Emergency Generator

[CathyInBlue]

Here's a question for an Electrician: Can I feed my 12.5K watt/50 amp manual transfer switch with a 7.8K watt/30 amp genny?

As long as you feed the xfer switch from the 30 A genny with the same cable as you would feed with the 50 A genny. The 50 A breaker in the xfer switch would not protect a 30 A conductor from the 30 A genny if the genny were to overload and begin supplying more than 30 A. In fact, the 50 A xfer switch won't protect the 30 A genny at all, but then, it wouldn't prevent a 50 A genny from eating itself either. The issue is in protecting the cabling therefrom. A >30 A load placed on the system would be passed merrily by the xfer switch, and attempt to be met by the genny, but if the conductors between the two are insufficient, they can overload, overheat, and cause a fire. Of course, that is, unless the genny eats itself first, thus reducing the power on the wires to zero regardless of wouldbe load.

 

[indyjohn]

As long as you feed the xfer switch from the 30 A genny with the same cable as you would feed with the 50 A genny. The 50 A breaker in the xfer switch would not protect a 30 A conductor from the 30 A genny if the genny were to overload and begin supplying more than 30 A. In fact, the 50 A xfer switch won't protect the 30 A genny at all, but then, it wouldn't prevent a 50 A genny from eating itself either. The issue is in protecting the cabling therefrom. A >30 A load placed on the system would be passed merrily by the xfer switch, and attempt to be met by the genny, but if the conductors between the two are insufficient, they can overload, overheat, and cause a fire. Of course, that is, unless the genny eats itself first, thus reducing the power on the wires to zero regardless of wouldbe load.

Okay. So I'm going to infer from your post that the 30 A breaker will govern the load from the genny, and if the draw from the xfer switch (@ 50 A) exceeds 30 A, the breaker on the genny will pop.

Yes, dear, I was hoping to get your attention because you appear to know a thing or two about this stuff.

 

[GodFearinGunTotin]

Okay. So I'm going to infer from your post that the 30 A breaker will govern the load from the genny, and if the draw from the xfer switch (@ 50 A) exceeds 30 A, the breaker on the genny will pop.

Yes, dear, I was hoping to get your attention because you appear to know a thing or two about this stuff.

If your generator has a breaker to which you'll be connecting the transfer switch to and all the downstream load, whether your 50A transfer switch has a breaker or fuse in it doesn't matter (assuming it's greater than your generator's output circuit protection).

 

[indyjohn]

If your generator has a breaker to which you'll be connecting the transfer switch to and all the downstream load, whether your 50A transfer switch has a breaker or fuse in it doesn't matter (assuming it's greater than your generator's output circuit protection).

This issue has come about due to wattage. The generator I'm looking to buy is 7800 running watts, 13500 startup watts. The first transfer switch I looked at was rated at 7500 watts max. So... I'm now looking at a 12,500 watt / 50 amp switch.

Just want to make sure I'm not making a mistake.

 

[CathyInBlue]

A 30 A breaker on the genny will protect… the genny. It will not necessarily protect the conductor from the genny to the xfer switch which is equipped with a 50A breaker. But, if that conductor is rated for 50 A, then the 50 A breaker in the xfer switch will protect it, leaving the 30 A breaker in the genny to protect the genny.

 

[GodFearinGunTotin]

This issue has come about due to wattage. The generator I'm looking to buy is 7800 running watts, 13500 startup watts. The first transfer switch I looked at was rated at 7500 watts max. So... I'm now looking at a 12,500 watt / 50 amp switch.

Just want to make sure I'm not making a mistake.

Does your transfer switch even have a circuit breaker or fuses in it? My background is in industrial electrical and the ones we typically used did not--I'm not as familiar with residential versions.

Like I said above, in general, you'll never go wrong with over sizing conductors, switches, etc. Assuming your generator has a breaker protecting its output, I wouldn't worry about it. If somebody cuts your cable (shorts it out), the breaker in your generator should trip--protecting your cable and the generator.

Cathy's right in that, if your transfer switch did have a 50A breaker, the conductors down stream will be protected by the 50A circuit but your generator's breaker will usually trip out first anyway, since it's 30A.

 

[Brian Ski]

This issue has come about due to wattage. The generator I'm looking to buy is 7800 running watts, 13500 startup watts. The first transfer switch I looked at was rated at 7500 watts max. So... I'm now looking at a 12,500 watt / 50 amp switch.

Just want to make sure I'm not making a mistake.

You are doing fine. The bigger the transfer box the more circuits you will be able to use. (maybe not all at the same time) It should have more circuits in it. On the other hand the generator has it own breaker to protect itself and the 30 amp wiring from it.

BTW if the transfer box has two meters on it you can watch the load to see how much you can run. Since you are using a 7800 watt generator, you get to use 3900 watts per leg (per meter) max. You can't use 5000 on one and nothing on the other. It is better on the generator if they are kind of close to balanced.

 

[CathyInBlue]

This issue has come about due to wattage. The generator I'm looking to buy is 7800 running watts, 13500 startup watts. The first transfer switch I looked at was rated at 7500 watts max. So... I'm now looking at a 12,500 watt / 50 amp switch.

Just want to make sure I'm not making a mistake.

As long as you're feeding that 50 A xfer switch with conductors that can handle that ampacity, everything else is academic. If you then fed it with a genny that was rated for 50 A, everything's kosher. If you fed it with a genny that was only capable of 30 A, it's kosher, as there's no way for the lower rated genny to over load the conductor, and if it did, it'd be protected by the 50 A breaker in the xfer switch. If you then fed it with a genny rated for more than 50 A, that would be verbotten, but not overly dangerous, as there would still be the 50 A breaker in the xfer switch to protect the conductor from the greater than 50 A current draw that the genny could readily provide.

 

[indyjohn]

Does your transfer switch even have a circuit breaker or fuses in it? My background is in industrial electrical and the ones we typically used did not--I'm not as familiar with residential versions.

Like I said above, in general, you'll never go wrong with over sizing conductors, switches, etc. Assuming your generator has a breaker protecting its output, I wouldn't worry about it. If somebody cuts your cable (shorts it out), the breaker in your generator should trip--protecting your cable and the generator.

Cathy's right in that, if your transfer switch did have a 50A breaker, the conductors down stream will be protected by the 50A circuit but your generator's breaker will usually trip out first anyway, since it's 30A.

This is what I'm looking at. The genny is 240V / 30 A feed.

This is what I'm looking at. The genny is 7800 amp running / 13500 surge at 240V / 30 A feed.
http://www.steadypower.com/products.php?product=Reliance-51410C-Indoor-Transfer-Switch-(50A)

 

[CathyInBlue]

Does your transfer switch even have a circuit breaker or fuses in it? My background is in industrial electrical and the ones we typically used did not--I'm not as familiar with residential versions.

I guess I am making assumptions. If the xfer switch is just a dumb xfer switch, manual type, then there's no protection for the genny-xfer switch conductors except the breaker on the genny, and possibly protection downstream of the xfer switch, such as an associated power panel/breaker box.

All of the residential xfer switches I've seen were formed from two main breakers oriented in opposition, feeding the same set of breaker busses with a sliding blocker bar set between the main breaks' paddles such that only one of those main breakers may actually be feeding power to those breaker busses at a time. As a consequence, my concept of a xfer switch integrates the idea that it has a breaker. If it doesn't, much of what I've said immediately up-thread should be read skepticly.

 

[CathyInBlue]

This is what I'm looking at. The genny is 240V / 30 A feed.

It will feed household voltage at 30 A, but it should do so into a conductor rated for the xfer switch's breaker, not the genny's.

 

[indyjohn]

I guess I am making assumptions. If the xfer switch is just a dumb xfer switch, manual type, then there's no protection for the genny-xfer switch conductors except the breaker on the genny, and possibly protection downstream of the xfer switch, such as an associated power panel/breaker box.

All of the residential xfer switches I've seen were formed from two main breakers oriented in opposition, feeding the same set of breaker busses with a sliding blocker bar set between the main breaks' paddles such that only one of those main breakers may actually be feeding power to those breaker busses at a time. As a consequence, my concept of a xfer switch integrates the idea that it has a breaker. If it doesn't, much of what I've said immediately up-thread should be read skepticly.

See my edited post.

 

[Brian Ski]

I guess I am making assumptions. If the xfer switch is just a dumb xfer switch, manual type, then there's no protection for the genny-xfer switch conductors except the breaker on the genny, and possibly protection downstream of the xfer switch, such as an associated power panel/breaker box.

All of the residential xfer switches I've seen were formed from two main breakers oriented in opposition, feeding the same set of breaker busses with a sliding blocker bar set between the main breaks' paddles such that only one of those main breakers may actually be feeding power to those breaker busses at a time. As a consequence, my concept of a xfer switch integrates the idea that it has a breaker. If it doesn't, much of what I've said immediately up-thread should be read skepticly.

I took it as the manual transfer switch with a lot of small double pole switches. I have not seen a house fed with a 50 amp service. As a 50 amp service transfer switch would not work... I am pretty sure he has one of these, but with a 50 amp box.

Shop Reliance 6-Circuit Transfer Switch Kit with 30-Amp Inlet Box at Lowes.com

 

[GodFearinGunTotin]

I guess I am making assumptions. If the xfer switch is just a dumb xfer switch, manual type, then there's no protection for the genny-xfer switch conductors except the breaker on the genny, and possibly protection downstream of the xfer switch, such as an associated power panel/breaker box.

All of the residential xfer switches I've seen were formed from two main breakers oriented in opposition, feeding the same set of breaker busses with a sliding blocker bar set between the main breaks' paddles such that only one of those main breakers may actually be feeding power to those breaker busses at a time. As a consequence, my concept of a xfer switch integrates the idea that it has a breaker. If it doesn't, much of what I've said immediately up-thread should be read skepticly.

In the case of the latter, the transfer switch breakers will likely never have a reason to trip when powered by a 30A limited source. (I've seen wierd things happen with breakers not coordinating during short circuit events). But if the generator is limited by the generator's breaker, everything downstream is limited.

 

[indyjohn]

It will feed household voltage at 30 A, but it should do so into a conductor rated for the xfer switch's breaker, not the genny's.

I'm ok with that. I can built that patch cable.

 

[Brian Ski]

I'm ok with that. I can built that patch cable.

You may want to post what your transfer switch is. I posted one above, they are thinking it is something completely different. Want to make sure you are on the right track. A 30 amp cord would work fine for the one that I linked to. As long as you stick to the 30 amp generator.

 

[GodFearinGunTotin]

I took it as the manual transfer switch with a lot of small double pole switches. I have not seen a house fed with a 50 amp service. As a 50 amp service transfer switch would not work... I am pretty sure he has one of these, but with a 50 amp box.

Shop Reliance 6-Circuit Transfer Switch Kit with 30-Amp Inlet Box at Lowes.com

I've never seen one of those before. I guess each, individual ciruit is transfered, singularly?

 

[Brian Ski]

I've never seen one of those before. I guess each, individual ciruit is transfered, singularly?

Yes you can switch each circuit on one at a time in case to many would over load a generator. That way you can switch back and forth to run more loads but not all at the same time. I found a 50 amp one with about a dozen circuits. It is like the one I have. It takes more work to install but easier to use in the long run. No main and you can leave the rest of the circuits you don't want to switch, hooked on to the main box... Hooked to the pole power.

 

[Brian Ski]

They also have little bars to double tie the breakers if you have a 220 (double pole circuit), that you want to switch.

 

[GodFearinGunTotin]

Yes you can switch each circuit on one at a time in case to many would over load a generator. That way you can switch back and forth to run more loads but not all at the same time. I found a 50 amp one with about a dozen circuits. It is like the one I have. It takes more work to install but easier to use in the long run. No main and you can leave the rest of the circuits you don't want to switch, hooked on to the main box... Hooked to the pole power.

Yeah, the ones I'm used to are basically a double throw switch, where you select the source with the swich and a single output connection that goes to the load.