Demand factor for SDPs

[Dbronx]

Caculated MDP


The SOW specified 50 amp services for housing units. So a 3 phase 50 amp service at 80% would be 14,410.66 watts X 10 = 144,106.63 watts divided by 360 = 400.29619 X 1.25 = 500.37024
A 500 amp service with two cables in parallel 4C 250 with 2/0 ground.The 50 amp services will need #8 with #8 ground. Is that correct ?


It we assume that these are SFDs and subject to the NEC, then you could apply demand factors and have a smaller service load.

Annex D has some good examples.



Would the demand factor be 55% for housing units cosisting of lighting, receptical, and HVAC loads only?

 

[Smart $]

Caculated MDP


The SOW specified 50 amp services for housing units. So a 3 phase 50 amp service at 80% would be 14,410.66 watts X 10 = 144,106.63 watts divided by 360 = 400.29619 X 1.25 = 500.37024
A 500 amp service with two cables in parallel 4C 250 with 2/0 ground.The 50 amp services will need #8 with #8 ground. Is that correct ?


It we assume that these are SFDs and subject to the NEC, then you could apply demand factors and have a smaller service load.

Annex D has some good examples.



Would the demand factor be 55% for housing units cosisting of lighting, receptical, and HVAC loads only?

Before getting into allowable demand factoring we have to cover other points. First, did the SOW specify 1? or 3? services to each unit? It makes a very significant difference.

Second, spec'ing 50A "services" is fine as long as the calculated load is 50A or less.

Third, simply summing unit calculated loads is not the prescibed method of determining service load or conductor size as some demand factoring is based on total numbers, some such as motors on a single largest and not largest per unit.

Fourth, a true calculated load already factors in 125% for continuous loads, motors, and such.

Take the preceding into account and repost any remaining question(s)....

 

[Dbronx]

Yes 3 phase SPDs fed by 3 phase generator, the only motor is in a split air-condition and the manufactor gives MCA and MOP, in housing units. On my cable caculation I forgot to de-rate to 80% because there are 4 conductor.

 

[Smart $]

Yes 3 phase SPDs fed by 3 phase generator, the only motor is in a split air-condition and the manufactor gives MCA and MOP, in housing units. On my cable caculation I forgot to de-rate to 80% because there are 4 conductor.

Okay, 3? it is.

A/C motors are typically considered in the HVAC load as determined by the calculation method... so there would be no separate 125% (i.e. additional 25%) consideration for the largest motor on the service or feeder conductors. However, you will have to convert your MCA to VA and include in calculation as appropriate.

If one of your four cable conductors is a neutral, it is only counted as a current-carrying conductor if over 50% of the system load is non-linear (electronic balasts, electronic power supplies, for example) and connected line-neutral. I surmise there is no true need to de-rate for four conductors.

You are asking your questions on an NEC forum. The Code prescribes only one method for new installations, and that is a load calculation for service and feeder conductors per Article 220. This calculation sets the minimum ampacity or rating of the service and feeder conductors and equipment, respectively. However, there is nothing which prohibits oversizing the system. The method you used in the OP likely oversizes the main feeder... but the only way to know for certain is to do the load calculations.

 

[Dbronx]

The SOW states full load compacity for SPDs for future expansion, and there will be non linar loads flourescent lighting and general use outlets.

 

[jumper]

Caculated MDP


The SOW specified 50 amp services for housing units. So a 3 phase 50 amp service at 80% would be 14,410.66 watts X 10 = 144,106.63 watts divided by 360 = 400.29619 X 1.25 = 500.37024
A 500 amp service with two cables in parallel 4C 250 with 2/0 ground.The 50 amp services will need #8 with #8 ground. Is that correct ?

It we assume that these are SFDs and subject to the NEC, then you could apply demand factors and have a smaller service load.

Annex D has some good examples.

Would the demand factor be 55% for housing units cosisting of lighting, receptical, and HVAC loads only?

I highlighted the text you copied from the other thread. I posted it.

Where did you get 55% from?

 

[Smart $]

The SOW states full load compacity for SPDs for future expansion, and there will be non linar loads flourescent lighting and general use outlets.

I don't doubt there will be non-linear loads. The question is whether the non-linear loads that are L-N connected make up the majority of the entire load on the feeder or service cobductors.

Full capacity on SPD's... understood. 10 of these require not less than a 500A main. If derating for fourth conductor because of non-linear loads, that would mean a higher yet non-derated conductor ampacity... 500A (after) ? 80% = 625A (before) minimum. There is no need to multiply by 125% (1.25 in OP). The allowable current for continuous, noncontinuous combinations is already set by max capacity [500A (100%) for all loads non-continuouos, 400A (80%) for all loads continuous, and combinations fall somewhere in between automagically).


However, we have another conundrum if you want to apply a demand factor to the main. You cannot feed the SPD's to max capacity if you apply demand factors. If you want to apply demand to the main, now, and upgrade later for max capacity, you are back to doing a load calculation. Load calculations are where demand factors are applied.

 

[Dbronx]

I highlighted the text you copied from the other thread. I posted it.

Where did you get 55% from?

I looked through the expample D5(a) I supose is 200 amps @ 100% and above 200 amps @ 70%

 

[Dbronx]

I don't doubt there will be non-linear loads. The question is whether the non-linear loads that are L-N connected make up the majority of the entire load on the feeder or service cobductors.

Full capacity on SPD's... understood. 10 of these require not less than a 500A main. If derating for fourth conductor because of non-linear loads, that would mean a higher yet non-derated conductor ampacity... 500A (after) ? 80% = 625A (before) minimum. There is no need to multiply by 125% (1.25 in OP). The allowable current for continuous, noncontinuous combinations is already set by max capacity [500A (100%) for all loads non-continuouos, 400A (80%) for all loads continuous, and combinations fall somewhere in between automagically).


However, we have another conundrum if you want to apply a demand factor to the main. You cannot feed the SPD's to max capacity if you apply demand factors. If you want to apply demand to the main, now, and upgrade later for max capacity, you are back to doing a load calculation. Load calculations are where demand factors are applied.

Would table 551.73 be the right demand factor choice, since these are RLBs relocatable buildings, that can sleep up to 4 people.

 

[Smart $]

Would table 551.73 be the right demand factor choice, since these are RLBs relocatable buildings, that can sleep up to 4 people.

Article 551 is for Recreational Vehicles (RV's) and parks for such.

RLB ≠ RV

Do these RLB's have wheels for ease of relocating? If so, perhaps, Article 550.

Otherwise, Article 545 at best... which will take you back to doing Article 220 calculations.