Generator transfer switch

[bth0mas20]

I have a customer that is planing to buy a 20kw generator and have me wire it for him. Guess he dosent want my warranty or mark up costs.

He said he was going to buy a 200a auto transfer switch.
I went to his house and noticed he has 2 panels ( a 200amp and 150a) so I told him he will need a larger switch.


Is thier any recomendations for a 350a auto transfer switch that has double lugs for two main panels?

 

[Cow]

I'm not very familiar with generator installs but something doesn't seem right here. He wants to buy an 83 amp generator that you think needs a 350 amp transfer switch? I believe when you use an ATS the gen MUST be able to carry the full load of the dwelling.

Have you done a load calc?

 

[augie47]

Cow is correct.
The major flaw in the plan is 702.5(B).

You might come closer to using his switch on one of his panels dependant on the calculated load.

You would still need to address "grouping of service disconnects", etc.

 

[ceb58]

I have a customer that is planing to buy a 20kw generator and have me wire it for him. Guess he dosent want my warranty or mark up costs.

He said he was going to buy a 200a auto transfer switch.
I went to his house and noticed he has 2 panels ( a 200amp and 150a) so I told him he will need a larger switch.


Is thier any recomendations for a 350a auto transfer switch that has double lugs for two main panels?

You are now in for a screwed up experience. It has already been said 702.5 (B).
Your best bet is to go back and sit down with the HO and come up with what they NEED to run in an outage. Then do a calculation of those loads, keeping in mind, a 20kw will do a continuous load of 83 amps. Then talk him into something like a 16 circ. ATS . You can feed it from one of the panels and bring what they need into the ATS/panel.
The smart thing to do is let the HO buy the gen. Depending on the brand of Gen. if you are not an authorised dealer and you need teck. support they will not even talk to you. Let him buy it and register the warranty. When something goes wrong with the gen. it will be his responsibility to find a authorised dealer to service the unit.

 

[bth0mas20]

thanks I agree about him buying the generator. I will read up on the NEC tomorrow.

 

[bullheimer]

i agree

email paulette at Paulette.Currington@generac.com and tell her i sent you.

not that generac walks on water but they have a good incentive program.

my limited understanding of washington state elec laws is not what it should be, but it is that any automatic transfer switch, because of it's location upstream of the main service, has to be rated at the same amount as the service. so since you wont, i think, get a 350A ATS you will get the 400A one

don't quote me on this, but i will try to come back (someday) with a link to this info from a wa state site. aaaaaahhhhhhh, here we go: http://www.lni.wa.gov/TradesLicensing/Electrical/files/currents/elc0710special .pdf



on manual transfer switches i have installed, a generator was good for about 40 Amps so i used a 60A Service Rated disconnect, i forget what i fused them at.

 

[ksmith846]

Here we are allowed to install a MTS on the service. This allows us to use a smaller Generator than the entire service size requires. If we use an ATS then we have to size the Generator per the total Service size.

So we could install the 20 KW generator and install a 400 Amp SE MTS prior to the existing disconnects. We are then required to provide instructions for a sequence of events for the end user to follow for turning on their generator, MTS and correct circuit breakers for the predetermined power consumption required in the household.

Or you could install a smaller MTS and feed either of the 2 existing panels from it with the same end user instructions.

 

[ceb58]

If we use an ATS then we have to size the Generator per the total Service size.

Why? That is not what 702.5 (B) states,unless it is a local thing. It states the calculated load in accordance with Art. 220. Just because you have a 200amp service doesn't mean it has a calculated load of 200amp

As far as the OP he can use a 200amp SE rated ATS. With a 400amp (320) meter base it has parallel lugs. One set could be brought into the ATS to feed one panel only. The only thing is the feeders leaving the ATS going to the panel could not go back through the meter base. They would need to be routed differently.

 

[jzadroga]

What is the reason a generator with an automatic transfer switch has to be sized to carry the whole load of the house? I assume that means everything turned on like all the lights and appliances. I could see if it was a criticle care generator, but an optional standby generator for a residence? Whats the worst thing that would happen if the load was to much for the generator? The only thing I could see is the breaker for the generator trips. Not a safety hazard just a nusiance like AFCI's. Most houses could run very well on a smaller generator as the usual load is rarely the entire connected load.

 

[ActionDave]

I feel the same. I can't see the reason for it in terms of safety or reliability.

 

[goldstar]

You can make one of the existing breaker panels the emergency panel. You may have to swap a few circuits though.

 

[laketime]

You can also use automatic load shedding to use a smaller generator to back up the entire service. 702.5 (B) (2)

 

[hurk27]

You can make one of the existing breaker panels the emergency panel. You may have to swap a few circuits though.

he can only do this if he installs another disconnect for the 150 amp panel 230.72

The OP said he has one meter feeding two main breaker panels, placing a service rated ATS will put one main disconnect outside and the other inside unless he locates the ATS inside next to the panel.

 

[renosteinke]

My best generator experiences have been when the generator dealer is the primary contact. Let him 'package' the generator, transfer switch, exercizer, and load bank together. Then you work with him to get it going.

At 20kW, the generator is big enough to require periodic operation, under load, to operate properly. My customers, for their convenience, exceeded the minimum requirements. For example, they set the exerciser to operate the thing every fourth Tuesday, rather than every other other month. Or, they ran it for 30 minutes rather than the 15 minute minimum.

This periodic operation is required to keep everything in the generator lubricated, to get 'stale' fuel out of the lines, and otherwise make sure everything worked. The extended run is required to get everything up to full operating temperature.

Running under load is important as well. While the actual percentage varies, genny makers usually say something like 'the genny should be run at 80% full load.' Thus, the genny should not be too large.

The load can be provided in two ways. Some places will actually transfer the facility to generator power during the maintenance run. Others will install a 'load bank' -that is, a giant toaster- on the genny.

Now, for sizing ...

There are two conflicting ways you need to use.

You size the genny to the actual load, which you often get by actually measuring it.

The transfer switch needs to be able to switch the full service-size amps.

Thus, it's common to find a 200 amp service backed up by an 85 amp genny.

 

[bill j]

The customer could use a 20kW Generac air-cooled with 2 200 Amp Nexus service rated transfer switches with load shedding. Each switch can drop out 4 contactors ( loads ) plus 2 A/C's. That might be enough too meet NEC requirements for his situation.

 

[ceb58]

What is the reason a generator with an automatic transfer switch has to be sized to carry the whole load of the house? I assume that means everything turned on like all the lights and appliances. I could see if it was a criticle care generator, but an optional standby generator for a residence? Whats the worst thing that would happen if the load was to much for the generator? The only thing I could see is the breaker for the generator trips. Not a safety hazard just a nusiance like AFCI's. Most houses could run very well on a smaller generator as the usual load is rarely the entire connected load.

I feel the same. I can't see the reason for it in terms of safety or reliability.

By both of your profiles you are electricians just as me. We know what is what when it comes to these things. Now pretend you are elderly and know nothing about it. Some one sells you a gen. package with out caring or knowing what should be done. You have an outage, gen. cranks and transfers but trips out the breaker because the unit is undersized. What then? Why doesnt it then become a safety issue. You paid for a system that is supposed to work. Why is it not a reliability issue?

 

[augie47]

adding to ceb58's comments:
You pay to have an automatic generator back up system and happen to be out-of-town when the power fails. You return home to find the generator breaker tripped and your food spoiled and pipes frozen due to an undersized back-up.

 

[hurk27]

most breakers on the generator do not protect the generator from over loads, they are there only to protect the conductors from the generator to the transfer switch, a few weeks ago I had the sad option of having to tell home owned that he voided the warranty when he took it upon himself to add his central air and 240 volt 40 amp oven to the generator sub panel, as the code in the generator s black box pointed to an over load and the windings was cooked, I even spoke with a tech at Generac and he confirmed this with the homeowner, who opted to purchase a 25kw unit to replace the burned up 14kw.

 

[ceb58]

most breakers on the generator do not protect the generator from over loads, they are there only to protect the conductors from the generator to the transfer switch, a few weeks ago I had the sad option of having to tell home owned that he voided the warranty when he took it upon himself to add his central air and 240 volt 40 amp oven to the generator sub panel, as the code in the generator s black box pointed to an over load and the windings was cooked, I even spoke with a tech at Generac and he confirmed this with the homeowner, who opted to purchase a 25kw unit to replace the burned up 14kw.

I will say that a lot of the smaller units do not have a breaker as we know it. It is like the A/C disconnect units. It looks like a breaker, has an amp rating stamped on it. But if you look at it it will state "unit dose not have overload protection". So in fact it dose nothing to protect the feeders or the gen.

 

[ActionDave]

By both of your profiles you are electricians just as me. We know what is what when it comes to these things. Now pretend you are elderly and know nothing about it. Some one sells you a gen. package with out caring or knowing what should be done. You have an outage, gen. cranks and transfers but trips out the breaker because the unit is undersized. What then? Why doesnt it then become a safety issue. You paid for a system that is supposed to work. Why is it not a reliability issue?

The way I see it if you size the generator to the actual load there is no issue with safety or reliability. Hurk brings up a point that the breaker on the gennie only protects the conductors, but then again I refer back to the preceding statement.

adding to ceb58's comments:
You pay to have an automatic generator back up system and happen to be out-of-town when the power fails. You return home to find the generator breaker tripped and your food spoiled and pipes frozen due to an undersized back-up.

Then you are a cheap, price only, bottom feeding, son of a female canine that got what you deserved.

Moreover, the scenario you pose only reinforces my rationale. When, if ever, are you going to pull the max load on your house when no one is home? When have you ever seen a house pull the calculated load with everybody home? (Jim in Tampa don't answer this).