Service Calculations

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rezendejas

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I have the following connected loads which I am sizing a service (208V, 3 ph, 4 w)for:

Lights 20kVA
Recepts 50kVA
Motors 10 kVA

The demand totals 65 kVA (181 amps).

Can the service be 200 Amps and can this be fed from a 200A/ 3P main circuit breaker?
 
Re: Service Calculations

First, what type of consulting do you do?

You can figure this out using article 220.

There is not enough information given to answer the question.

Roger
 
Re: Service Calculations

I would not cut it that close. Reasons:
1. It leaves little room for future expansion.
2. I'm not sure where you are getting the demand factor. You can only use the demand factor for very specific instances (see table 220.11 for example).
3. What if the motors actually wind up being 15Kva when they are installed. Then the service is too small.
4. What if other loads are added later(sign lighting, or site lighting, ect.)
5. The lighting is a continuous load and I think it should be multiplied by 1.25 since the main breaker will only be 80% rated. See 230.42.

Steve
 
Re: Service Calculations

I am an electrical engineering consultant with 20 years experience.

What I am really looking for is opinions regarding to service size versus breaker ratings.

Presume the demand factors are appropriate per the NEC for the sake of this discussion. Ignore any spare or future capacities.

If the demand load calculations (per the appropriate NEC articles) total 181 amps, and there is only a 200 amp meter socket left for the tenant, can the service be 200 amps. Someone raised the point the service can not be 200 amps since the circuit breaker rating is only rated for 80%, therefore the service size must be 181 amps times 1.25 equalling 226 amps, therefore a service larger than 226 amps must be provided.

I contend if all of the NEC load calculations are met and they total to 181 amps, the 200 amp meter socket can be used.

I appreciate your thoughts on the subject.
 
Re: Service Calculations

rezendejas
Lets start withe what the NEC allows. If your calculated load is 181 amps, can you install a 200 A OCPD for the service disconnect?
 
Re: Service Calculations

You can use a 200 amp switch with a 200 amp calculated load.

As a matter of fact, you can use six 200 amp switches on a calculated load of 200 amps.

[ December 31, 2003, 12:14 PM: Message edited by: bennie ]
 
Re: Service Calculations

yes according to the NEC

Size the overcurrent protection device in accordance with Sections 210-20(a), 215-3, and 384-16(d). These three NEC rules required the overcurrent protection device (breaker or fuse) be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load. Section 240-6(a) contains the list of standard size overcurrent protection devices.
 
Re: Service Calculations

The continuous load only has to be applied once. The calculated load should include the continuous load adjustment.
 
Re: Service Calculations

I believe you are ok as well, if you are worried about it, specify a 100% rated circuit breaker.

[ December 31, 2003, 01:06 PM: Message edited by: bwyllie ]
 
Re: Service Calculations

I am not 100% sure, but I do not believe that there are 100% rated breakers to be found at the 200 amp rating - could be wrong - would like to know from someone who knows for sure.

Yes you can use the 200 amp service for your calculated load. That is why we use Article 220 for calculations.

Pierre
 
Re: Service Calculations

After reading the specifications on Cutler Hammer panel boards, I am under the impression that the rating of the breaker determines the panel rating when used as service equipment. There is no reference to making adjustments for continuous load.

My service panel has two 100 amp breakers paralleled on each line. This makes a 200 amp main with four poles.
 
Re: Service Calculations

Originally posted by rezendejas:
I have the following connected loads which I am sizing a service (208V, 3 ph, 4 w)for:

Lights 20kVA
Recepts 50kVA
Motors 10 kVA

The demand totals 65 kVA (181 amps).

Can the service be 200 Amps and can this be fed from a 200A/ 3P main circuit breaker?
Click to expand...
I'm worried about the demand factors as well. However, if it's a non dwelling the recept load can be sized down to 30 KVA (10kva@ 100% and the rest at 50%). Also if lighting is that much of a load, and it's fluorescent then you might consider it being partly harmonic.

With a diversity of about 75% then I think you will be fine. Of course the utility will make us all look stupid and give you a 30 KVA transformer. :)
 
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