Amps vs Amps per hour

[binwork91]

It is a stupid question. It kinda bother me.
For example, when we do the load calculation for service, we said we need 1000A， it mean 1000A per phase.
But utility company ruling said "The service take-off(s) are each limited to 400 amperes
per phase per half hour demand and the entire vault installation is not to exceed 400 amperes per phase per
half hour demand."
The "400 amperes per phase per half hour demand" is kinda bother me. Does it mean 800A per phase per hour? so does utility only provide max 800A? or 400A?

(I made up the number, just want to bring up an example.)

 

[ron]

That means 400A on each of the ungrounded phases. You would use a 400A service switch and meter.

 

[wwhitney]

The "400 amperes per phase per half hour demand" is kinda bother me.

The only meaning I can come up with is they will be unhappy if your average current over a half-hour period exceeds 400A. Which implies your peak current may exceed 400A.

Cheers, Wayne

 

[ggunn]

It is a stupid question. It kinda bother me.
For example, when we do the load calculation for service, we said we need 1000A， it mean 1000A per phase.
But utility company ruling said "The service take-off(s) are each limited to 400 amperes
per phase per half hour demand and the entire vault installation is not to exceed 400 amperes per phase per
half hour demand."
The "400 amperes per phase per half hour demand" is kinda bother me. Does it mean 800A per phase per hour? so does utility only provide max 800A? or 400A?

(I made up the number, just want to bring up an example.)

If the current on a balanced set of conductors is 1000A per phase, it's just 1000A total line current whether it is single phase or three phase. The more than 400A per phase per half hour demand means an average of more than 400A over a half hour period. The "per phase" means, I suppose, that in the unbalanced case where the phase currents are not equal due to unequal single phase loads, the demand is determined by the larger/largest phase current.

 

[binwork91]

So the answer is utility only will able to provide 400A max, even if my demand load is 1000A? Which mean if my building use 500A, then utility side OCPD will trip? Because they are not going to provide 500A.

Then is it mean the voltage drop calculation base on demand load is kinda useless, because you won't see full demand load.

 

[tortuga]

Whats the kva of the utility transformer and the voltage?

 

[binwork91]

Whats the kva of the utility transformer and the voltage?

I make up the number, just want to bring up the idea. Assume secondary side of utility transformer is 208V, and building need 208V.

 

[tortuga]

I make up the number, just want to bring up the idea. Assume secondary side of utility transformer is 208V, and building need 208V.

Well if my NEC load calc for a multifamily building is 1000A @ 208V, I'd expect the utility will usually come in right around 300kVA padmount. You might see a 225kva sneak by, but not a 150kva.

 

[Tainted]

Essentially what this means is that you cannot run 400 amps+ for more than half an hour. You can have some spikes that are more than 400 amps but do not exceed 400 amps+ for more than half an hour.

 

[Tainted]

So the answer is utility only will able to provide 400A max, even if my demand load is 1000A? Which mean if my building use 500A, then utility side OCPD will trip? Because they are not going to provide 500A.

Then is it mean the voltage drop calculation base on demand load is kinda useless, because you won't see full demand load.

You must size your equipment per code, not what the utility can give you.

Are you dealing with Con-Edison?

In my recent project I had 3 sets of 4#500KCMIL from Con-Ed which is around 1200 amps, but I sized my switchboard at 3000 amps. I had 3 sets of 500s coming into the service end box and 8 sets of 500s coming out going into the 3000 amp service switch.

 

[binwork91]

You must size your equipment per code, not what the utility can give you.

Are you dealing with Con-Edison?

In my recent project I had 3 sets of 4#500KCMIL from Con-Ed which is around 1200 amps, but I sized my switchboard at 3000 amps. I had 3 sets of 500s coming into the service end box and 8 sets of 500s coming out going into the 3000 amp service switch.

Yes, Con ed. Regarding to NYC electrical code 215.2 exception no.4 for voltage drop.
Exception No. 4: Where the distance between the utility service point and the service disconnecting means exceeds 15.2 m (50 ft), the voltage drop between the service point and the service disconnecting means may be calculated utilizing the service capacity limits defined by the utility company in lieu of the computed load. The distance between the service point and the service disconnecting means, the computed load and a letter on utility company letterhead indicating service limits shall be submitted for department approval.

Coned never provide the service capacity to meet your demand load.

 

[kwired]

So the answer is utility only will able to provide 400A max, even if my demand load is 1000A? Which mean if my building use 500A, then utility side OCPD will trip? Because they are not going to provide 500A.

Then is it mean the voltage drop calculation base on demand load is kinda useless, because you won't see full demand load.

Something tells me if you exceed 400 amps for longer than half hour then you will start seeing demand charges on your billing.

If you plan to have that kind of load regularly you may need to negotiate different demand arrangements with them. Likely means higher base fee but higher demand is allowed before demand charges are billed. Could be equipment/construction fees involved as well if they need to upgrade things to be able to supply what you want.

 

[tortuga]

If your working with this utility often I'd ask them to explain more what 1/2 hr rating means to them, even if its not your job to know it helps plan big projects. Is it fed from a network of transformers or a single unit? A simple example would be if you had just one building on one single 208Y transformer, with a given transformer type and installation conditions one could model a table of possible transformers and the peak load they can carry for 1/2 hour before the core overheats, and I'd suspect thats what they size NEC calculated load to. For an example;

208V 3PH​	Secondary Amps​	​
Transformer kVA, Padmount Liquid cooled​	½ hour rating​	1 hour rating​	24/hr rating​	Primary ½ Hr rating​
37.5​	208​	130​	104​	4​
75​	416​	260​	208​	6​
112.5​	624​	390​	312​	10​
150​	832​	520​	416​	14​
200​	1110​	694​	555​	18​
225​	1250​	781​	625​	20​
300​	1666​	1041​	833​	28​
ASTM D-3487, TYPE II mineral oil.​	​	​	​	​

 

[binwork91]

If your working with this utility often I'd ask them to explain more what 1/2 hr rating means to them, even if its not your job to know it helps plan big projects. Is it fed from a network of transformers or a single unit? A simple example would be if you had just one building on one single 208Y transformer, with a given transformer type and installation conditions one could model a table of possible transformers and the peak load they can carry for 1/2 hour before the core overheats, and I'd suspect thats what they size NEC calculated load to. For an example;

208V 3PH​	Secondary Amps​	​
Transformer kVA, Padmount Liquid cooled​	½ hour rating​	1 hour rating​	24/hr rating​	Primary ½ Hr rating​
37.5​	208​	130​	104​	4​
75​	416​	260​	208​	6​
112.5​	624​	390​	312​	10​
150​	832​	520​	416​	14​
200​	1110​	694​	555​	18​
225​	1250​	781​	625​	20​
300​	1666​	1041​	833​	28​
ASTM D-3487, TYPE II mineral oil.​	​	​	​	​

Thank you for the suggestion. I work for contractor, sometime it is hard for us to get more information from utility. But I will try in the future.
Btw, where is the table from?

 

[jhink]

I believe the 24hr data is from transformer manufacturers FLA. i reference 2 manufacturers, Solomon and Hammond.