generator larger than service

[mjmike]

Odd situation question. Current project has an old 60A 120/240V 1-phase service. load consists of a couple lights and a larger motor across the line start. The project is to incorporate an emergency generator to back up entire service. due to age of service, will be replacing with 100A service. When sizing the generator, due to the motor, the genrator is larger than the service (won't be installing a VFD). The generator breaker will be 150A-2P hence a 150A rated transfer switch. However, the service disconnect will be 100A fused, then the 150A transfer switch, and then a single panel on transfer switch output with a 100A main breaker. Never had a situation where the running load is so small yet the motor inrush requires a large genset.

So, should the service and panel be changed to 150A to keep the system common per say? Or is a 100A service with a 150A rated genset and 150A transfer switch ok? Anybody run into this before? I just can't see a 150A service for such a small load. Heck, the peak utility demand over the past year has only been about 9Kw. Maybe I should make everything 150A rated and be done with it.
thanks.

 

[Dennis Alwon]

Have you looked at 445.13

445.13 Ampacity of Conductors. The ampacity of the conductors from the generator terminals to the first distribution device(s) containing overcurrent protection shall not be less than 115 percent of the nameplate current rating of the generator. It shall be permitted to size the neutral conductors in accordance with 220.61. Conductors that must carry ground-fault currents shall not be smaller than required by 250.30(A). Neutral conductors of dc generators that must carry ground-fault currents shall not be smaller than the minimum required size of the largest conductor.
Exception: Where the design and operation of the generator prevent overloading, the ampacity of the conductors shall not be less than 100 percent of the nameplate current rating of the generator.

 

[mjmike]

Thanks, that helps some. To me, its saying that the condutors from the generator thru the transfer switch, to the panel, by exception need to be rated for 100% the generator nameplate rating. If I am reading it correctly, the panel and service can remain 100A with 100A rated service conductors thru the disconnect to the transfer switch.

 

[hillbilly1]

The breaker in the generator can be downsized to 100 amp, or set a disconnect outside the generator, using the tap rule, and fuse down to 100 amps, leaving the exsisting breaker in the generator. The T-switch can remain at the 150 amp rating, as long as it's not the service disconnect.

 

[Smart $]

Thanks, that helps some. To me, its saying that the condutors from the generator thru the transfer switch, to the panel, by exception need to be rated for 100% the generator nameplate rating. If I am reading it correctly, the panel and service can remain 100A with 100A rated service conductors thru the disconnect to the transfer switch.

Why not just use a 100A service-entrance-rated ATS?

With the first breaker at the gennie, you can use tap rules and size your conductors from the gennie to the ATS for 100A.

 

[mjmike]

Well I guess I could make the breaker at the genset 100a. Since the genset would normally be designed for 150a, I'm thinking I would keep the emergency system at 150a. Why limit it's capability. So if the genset breaker, transfer switch, and conductors are 150a, do you think I should make the service 150a to match.

The reason for not using a service entrance ats varies. I have used them and like them, but not in this particular situation.

 

[hillbilly1]

Well I guess I could make the breaker at the genset 100a. Since the genset would normally be designed for 150a, I'm thinking I would keep the emergency system at 150a. Why limit it's capability. So if the genset breaker, transfer switch, and conductors are 150a, do you think I should make the service 150a to match.

The reason for not using a service entrance ats varies. I have used them and like them, but not in this particular situation.

Sure your genset capability would not be limited if you keep the 150, but your load would be if you stay with the 100 amp service. No use staying with the 150 amp genset breaker if your service is 100 amp.

 

[mjmike]

Sure your genset capability would not be limited if you keep the 150, but your load would be if you stay with the 100 amp service. No use staying with the 150 amp genset breaker if your service is 100 amp.

True. Ok, maybe a 100a breaker on the genset would be good. If the generator is required to be a certain size hence the 150a breaker, is it ok to put a 100a breaker on it? Or, am I getting the overcurrent protection confused with the motor starting capability (inrush) of the genset?

Next, can my conductors from the genset have to be rated for the nameplate or can that be based on the genset breaker?

 

[hillbilly1]

The generator oversizing has more to do with HP of the generator than anything else, the more HP, the more resistance to the change of RPM and voltage output under load. With the high starting amps of the electric motor, this tends to reduce voltage sag. It's just like dropping a 100 amp load on a 100 KW generator, you will hear the RPM's change, drop a 100 amp load on a 200 KW generator, and you will likely not notice an appreciable change in RPM. The load hasn't changed, it's just the capacity of the generator to handle the load. Manufactures often put in smaller breakers than required for specific uses, the Orange box for example, has two breakers in the generator, one for the emergency lighting loads, the other for the non-emergency. The total of the two breakers may be the rated output, but the wire is sized for each breaker rating.

 

[ceb58]

Well I guess I could make the breaker at the genset 100a. Since the genset would normally be designed for 150a, I'm thinking I would keep the emergency system at 150a. Why limit it's capability. So if the genset breaker, transfer switch, and conductors are 150a, do you think I should make the service 150a to match.

The reason for not using a service entrance ats varies. I have used them and like them, but not in this particular situation.

If you do not use a SE rated ATS then you will need to install a disconnect between the meter and ATS.
You keep referring to it as a emergency system is it really or is it just an optional system?
Ether way you will need a disconnect on the generator feeder so you can use a fusible disconnect and down grade the amp rating there.

 

[mjmike]

If you do not use a SE rated ATS then you will need to install a disconnect between the meter and ATS.
You keep referring to it as a emergency system is it really or is it just an optional system?
Ether way you will need a disconnect on the generator feeder so you can use a fusible disconnect and down grade the amp rating there.

I have a 100a fused disconnect located after the meter before the ats to serve as the service disconnect. You are correct in my nomenclature, this is not a life safety genset. It is an optional genset.

I will have the breaker at the generator so my conductors after this breaker to the ats located indoors can be based on the genset breaker correct?

I am leaning more towards the 100a breaker at the genset and making the whole system 100a rated.

 

[kwired]

The generator oversizing has more to do with HP of the generator than anything else, the more HP, the more resistance to the change of RPM and voltage output under load. With the high starting amps of the electric motor, this tends to reduce voltage sag. It's just like dropping a 100 amp load on a 100 KW generator, you will hear the RPM's change, drop a 100 amp load on a 200 KW generator, and you will likely not notice an appreciable change in RPM. The load hasn't changed, it's just the capacity of the generator to handle the load. Manufactures often put in smaller breakers than required for specific uses, the Orange box for example, has two breakers in the generator, one for the emergency lighting loads, the other for the non-emergency. The total of the two breakers may be the rated output, but the wire is sized for each breaker rating.

The POCO's generator also slows down when you start your load. It is such a large generator that it almost can not be seen or measured how little it slows it down however.

With small generator the energy needed to start a motor is still the same as with utility power, the generator typically can not provide as high of current as utility and therefore the starting duration will last longer. A larger breaker or one with different trip characteristics may be needed to allow motor starting.

 

[mjmike]

To follow-up, can't downsize the genset breaker therefore the service will be 150A.

 

[kingpb]

As others have pointed out, you will need the capacity of a generator capable of starting the motor. The motor starting power factor needs to be known. This will allow you to size a generator capable of starting the motor. The next issue, is with using an over sized generator, or looking at it another way, a lightly loaded generator; the generator is subject to wet stacking. This can have major issues with performance and required maintenance. Putting a RVS on the motor to keep the gen size down would be most beneficial.

 

[mull982]

As others have pointed out, you will need the capacity of a generator capable of starting the motor. The motor starting power factor needs to be known. This will allow you to size a generator capable of starting the motor. The next issue, is with using an over sized generator, or looking at it another way, a lightly loaded generator; the generator is subject to wet stacking. This can have major issues with performance and required maintenance. Putting a RVS on the motor to keep the gen size down would be most beneficial.

Does the motors power factor need to be known as opposed to simply the motors starting current since most of this if not all will be reactive power anyway? Cant it be assumed that pf will be very small?

 

[kingpb]

Not sure what size your motor is, or what type. After thinking about it, the starting pf may not be all that low, especially if it is a capacitor start/induction run, general purpose motor. Which means the cap would provide quite a bit of the starting reactance. Never the less, your still looking at an inrush of on the gen that would be 3.5-4 times the motor rated value, plus the other load which means you have a generator that is maybe 40% loaded. You would need to talk to the gen manufacturer, and see of a small load bank would be needed.

For example, CAT program says for a 7.5Hp, 240V motor, and 9kW worth of misc linear load, and you want to limit VD to 10%, then it will require you to go with a 46.6KW (model D50). I think that's going to put you up into, at least, the 200A CB/panel rating.

Without more details this is just speculating.

 

[kwired]

Not sure what size your motor is, or what type. After thinking about it, the starting pf may not be all that low, especially if it is a capacitor start/induction run, general purpose motor. Which means the cap would provide quite a bit of the starting reactance. Never the less, your still looking at an inrush of on the gen that would be 3.5-4 times the motor rated value, plus the other load which means you have a generator that is maybe 40% loaded. You would need to talk to the gen manufacturer, and see of a small load bank would be needed.

For example, CAT program says for a 7.5Hp, 240V motor, and 9kW worth of misc linear load, and you want to limit VD to 10%, then it will require you to go with a 46.6KW (model D50). I think that's going to put you up into, at least, the 200A CB/panel rating.

Without more details this is just speculating.

The real issue may be not be so much the kw of generator, but what size of prime mover is powering the generator - can the prime mover keep the generator supplied with enough power to start the motor without a too significant voltage drop.

If you go too large on an engine driving the generator to compensate for this you will have a unit that uses more fuel than maybe is necessary.

Soft starting large motors will help the situation where possible.

 

[kingpb]

Agree with soft starting, would probably more than pay for itself in a situation like this.