15Kw generator on 7.5Kw transfer switch

[robstaples]

Forgive my reply after settlement, but as a newbee to the forum and retired utility elec engineer I thought this analogy might help. The governor on an engine/car/tractor will limit its rpm or speed. A breaker or fuse is a current limiting device like a governor. The 30 amp breaker in both legs of any size generator will limit the output to 30 amps ( ignoring short term transcient effects). A panel has a power rating and has to be protected to not exceed that rating. Most new homes have 200 amp main breaker at the top( electrcal input) to protect the panel and beyond. The vendor and NEC has to try to keep the uninformed person in this example from replacing the 30 amp breaker with a 60 amp and no other changes which with additional loading of the panel would lead to trouble.

 

[Midniteoyl]

And there in lies the problem.. even IF the switch was fused at 30 amps a leg, feeding it with a larger genset just creates too much temptation for the uninitiated to replace or by-pass it.

 

[Mace Canute]

Forgive my reply after settlement, but as a newbee to the forum and retired utility elec engineer I thought this analogy might help. The governor on an engine/car/tractor will limit its rpm or speed. A breaker or fuse is a current limiting device like a governor. The 30 amp breaker in both legs of any size generator will limit the output to 30 amps ( ignoring short term transcient effects). A panel has a power rating and has to be protected to not exceed that rating. Most new homes have 200 amp main breaker at the top( electrcal input) to protect the panel and beyond. The vendor and NEC has to try to keep the uninformed person in this example from replacing the 30 amp breaker with a 60 amp and no other changes which with additional loading of the panel would lead to trouble.

This is exactly the problem that allowing the tenant to use a 15Kw generator might lead to. If nothing was changed it wouldn't be a problem but how long would it be before he decided that he might as well get full use from the 15Kw generator and add extra load?

Best bet is to definitely limit the replacement generator to 7.5 Kw.

 

[daybreak1998]

Howdy,
A lot of the difference would be how is it connected.

If you have a generator with a properly available 30amp twistlock socket, then it does not matter on the size of the generator.

Multiquip 20kw genset
2 120 duplex sockets
1 30amp 120/240 socket L14-30R
1 50amp 120/240 socket CS6364
higher power connections to be hard wired off lugs

Even though the generator is more than 7500w, it has the proper twistlock to use with that panel.

The limiting item with be the L14-30R socket. They are made with 30amp max use with exactly that 30amp dual pole breakers protected.

Potential is 30amps

Use is 30amps

 

[techman]

A simple and code legal solution is to come out of the generator with a cable good for 60+ amps (15 KW) and go into a 2-pole breaker box (they are made for branch circuit protection). Use a 2-pole 30 amp breaker in that box. Come out of that breaker box with a 30 amp cable (10 ga or larger) and terminate it with a plug to connect to the transfer switch. You will have circuit protection to meet code. It will cost under $ 100 for everything.

paul

 

[DarkBlack]

:confused2:

I'm not taking anyone's advice except for the company that makes the transfer switch. Here is what their answer is regarding using a generator rated to higher output then the transfer switch.
I did want to hear people's feedback on my question; that does NOT mean I'm doing electrical decision making based on a tractor forum. Stop assuming what I'm going to be doing- I'll be the one who decides what, AND the answer should send a lot of misinformation to the curb for others looking for the answer to this question.

FROM: Gentran Corporation: Generator Transfer switches for home & business

Can I use a generator whose wattage exceeds the wattage on the Transfer Switch?
NO. While it might be possible to connect a larger generator to the transfer switch, this is an unsafe and dangerous practice. Supplying more than the rated wattage through the transfer switch could damage the wattmeters, damage the neutral wire, and could overheat the main wiring in the transfer switch, which could cause a fire.

The above statement by the company who manufacturers the transfer switch is what the tenant will comply with.

Thanks again to all who contributed. This is basically what I told my tenant at the outset- generator MUST be 7500 watts or less.

Case closed.

CM - out.

That's misleading electrical advise from the switch mfg.
It's not the electrical generation source's responsibility to be power limited by a down stream device. Typical of a couple of this forum's perpetual safety posters, to advocate decisions based on "perceived" safety, of such ideas, with obviously no real knowledge. If we followed that bad "logic" then the utility companies would limit their generator supply to your house 100, or 200 amp panelboard.:confused2:
Sorry, It's not aimed at you. It's for some of the "expert" posters who pose as electrical safety experts here. The oversized generator will hold waveform and voltage, and actually make a quicker/safer breaker tripping environment than a smaller unit. Yes.. counter intuitive. The woman said "Supplying more than the rated wattage": In that context she is right., but you are talking about a higher RATED wattage generator, which is totally different.

 

[Midniteoyl]

All of that is correct... BUT: as said, the temptation to pull more from the genset than the switch is rated at is the major concern. All the tenant has to say is 'he let me use the 15kw genset, so I thought it was ok'. You gotta think like a tenant does - it not their house on the line, so they dont care. Then, when something DOES happen, they either disappear leaving you hanging (just try to collect on a judgement - it doesnt work), or, they blame you and try to collect from you. Been through it enough with many tenants. The easiest and best way is to simply say 'No' and be done with it.