50 Amp Power Distribution "Spider" Box

[cdcengineer]

Hey all. I have always seen those spider boxes used for temp power. Usually the contractor runs 50 Amp, 4-wire as the feed for these boxes even though the receptacle configuration can vary on each box and the boxes usually have 50 Amp feed thru receptacles for powering additional spiders.

My question is, what type of load should we consider these spiders? The closest thing I can think of is NEC 551 using 9,600 VA for the 50 Amp feed. I am working on a project and they are installing receptacles to power temp spiders during events. This is a town project and they hold events throughout the year. No one really knows what loads the vendors will actually have, but they know that having spiders fed from generators in the past is what has worked.

Any feedback is greatly appreciated. Thanks.

 

[Rick Christopherson]

For planning out portable power distribution, you do not consider the spider as the load, but the vendor's load as the load. You start at the end and work back to the source. For a properly planned event, each vendor should be renting a specific amount of power, and this should be listed in their application form for setting up at the event (or specified maximum by the event coordinator). You base it all on what is used. For example, just because a booth has a 20-amp receptacle does not mean they can use 20-amps. Their contract may limit the amount of power to just 10 amps.

Spiders come in a wide variety of configurations, but a 50-amp 120/240 spider typically has 6 120V outputs. The feedthrough function on a 50-amp spider is rarely ever used unless it is known that the load connected to each spider is very low. On the other hand, a 200-amp 3Ph spider will frequently use the feedthrough. (Some spiders will even have 200A 3Ph in and 3x 200A 3Ph (unfused) outs. So you always protect your feeders from upstream.)

Each spider is placed near a cluster of vendors whose total load does not exceed the total available power by that spider. If the booths just have some lights or video monitors, that can be quite a few booths. However, if it is food, it takes few booths to reach capacity.

Upstream from the spider, you would have a larger distribution panel, also available in many different configurations. These will typically be 3-phase panels in either 100-amp or 200-amp inlets. You size this panel's main based on the total load it needs to serve, and the number of outlets depending on how many spiders it feeds.

For really large events, it probably all begins back at an I-Line distribution panel located close to the generator, and feeding off to the smaller mid-distribution panels.

Depending on where you are located, you probably already have a local company that not only rents all of the panels and cables, but can even assist you in laying out the distribution plan.

 

[cdcengineer]

Rick,

Thanks for the response. This is a town project where the power will be installed permanently. The town owns spiders and will wheel them out to pre-installed receptacles for events as needed. They do not charge vendors based on usage, but more so on a 50A receptacle or 20A receptacle flat fee. And we do not know loads. The town wants each receptacle dedicated, but I cannot see that every receptacle will draw the max allowable full load all the time. I just don't know if using the demand factors from NEC 551 make sense and considering each 20A or 50A receptacle to be 2,400 & 9,600 VA respectively.

I'm working through it with the town, I just wanted to see if anyone had any experience sizing this type of installation.

 

[texie]

Rick,

Thanks for the response. This is a town project where the power will be installed permanently. The town owns spiders and will wheel them out to pre-installed receptacles for events as needed. They do not charge vendors based on usage, but more so on a 50A receptacle or 20A receptacle flat fee. And we do not know loads. The town wants each receptacle dedicated, but I cannot see that every receptacle will draw the max allowable full load all the time. I just don't know if using the demand factors from NEC 551 make sense and considering each 20A or 50A receptacle to be 2,400 & 9,600 VA respectively.

I'm working through it with the town, I just wanted to see if anyone had any experience sizing this type of installation.

I'm assuming that you are going to supply each 50 amp recep. with a 50 amp branch circuit and you are trying to calculate the feeder load for the group of receps. I think your approach to use 9600 VA is reasonable for an application like this. There are no hard and fast code rules for this. Rather I think sound engineering judgment applies here and this seems reasonable.

 

[cdcengineer]

I'm assuming that you are going to supply each 50 amp recep. with a 50 amp branch circuit and you are trying to calculate the feeder load for the group of receps. I think your approach to use 9600 VA is reasonable for an application like this. There are no hard and fast code rules for this. Rather I think sound engineering judgment applies here and this seems reasonable.

Yeah Tex

I am sizing branch circuits for full load, I was just wondering if I could get away with using NEC table 551.73(A) demand factors. I am not comfortable sizing the service at 45 to 47% based on my quantities. I may either take each receptacle at full load i.e. 2400 (20A at 120V) & 10400 (50A at 208V) and then apply an arbitrary 80% demand factor, or I may use 1920 VA (per 20A) & 9600 VA (per 50A) and take them at 100%. It's close either way. I just don't want to run into any trouble with tripping the main during a big event.