Service Sizing

[aelectricalman]

What factors are considered with sizing wire for an electrical service? What I mean by this is, if I have a 600amp service, why should I have to have all three legs of a wye system sized to be able to withstand 600amps, knowing the system will never have 600amps on any leg at any time. I did not respond to the inspector because I didnt know what to say at that point! This is a sanity check for my own good. Thanks again. Is there nothing to be said about a balanced system?

 

[Finite10]

Is this an article 220 feeder calc issue?

 

[aelectricalman]

We did a load calc. but....

We did a load calc. but....

Is this an article 220 feeder calc issue?

was told that no matter the size of service we install, the feeders for each leg had to add up to be the voltage of the system times three.

Basically, we are installing a 120/208Y - 125kW generator to an 800amp Service with a 116kW peak demand.

Assuming .80 PF, we have 433 Amp to account for. He is telling me I need to use 600MCM feeders or parellel them (but thats not the point) to supply a 433 Amp Gen output? What?

 

[aelectricalman]

was told that no matter the size of service we install, the feeders for each leg had to add up to be the voltage of the system times three.

Basically, we are installing a 120/208Y - 125kW generator to an 800amp Service with a 116kW peak demand.

Assuming .80 PF, we have 433 Amp to account for. He is telling me I need to use 600MCM feeders or parellel them (but thats not the point) to supply a 433 Amp Gen output? What?

Basically accounting for 433 Amps per leg.

 

[augie47]

Obviously your generator would need to supply the load.
The normall basics would be to (a) determine the load, (b) size the conductor for the load and (c) then size the OCP device to protect the conductors. In your case you probably already have the OCP sized by the generator manufacturer and you must take that into cosideration. The size of your generator OCP device, along with the load, should be the basis for sizing the conductors. 125 kw, possibly a 450 amp breaker. A 600 kcmil would be adequate for a 420 amp load and a 450 amp breaker.

 

[Finite10]

was told that no matter the size of service we install, the feeders for each leg had to add up to be the voltage of the system times three.

Basically, we are installing a 120/208Y - 125kW generator to an 800amp Service with a 116kW peak demand.

Assuming .80 PF, we have 433 Amp to account for. He is telling me I need to use 600MCM feeders or parellel them (but thats not the point) to supply a 433 Amp Gen output? What?

Could it be worse?....
445.13; 115% NPR
1.15 x 125,000W = 143,750W

143,750/208V x 1.732 x .8
143,750/288
=499A

Are the lugs 75 degree?

 

[kwired]

Are the conductors in question the conductors from the generator, from the utility or the common feed off the transfer switch? What is overcurrent protection on these conductors, during both positions of the transfer switch?

If you only have a load calculation of less than 433 that is fine when the generator is the supply, but if the same conductor is protected at 600 amps when running on utility supply then the conductor needs to be sized larger or the overcurrent device from the utility supply needs to be reduced.

 

[david]

was told that no matter the size of service we install, the feeders for each leg had to add up to be the voltage of the system times three.

Basically, we are installing a 120/208Y - 125kW generator to an 800amp Service with a 116kW peak demand.

Assuming .80 PF, we have 433 Amp to account for. He is telling me I need to use 600MCM feeders or parellel them (but thats not the point) to supply a 433 Amp Gen output? What?

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 . 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.

nameplate current rating of the generator?

 

[jbelectric777]

Be careful what some inspectors say, me being one Im always for the contractor and laugh at some stuff I hear. All you have to do is size it in accord with your load calculation, period, thats it. One time a buddy told me the inspector wanted the following for an apartment building. "now this is funny" 100% for the first panelboards main and 50% for each additional. So if its 10 apts with 100amp each he wanted the main SE to be 550 amps, a total and complete WASTE OF MONEY ! I ended up doing the inspection because it was private back in those days but the guy was using a "rule of thumb" because he was too lazy to look it up.

 

[david]

1. Be careful what some inspectors say, me being one Im always for the contractor and laugh at some stuff I hear. All you have to do is size it in accord with your load calculation, period, thats it. One time a buddy told me the inspector wanted the following for an apartment building. "now this is funny" 100% for the first panelboards main and 50% for each additional. So if its 10 apts with 100amp each he wanted the main SE to be 550 amps, a total and complete WASTE OF MONEY ! I ended up doing the inspection because it was private back in those days but the guy was using a "rule of thumb" because he was too lazy to look it up.


I don’t think I could say it with that simplicity. For instance the op in this thread said it was a 600 amp service in his opening remarks. My thought would be what makes it a 600 amp service. He stated the load was only 420 amps. So I have to assume the service equipment has overcurrent protection of 600 amps. I say if that’s the case the service entrance conductors have to be sized for 600 amps.

I do agree that services only have to be sized for the calculated load but they also have to be sized a min of the overcurrent protection for the service.

As this thread went on it was stated that a generator feed was involved. The 420 amp load is being supplied by a gen. There is a 115% increase required for the Gen feed to the first overcurrent protection.

So in this case again the conductor’s sizes have other considerations then the calculated load they are supplying. The 115% of the name plate rating of the gen regardless iof the calculated load is less than that.

Am not saying you’re wrong. I agree that the service doesn’t have to be larger than the calculated load.
I just do not think it can be stated that simply.

clarification
I am aware of 230.90 exception three but that information isn't given in this post

 

[aelectricalman]

I have a 125kW generator. The peak demand load is 116kW. The Manual Transfer Switch will be a SER transfer switch and will serve as the buildings new OCPD since it is before the MDP. The building service is sized for 800amp 120/208Y 3 ph 4 wire. I don't see why the service size matters since the peak load is 116kW and the generator is sized to accomodate that. The SER XFR is rated for 800amps and the generator has OCPD built in for the size of the GenSet. The only question I'm really concerned with now(not related to the post) is does the SER XFR have to have an equipment ground to the MDP since the MDP is now a subpanel?

Regarding the initial post. I was told I would have to have a ganerator that is capable of powering 800amps continuous on three legs. I told him I would like another inspector!

 

[aelectricalman]

3 phase generator manufacturers use a .80 PF

3 phase generator manufacturers use a .80 PF

Why would a manufacturer use a .80 PF for a three phase gen and resonance for single phase. Seems like the rating for 3 P would be closer to 1. .80 just seems awful. Anyone have any thoughts?

 

[david luchini]

Why would a manufacturer use a .80 PF for a three phase gen and resonance for single phase. Seems like the rating for 3 P would be closer to 1. .80 just seems awful. Anyone have any thoughts?

I'm not sure I understand your question...but, how do you know what the power factor of your loads are going to be? If the manufacturer gave you a 125kW, 120/208V generator with a specified power factor of 1.0, then the rated generator current would be 347 amps.

Now what if I connected a load of 122kW to the 125kW generator, but the load had a pf of 0.9. The load current would be 122kW/208V/1.732/0.9 = 376. My load current exceeds the generator rated current, even though my load kW is less than the generator kW.

In a simplistic sense, the 0.8 pf of the generator allows it to supply 125kW of load from 0.8 to unity power factor (or anything in between) without exceeding its rating.

 

[kwired]

I have a 125kW generator. The peak demand load is 116kW. The Manual Transfer Switch will be a SER transfer switch and will serve as the buildings new OCPD since it is before the MDP. The building service is sized for 800amp 120/208Y 3 ph 4 wire. I don't see why the service size matters since the peak load is 116kW and the generator is sized to accomodate that. The SER XFR is rated for 800amps and the generator has OCPD built in for the size of the GenSet. The only question I'm really concerned with now(not related to the post) is does the SER XFR have to have an equipment ground to the MDP since the MDP is now a subpanel?

Regarding the initial post. I was told I would have to have a ganerator that is capable of powering 800amps continuous on three legs. I told him I would like another inspector!

I don't blame you for wanting another inspector. 800 amps would mean the generator needs to be at least 288 kW. If it is a manual transfer switch there is no reason you can't use a 5 kW generator and the user is allowed to select the load, as long as the generator has proper overcurrent protection. May not be a great design but would still be code compliant.

 

[Sangre]

Is 445.13 referring to the conductors which are commonly installed by the factory from the terminals to the built-in generator main breaker which could be considered the first distribution device and overcurrent protection? Or does it refer to the conductors from the genset main breaker to an external distribution panel?