Equiptment Grounding Conductor will be "Bonded" to several Grounding-points at both "ends"-- shed/house-- of the system.

Good Luck & Enjoy the Experience!!!!!!!
Old 03-19-04, 05:26 PM
Speedy Petey's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Nov 2003
Posts: 2,262
Upvotes: 0
Received 0 Upvotes on 0 Posts

It's true the conductors can occupy the same conduit according to code. The problem I usually run into is that the different sets of conductors need to terminate at different physical locations. Having them in the same disconnect/enclosure at the genset can be a real hassle. Unless of course you run separate conduits/flex from that to the gen enclosure.
Also, most of the gensets I do require the control wires to NOT be in the same conduit as the 120 volt accessory circuit.
Old 03-19-04, 08:26 PM
Visiting Guest
Posts: n/a
First off thanks for the great information.

The Transfer switch was designed to work with the generator so I do not for see any issues there.

Any thoughts on the proper disconnect switch? As I mentioned in my previous post there seems to be many different choices for 100 AMP single phase disconnect switches.

Would the following item seem reasonable?

General Duty Safety Switch, Current Rating 100 Amps, Power Rating 15 HP, NEMA Type 1 Enclosure, Height 17 1/2 Inches, Depth 6 1/2 Inches, Width 8 1/2 Inches, 240 VAC, Without Handle, 3 Wire, 2 Blades and Fuse Holders, 1 Solid Neutral, For Corner Grounded Delta System, Series F, Cartridge Type Fuse

Square D Model D223N

Thanks again
Old 03-19-04, 10:25 PM
HandyRon's Avatar
Join Date: Dec 2001
Location: New York
Posts: 1,287
Upvotes: 0
Received 0 Upvotes on 0 Posts
Don't forget that 225.31 & 225.32 require a disconnect located where the circuit enters your home, somewhat like a service disconnect (within a few feet of entering the home).
Old 03-20-04, 08:13 AM
Join Date: Feb 2002
Location: port chester n y
Posts: 1,983
Upvotes: 0
Received 0 Upvotes on 0 Posts
A 2-pole 100-amp circuit-breaker can serve a dual-purpose-- a dis-connect and over-current protection, similiar to a Service "Main- breaker-- no need to stock & re-place fuses----

S-P-- you state--- "Conductors need to terminate at different -- locations"

That's EXACTLY the question I adressed in my Posting, when I advised M-D to use a Wireway-----
Old 03-20-04, 08:46 AM
Sharp Advice's Avatar
Admin Emeritus
Join Date: Feb 1998
Location: The Shake and Bake State USA
Posts: 9,927
Upvotes: 0
Received 7 Upvotes on 6 Posts
Hello: Gents

I am not a licensed electrician nor do I have experience with connect any generator to an existing electrical system.

The point I would like to make is regarding the safety issues concerning power company linemen whom will be working on the electrical lines during power outages.

Generators connected to the main electrical panels used for power during outages must not allow current to flow into the power companies lines. Workers safety is the issue.

However any alternate or backup electrical systems are to be installed must be approved and meet all existing codes. Whom ever does the final connections is the person(s) whom must insure the safety and proper operation(s) of the system.

I am sure and hope each of you would agree, final inspection by a licensed electrical service person should be hired to at least oversee the project to ensure safety to all concerned parties.

As a utility service person myself, (24 yrs) in the natural gas industry, per company policy, we require each and every service person to use the company supplied electrical tester before working on any gas company equipement.

Electrical current in the gas lines is that much of a problem in many areas. Serious injuries and deaths have happened to service personal as a result.

Several cases it has been an improperly installed backup gen set in the electrical system, of which was unknown to exist. Other cases improperly grounded appliances and or main electrical panels grounded to underground gaslines, etc.

For the safety of all concerned parties, all gen sets installed should have a final inspection and meet all codes prior to initial startup and testing please.

Sharp Advice
Web Site Host & Forums Monitor.
Old 03-20-04, 10:29 AM
Visiting Guest
Posts: n/a
I agree with the safety issues raised in this thread. It has always been my intension to have a licensed electrician inspect my work before the generator is used or even tested. I have spoken to one already and have picked his brain for design ideas and requirements.

In my case natural gas is not available so I will be using Propane. All the gas work is being done by my local gas company who will be supplying the tank.

I posted on this board to get other opinion it has been my experience that you can always benefit from getting a few different perspectives and hopefully learn from others mistakes. At the same time I have a few personal goals I want to achieve in this project. 1) Learn about the installation and operation of my generator and electrical service. 2) Ensure that the job is done in a high quality manner meeting or exceeding the codes involved. 3) Save some money. I believe in all trades just because someone is licensed does not always mean they do the job right (Most do not all however). I believe by educating my self in this area I will ensure that this job gets done right and provides years of reliable service & save service.

Again I appreciate all the advice that has been offered.

Old 03-20-04, 04:04 PM
hornetd's Avatar
Join Date: Aug 2001
Location: Maryland
Posts: 646
Upvotes: 0
Received 0 Upvotes on 0 Posts
Two or three pole transfer switch?

Many generators of that size have a built in circuit breaker. If yours does you will not need a disconnect at the shed.

If the transfer switch is not listed as service equipment you will need a disconnect for the generator supply at the house end of the generator supplied feeder and a separate disconnect at the house that is ahead of the transfer switch for the public utility power. If your present service disconnecting means is a single breaker that is located in your main lighting and appliance panel board you may be able to save some money by buying an enclosure for your present main breaker and a main lug kit for your panel in order to relocate your existing main breaker ahead of the transfer switch. If the transfer switch is listed as service equipment; as many are; you will not need either additional disconnect at the house.

If the transfer switch is three pole, one pole of which is used to transfer the grounded current carrying conductor (read neutral), than you will only need a three wire generator feeder. If the transfer switch is two pole with a solid connection for the neutral conductor you will need to remove the bonding connection that may be present between the neutral of the generator winding and the frame of the generator set. You will then attach the frame of the generator to the Equipment Grounding Conductor that is run with the generator supplied feeder conductors. You can use the same conductor as the EGC for the feeder or branch circuit that supplies the loads in the shed as well as the generator supplied feeder that will supply the house under emergency conditions as long as it is sized for the highest amperage rating of the two over current protective devices.

The reason that you must have a separate EGC if the neutral is solidly connected at the transfer switch is that the bond between the generator winding and the frame of the generator is a connection of the neutral to ground on the load side of the service disconnecting means under normal operation. Such grounding connections on the load side of the service disconnecting means are specifically prohibited by the US NEC.

In either case you must run an EGC with the feeder or branch circuit that supplies the loads in the shed because the generator supplied feeder constitutes a metallic pathway between the two buildings that forces the use of a separate EGC in the supply to the shed and keeping the neutral aloof from ground in the shed.
Tom H

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: