Load Calculation to determine minimum size transformer - Mobile Home/s

[mwsjgb]

I have recently calculated loads for all-electric mobile home units in order to determine transformer size/s for suppling from one to four of the same size mobile homes, all fed from one transformer. I would like to see if any of you would like to do your own calculations to see if you arrive at the same transformer sizes that I did. The mobile home size and loads are listed below. I can provide my findings using NEC 550-18 as a guide, but would like to see what some of you come up with first. Please let me know if you would like additional information.

Mobile home layout: (14'x 70') 980 sq ft., 3 bedroom, 2 bath. Full kitchen, dining/den, central HVAC with resistance heat (not a heat pump), washer and dryer circuits.

Resistance Heating unit: 10000 VA (AC size not listed as it is much smaller than the heating load and cannot run concurrently)
HVAC blower motor: 500 VA
Freestanding Oven/Range: 10000 VA
Water Heater: 4500 VA
Clothes Dryer: 5500 VA
Bath Fans: 150 VA
Microwave: 1200 VA
Refrigerator: 700 VA
Dishwasher: 1800 VA
Garbage Disposal: 900 VA

Thanks in advance.

Mike

 

[Little Bill]

@mwsjgb
Could you update your profile to include what you are retired from?

 

[tortuga]

I have recently calculated loads for all-electric mobile home units in order to determine transformer size/s for suppling from one to four of the same size mobile homes, all fed from one transformer. I would like to see if any of you would like to do your own calculations to see if you arrive at the same transformer sizes that I did.

I come up with 34.2kVA for a single home and 53.4 kVA for four homes.
Side note: seems like there is a type'o in my 2023 NEC under 550.18(B)(3) is asking for 25% of the largest motor in Table 550.18(B) which is the range table.

 

[Little Bill]

@mwsjgb
Could you update your profile to include what you are retired from?

@mwsjgb
Thanks!

 

[suemarkp]

Are you calculating the service and equipment size, or transformer size? The NEC load calculation usually isn't used for transformer sizing, especially in residential type loads, as the calculations give numbers that are too large. Is there not a power utility supplying these mobile homes? Their sizing methodology gives much smaller numbers. They tend to care about the largest motor LRA to make sure the transformer can supply that. They also seem to allow about a 100% overload because loads don't tend to last that long. When I put a 400A service on my last house, the calculated load was in the high 300's (so about 90KW) but the utility was going to leave the 25 KVA transformer (and they did not ask about my load other than the largest LRA). I paid extra to get a 50 KVA installed. And then there is the 1/0 aluminum service drop for all that load...

 

[tortuga]

Are you calculating the service and equipment size, or transformer size? The NEC load calculation usually isn't used for transformer sizing, especially in residential type loads, as the calculations give numbers that are too large. Is there not a power utility supplying these mobile homes? Their sizing methodology gives much smaller numbers. They tend to care about the largest motor LRA to make sure the transformer can supply that. They also seem to allow about a 100% overload because loads don't tend to last that long. When I put a 400A service on my last house, the calculated load was in the high 300's (so about 90KW) but the utility was going to leave the 25 KVA transformer (and they did not ask about my load other than the largest LRA). I paid extra to get a 50 KVA installed. And then there is the 1/0 aluminum service drop for all that load...

Yeah good point, there are extra large sites, for example a large YMCA camp comes to mind, that are primary customers and run their own 7.2, 4.16 or even good old 2.4 MV underground distribution. And in that case the transformer sizing would be per the NEC, but notice the NEC 450 does not explicitly say the transformer nameplate has to be sized to the calculated load either. Just the feeders.

 

[augie47]

Very unusual mobile home. .far more electric than the ones I see
If you calculated it as a dwelling it would be around 36kw whereas the NEC figures 16kw per unit (Art 550.31)

 

[mwsjgb]

I come up with 34.2kVA for a single home and 53.4 kVA for four homes.
Side note: seems like there is a type'o in my 2023 NEC under 550.18(B)(3) is asking for 25% of the largest motor in Table 550.18(B) which is the range table.

Thanks. That is what I came up with as well. However, as others have pointed out, NEC 550-18 is used for calculating service equipment size and is not a transformer sizing tool or requirement. The only guidance I can find as far as the transformer size selection is general practices of utility companies which for the most part indicate using a 50 KVA for 4 to 6 standard size homes with 200 Amp services.

 

[mwsjgb]

Are you calculating the service and equipment size, or transformer size? The NEC load calculation usually isn't used for transformer sizing, especially in residential type loads, as the calculations give numbers that are too large. Is there not a power utility supplying these mobile homes? Their sizing methodology gives much smaller numbers. They tend to care about the largest motor LRA to make sure the transformer can supply that. They also seem to allow about a 100% overload because loads don't tend to last that long. When I put a 400A service on my last house, the calculated load was in the high 300's (so about 90KW) but the utility was going to leave the 25 KVA transformer (and they did not ask about my load other than the largest LRA). I paid extra to get a 50 KVA installed. And then there is the 1/0 aluminum service drop for all that load...

Understand. I was trying to correlate the 550 load calculations to typical transformer sizing that I routinely see deployed (4 to 6 homes per 50kVA), which doesn't match up. I was just wondering if I was missing something. But as you and others imply, there is no NEC sizing methodology for transformer sizing.

 

[tortuga]

I was trying to correlate the 550 load calculations to typical transformer sizing that I routinely see deployed (4 to 6 homes per 50kVA), which doesn't match up.

Are these utility owned or privately owned transformers? And if privately owned is the primary over 1000V?

 

[mwsjgb]

Are these utility owned or privately owned transformers? And if privately owned is the primary over 1000V?

Privately owned. Primary voltage 7200 Volts

 

[tortuga]

Privately owned. Primary voltage 7200 Volts

Alright cool we can keep using the NEC then.
What did you come up with for four identical homes?
Going with my 53.4 kVA and If they are all fed off one drop thats a minimum feeder size of 223A @ 240V
and a minimum feeder size of 7.5A @ 7.2kV
And at 7.2kV I presume its a supervised installation so no secondary protection.
primary current of transformer:
@ 50kVA =
50kVA/7.2kV = 6.9 A
fuse can be 250%
6.9 X 2.5 = 17.36A
I think typically they recommend a 10A fuse for a 50kVA transformer.