View Full Version : standby generator - what gauge wire???

September 20th, 2003, 08:45 PM
I've enjoyed reading the posts to this forum, and they have been helpful.
I am getting ready to purchase a 13KW Natrual Gas Standby generator for my home and have some questions. I plan on using this generator to drive heat, air conditioning, refridgeration and lights thoughout the house.

1) What would be the recommended gauge for a 100 foot run carrying 108-125amps?
:arrow: The Guardian transfer box that comes with this unit includes a 6 guage 30ft wire. But I don't think 6 guage is adequate for this length. I need to confirm with the manufacturer, but a distributer thinks the transfer box will support 4 gauge.

2) Is copper preferred over aluminum?

3) The transfer box with the unit I'm looking at comes with 12 positions and has a max 30amp breaker size. My gas pack is 240 w/35 amp two pole breakers in my existing breaker box. Is there a way to determine if 30 amp two pole will suffice w/starting amp requirements?

4) Any opinions on the Generac Guardian Air Cooled Generators?

Thanks in advance -

September 21st, 2003, 01:33 AM
Something isn't adding up here. A 13 KW generator should produce about 55A at 240V. You won't get a single 120V 110A circuit out of the generator, you'll get a split 120/240 with 55A max at each pole.

Next, what are we sizing -- the generator output to the transfer switch (i.e. what is at the end of the 100' run)? There are many issues in installing a generator, so you may want to ask here if you're not sure of the rules for grounding and whether this is or isn't a separately derived system (depends on your transfer switch).

If the generator has no overcurrent protection, you need to size to 115% of the output. So worst case, we're looking at around a 65A circuit. If the lugs of the generator and the terminating devices are all rated at 75 degrees C, then you could use #6 wire. Since 65A breakers aren't standard, you could have your disconnect rated at 70A while still using #6. If you use an equipment grounding conductor, it needs to be #8.

However, with a 100' run, I'd be inclined to stay with the 60 degree wire rating column to aid voltage drop and this will require #4 copper. Copper is preferable to aluminum in just about all properties except cost and weight. If those aren't issues, I'd go with copper.

What is a "gas pack"? Does it have a nameplate indicating its current needs (such as FLA, MCA, or just amps)?

September 21st, 2003, 10:03 AM
Thanks for the reply suemarkp..

Based on your reply, I'm leaning towards #4 copper...

The generator is rated 13KW 120/240V with 108/54 amps respectively.

Gas pack is a term we use in NC for our forced air heat units with built in cooling unit. It's a self contained heater/air conditioner/compresser that sits outside of the house. The MCA on it is 20.4. Reading this morning, it sounds like a general rule of thumb is to take the MCA and multiply by 3 to get the correct breaker size. In this case, that would be just over two 30 amp breakers. Do you think a hard start kit may help with the compressor load?

If anybody out there has experience with the Generac Guardian generator, please let me know if it's been a positive or negative experience.

Thanks, binster

September 21st, 2003, 01:41 PM
Where you found the Minimum circuit ampacity rating your branch circuit conductor size for you heater/ac unit you should find a maximum overcurrent [breaker or fuse] size. This will tell you the maximum breaker or fuse size allowed on your unit.

Generac is a common generator. Brand name is not the key item you should be concerned about. Type of fuel is a concern as well as size capacity in amps of the generator. You need to consider that a generator often sets for a year or more without need of use. If your generator is a gas or deisel generator often times the carborator or injectors will be plugged up and the generator will not start when you need it due to lack of use and plugged injector or carb. If it were me I would consider an LP driven motor type generator or if you have a lawn tractor with a pto drive on it consider a PTO driven generator without a motor using the PTO to drive the generator. Most often your lawn of farm tractor is used commonly keeping it in running condition for when the generator is needed.

YOu quote two non compliant facts. YOu state a 13 KW generator being maximum amp load rated at 54 amps. Then you speak of 108/54 amp rating. Can't be both. Something is wrong. I am suspection the 108 amp is starting capacity with normal electrical load at 54 amps max output of generator.

Confirm and let us know what you find.

You could install a transfer box rated in amps to match the main service of your house panel then connect the generator with a breaker on the generator not to exceed 115% of maximum output as previously replied. Then run you factory power through that transfer box as well as your generator power. Then you entire house may use the generator but only one load at a time not to exceed 240 volt 60 amps in the house. You may manually cycle which circuits can run to limit overload of the generator main breaker protection.

Hope this helps


September 21st, 2003, 03:17 PM
Thanks for the information. The maximum breaker size for my heater/ac unit is 30 amps. Looks like my builder oversized when he installed 35amp breakers, unless they were thinking of the additional draw during the hot summer months.

Either way, looks like I am ok with two 30 amp breakers. And if I have problems with tripping, I'll consider a hard start kit.

Ref the generator specs.. perhaps I am reading them wrong, but the math worked out for me.. So I thought I had it right
This is a single phase generator..
120v x108a = 13kw
240v x 54a = 13kw

To clear up any confusion that I'm introducing, here's the link for the specifications

Thanks, binster

September 22nd, 2003, 10:16 PM
A 13 Kw generator will produce 54 amps on each hot line being for 120 volt or 240 volts using both hot lines.

YOu generator is rated as 54 amps 240 volts.

If you installed a 6 awg copper wire from the generator to a transfer box rated in amps equal to the service size coming in from the power company then your entire house will have available the generated power. Then turn off all breakers, start your generator and only run 50 amps to 60 amps maximum at any given time limiting the load by using certain breakers at a time. Example water heater and house lights or heat pump and lights or range partial use and lights or water pump lights and dishwasher. Cycling what you need as you need it limiting not to exceed the amp rating or your generator.

Hope this helps


September 24th, 2003, 10:32 PM
It is required a portable generator be physically connected to earth ground.

Is it permissible to use the same grounding rod as the main panel box, or does it have to a physically separate rod?

September 24th, 2003, 11:36 PM
It depends on if this generator is a separately derived source without an equipment grounding conductor connecting the two power sources or if this generator is with a feeder to the transfer box with no equipment grounding conductor joining the two systems.

Not all generators require their own ground rod. If this is a separately derived source then it requires its own ground rod.

If the generated feed contains an equipment grounding conductor that joins the two power sources grounding systems then no ground rod is to be installed at the generator. See the rules in Article 445 of the NEC for generators then refer to Article 250 for grounding.

Hope this helps